Cammer: The Pontiac OHC Six


The major objections to overhead cams for mass-production engines had always been cost and complexity. Most gear-driven overhead cams were prohibitively expensive for non-racing use and unacceptably noisy to boot. Chain drive, used by most production OHC engines of the fifties, was somewhat simpler, but still entailed a relatively high level of mechanical noise, not to mention the challenges of maintaining proper chain tension and lubrication.

An intriguing alternative was using a cogged rubber belt, like the Gilmer belts used to drive mechanical superchargers. A belt is quieter than a chain or gear drive, weighs less and thus consumes little power, and requires no lubrication. Better still, it’s considerably cheaper than either gears or chains.

Pontiac OHC Six in a 1967 Firebird Sprint - front
The Pontiac OHC six’s timing belt did not normally require adjustment, but the tension could be adjusted if necessary by moving the front cover. Unlike some later OHC engines, the Pontiac cammer is not an interference engine. If the timing belt breaks, it will immediately stop the engine, but it will not normally cause valve or piston damage, a serious risk with some OHC layouts. (Photo © 2006 Robert Nichols; used with permission)

Belt-driven camshafts were not a new idea even then. In the mid-fifties, racing engine builders had begun experimenting with belt-driven DOHC heads, including a 1955 Cadillac V-8 conversion. Although those early efforts were not very successful, they attracted the attention of the United States Rubber Company (later known as Uniroyal), which sensed a potentially lucrative new market; Uniroyal started developing automotive timing belts around 1956. Pontiac began its own experiments in 1959, initially using stationary engines.

Around the time the OHC six project began in earnest, the German automaker Glas introduced the 1004-S coupe, the first production car with a belt-driven OHC engine. The Glas engine, initially displacing 993 cc (61 cu. in.) and eventually expanded to 1,682 cc (104 cu. in.), proved durable and reasonably dependable, although Glas engineers hedged their bets by recommending timing belt changes every 25,000 miles (40,000 km).

The Glas engine was encouraging, but developing a timing belt adequate for a torquey big-bore six still presented a problem, particularly since McKellar was determined to find a belt that would last the useful life of the engine. Simple rubber belts weren’t strong enough or durable enough; reinforcing the belt with steel cords provided adequate strength, but the steel would rust and eventually weaken. Using stainless steel cords eliminated the corrosion problems, but was much too expensive and showed worrisome signs of fatigue at high mileage.

Pontiac’s eventual solution, developed in collaboration with Uniroyal engineer Richard Case, was a 1-inch (25-mm) wide, fiberglass-reinforced, neoprene-impregnated nylon fabric belt, which proved to be strong and durable, demonstrating minimal wear in high-mileage testing. Unlike some later automotive timing belts, it was not overly sensitive to dirt and oil, although Pontiac ultimately decided to keep it covered to protect it from snow and road spray.

Another of the bugbears of early overhead cam engines was the need for periodic valve lash adjustment. That, too, was unacceptable to Pontiac, whose divisional policy mandated hydraulic valve lifters (which needed no adjustment in normal use and prevented over-revving) for all engines carrying a full factory warranty. Hydraulic lifters had never been seen as practical for OHC engines, but Pontiac developed a clever solution, a variation of a concept GM had developed and patented in the mid-fifties for pushrod engines. Although the OHC six’s camshaft was mounted almost directly above the valves, it actuated them through finger-type cam followers — essentially small rocker arms — each of which was pivoted on a small hydraulic sphere that functioned like a hydraulic lifter. The pressure exerted by the sphere served to maintain a constant zero valve lash, reducing mechanical noise and eliminating the need for routine valve adjustments without adding to reciprocating mass or inertia.

Pontiac OHC Six in a 1967 Firebird Sprint
The Pontiac OHC six had an iron block and head, but the cam cover (where the camshaft is actually mounted) and the timing belt cover are both die-cast aluminum, helping to keep total engine weight to 489 lb (222 kg). Note the location of the distributor; it’s driven by a shaft running along the side of the block along with the oil pump and fuel pump. (Photo © 2006 Robert Nichols; used with permission)

The rest of the engine was a study in compromise. The cast iron block was loosely based on that of Chevrolet’s 1962-vintage OHV six and shared the Chevrolet engine’s connecting rods and seven-main-bearing crankshaft. However, Pontiac extended the skirt below the crankshaft center line for greater rigidity, much as Ford had done with its old Y-block V-8. (The deep skirt also allowed the use of cross-bolted main bearings, although these were specified only for the more powerful iterations.) Bolted to the right side of the block was an aluminum carrier for the accessory drive, including the gear-driven distributor and fuel and oil pumps. The accessory shaft sprocket was driven by the timing belt and did double duty as a belt tension adjuster.

The cast iron cylinder head used wedge combustion chambers with side-by-side valves like those of Pontiac’s V-8s, but the camshaft was actually mounted in an aluminum cam carrier rather than in the head itself and had very wide lobes to minimize wear. The valves, shared with Pontiac’s V-8s, were quite large: Intake diameter was 1.92 inches (48.8 mm) while exhaust diameter was 1.60 inches (40.6 mm), the biggest the ports would accommodate.

Despite its novel features, the Pontiac engine was more mildly tuned than were most of its European contemporaries. The basic version had a modest specific output of 0.72 hp/cu. in. (44 hp/liter), compared to 1.08 hp/cu. in. (65 hp/liter) for the big Mercedes six. On the other hand, the Pontiac engine was designed to be dependable and free of temperament, which could not necessarily be said for its more exotic British, German, and Italian rivals. It was not unlike Hollywood remakes of popular European films: retaining the basic plot of the original, but recast with familiar faces and a bigger effects budget.


Prototypes of Pontiac’s OHC six were running on test stands by the spring of 1962, but development and testing of the production engine was protracted and it was not production ready for another two years. However, that didn’t stop Mac McKellar from applying some of its concepts on a considerably larger scale.

For the past few years, Pontiac had been a major player in NASCAR competition, working surreptitiously with private teams to get around GM’s official no-racing policy. By 1962, NASCAR had become an arms race between the major automakers, each of whom fielded an array of increasingly specialized engines and equipment. Pontiac’s most recent salvo was the Super Duty 421, a ferocious 6,902 cc engine laughingly underrated at 405 gross horsepower (302 kW) with two four-barrel carburetors. It was essentially a hand-built engine, offered to the public only in tiny numbers for homologation purposes.

Despite its power, the Super Duty was hard pressed by the latest Chrysler and Chevrolet engines, particularly the new Chevrolet Mark II “Mystery Motor” that appeared in early 1963. To remain competitive in NASCAR, Pontiac would need something more.

McKellar’s solution was an overhead cam conversion of Pontiac’s 389 cu. in. (6,372 cc) V-8, drawing on concepts developed for the OHC six. Where the six sacrificed outright sophistication in favor of lower production costs, the 389 had no such compromises; it had 32 valves, belt-driven dual overhead camshafts (using a more robust version of the six’s belt drive), a cross-ram intake manifold, and sequential fuel injection. Pontiac never released power figures for the DOHC engine, but it probably made well over 500 gross horsepower (373 kW).

Unfortunately, the twin-cam 389 never made it to the racetrack. In early 1963, GM chairman Frederic Donner issued a tersely worded memo reiterating the corporate ban on racing, adding that under-the-table participation would no longer be tolerated. Pontiac’s DOHC engine went back on the shelf, although the division continued to work on OHC V-8s on an experimental basis. Toward the end of 1963, McKellar developed a simpler SOHC 421 with 16 valves and one belt-driven cam per bank, capable of some 620 hp (462 kW) with Tri-Power carburetion. This was followed in 1965 by a 24-valve SOHC version of the newer 428 cu. in. (7,008 cc) engine.

McKellar showed off the experimental engines to Hot Rod editor Eric Dahlquist in 1968, but none of the OHC V-8s made it to even limited production. Forbidden to race, Pontiac had little need for them, and the growing safety lobby had left GM management wary of fielding very powerful engines. A 500 horsepower (373 kW) OHC V-8 would have been a provocative gesture as far as Washington was concerned, and the GM brass was in no mood for provocative gestures.


While it originated in DeLorean’s Advanced group, the OHC six, unlike the V-8s, was always intended as a production engine. Its prospects for production improved significantly in November 1961 when DeLorean was promoted to chief engineer, succeeding Pete Estes, who replaced Bunkie Knudsen as general manager. Although the six was destined to become the base engine in Pontiac’s A-body intermediate line, its first application was DeLorean’s most ambitious project to date: the two-seat Pontiac Banshee.

The Banshee project, known internally by its styling code, XP-833, began in August 1963. Designed by Roger Hughet and Ned Nickles of the Advanced Design Studio, it was a compact fastback coupe, looking something like a miniature Corvette Sting Ray. To minimize tooling costs, XP-833 used a fiberglass body with a steel floorpan, although it borrowed most of its running gear from the new A-body Tempest. The OHC six was to be the base engine, although the second prototype was powered by a Pontiac V-8. DeLorean conceived it as an inexpensive sports car, a competitor for the new Ford Mustang.

1964 Pontiac Banshee roadster side
Not counting the early non-running mockups, there were two Pontiac Banshee prototypes: a coupe and a roadster. The coupe was powered by a base one-barrel OHC six, while the roadster was originally powered by a 326 cu. in. (5,340 cc) Pontiac V-8, which probably would have been optional if the Banshee had made it to production. (Photo: “pontiac_banshee_2” © 2009 EvThoMcC; used with permission)

GM management was unenthusiastic about the Banshee, preferring Pontiac to instead join Chevrolet’s new F-body sporty-car program. Estes and DeLorean still believed the XP-833 was a viable concept, but they realized that the corporation would kill it if they continued developing it through normal channels, so DeLorean assigned Advanced Engineering chief Bill Collins to continue the project in secrecy.

In the summer of 1965, DeLorean was promoted to general manager of Pontiac. Seeing his opportunity, DeLorean had Collins show off the two fully finished XP-833 prototypes to senior management, along with a beautifully illustrated presentation that detailed the Banshee’s expected market position, tooling costs (well under $20 million), and projected sales (about 32,000 a year). With a starting price of $2,500, the Banshee would compete directly with the Mustang and would help to bolster Pontiac’s sporty image.

Unfortunately, Donner and GM president Jim Roche were not interested. They thought the XP-833’s lack of rear seats would limit its sales potential and worried that the car would cannibalize sales of the more expensive and more profitable Chevrolet Corvette. DeLorean continued fighting for the Banshee until the spring of 1966, but Ed Cole, GM’s executive vice president, finally ordered him to forget it and develop a Pontiac version of the F-body, which became the 1967 Pontiac Firebird.

To DeLorean and Collins’ great annoyance, not long after rejecting the XP-833 project, Roche and Donner approved production of the conceptually similar (and similar-looking) Opel GT, based on the European Opel Kadett sedan. The GT was roughly the same size as the Banshee, but it used a steel body and four-cylinder engines. To add insult to injury, it was sold in the U.S. through Buick dealers, not by Pontiac.


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  1. Great article Aaron! Milt Schornack of Royal Bobcat fame had some good words concerning the OHC six in his book. It appears they did some testing with headers and a tri-power setup on the sprint six engine. It would be quite the sleeper if it weren’t so loud.

    1. Pontiac did some similar experiments — the PFST project, developed by Herb Adams, used three Webers and headers. It was a pretty good setup, but it was too noisy to pass muster, and GM had banned multiple-carburetor setups.

      (Once interesting side note is that McKellar’s engine guys tried to create a common baseplate for the Tri-Power set-up so they could tell the corporation it was a single six-barrel carb. It didn’t work, though.)

  2. How does an engine designed by (presumably) capable, experienced engineers make it into production with a design flaw like this?

    1. Mac McKellar actually took pains at the design stage to reduce camshaft wear; the lobes were twice the normal width, for example, in an effort to reduce surface pressure. However, hand-assembled test engines may not reveal issues that crop up with assembly-line engines owned by people who only change their oil once a year.

      As I understand it, the camshaft damage to the ’66 and ’67 engines was usually caused by one of three things:

      1) Incorrect machining of the metering hole in the restrictor that that controls the flow of oil to the camshaft journals. A lot of ’66 and ’67 engines came through with too large a metering hole, effectively reducing oil pressure to the cam and lash adjusters. This problem could be exacerbated by an incorrectly machined or clogged primary oil passage (the line through which oil flows to the cam cover), which could happen with infrequent oil changes or poor-quality oil.

      2) Too rough a finish on the contact area of the cam follower, where the follower actually touches the cam lobe, scuffing the cam.

      3) Broken retaining clips. The ’66 and ’67 engines used little metal spring clips to hold the lash adjuster to the cam follower during assembly. This was just an assembly-line convenience; once the cam cover is assembled, it’s not necessary. However, they just left the clip in place on the assembled engines, which would occasionally break when the engine was running, damaging the cam and/or valves with the pieces. The later engines omitted the clips, and simply removing them from the 230 will avoid the problem.

      For the most part, these were manufacturing/assembly issues, rather than design problems. Without talking to old Pontiac engineers, I don’t know why they weren’t fully resolved until the ’68 model year; if they’d been taken care of in the first few months of production, I’d file them under “teething problems.” I assume it comes down to the fact that design engineers don’t control production, and vice versa, as happened with the con rod breakage on the Fiero engines years later. (In that case, Saginaw foundry division was aware of the metallurgical problems, but they had no incentive to fix them.)

  3. This engine should have been an option in the 73-74 Ventura GTO. With an appropriate suspension and steering it would have been an excellent road car for the time and sales would have exploded during the first oil embargo.

    1. If GM had let Pontiac keep that engine past 1969, it should have surpassed the pushrod Chevy I6 until the end of the 70’s. Had the people at GM known there was going to be a oil embargo in 73 it would have been a more perfect replacement for the Chevy I-6.

  4. I have a OHC 6 without a Z (code) build date I think is L076 (DEC. 7th 1966) But can not find any code starting with a Z? I was told this engine was never loaded into a car or frame and was sent to a school for testing? Do you thoink there would be any truth to this? Thank you Rick

  5. I thought the cammer poncho was awesome,–especially the Sprint, and I wonder–do blue prints/photos exist for the never-produced DOHC 389? Or even the SOHC 421 & SOHC 428? The tri-power OHC-sprint? Taking a page out of Govt., I wonder what “vices” those jerk-Globalist(imo) Board Members of GM had–evidently none that Delorean was able to exploit. I mention this because Pontiac is no more but for idiots that didn’t want to “ruffle” Govt. feathers, like the moribund Roach and the drooling Donnor-Dumber–two killers of Pontiac-Power, and Legend.

    1. I assume the blueprints for those engines still exist in the files somewhere (certainly for the SOHC — as the conclusion mentions, Mac McKellar ended up with one of the prototype engines). It’s possible some of the prototypes are in the Heritage Center, along with other abortive GM engines like the SOHC Cadillac V-12, but I haven’t checked.

      It’s easy to understand why the SOHC and DOHC V-8 projects ended up not going anywhere, regrettable as it may be. Pontiac already had engines more powerful than senior corporate management thought was prudent; the division didn’t have a NASCAR program where a hot SOHC 421/428 would be really useful; and price escalation and insurance rates were already making the really hot cars unaffordable to most of the kids who wanted them. And that’s without even getting into the emissions certification issues. If the SOHC/DOHC engines had made it out of experimental, they probably would have been roughly as attainable as the Ford SOHC 427 or Chevy’s early Z-11 427 “Mystery Engine.” For the street, a Ram Air 428 or 455 would have been a lot cheaper and probably more practical.

      Still, I would be lying if I said I didn’t find the idea of a Trans Am 303 with overhead cams intriguing…

  6. GM UK (Vauxhall) introduced a belt driven SOHC four engine in 1968, using some design cues from the Pontiac 6, the camshaft in an aluminium housing and large followers, but with solid lifters. Very few European engines had hydraulic lifters then.
    However it was slanted 45 degrees, more like half a V8, although it helped it fit under hoods more easily.
    It wasn’t a great design, no more refined or efficient than old fashioned ohv engines from contemporary Ford or BMC offerings, and nor easy to work on either.
    I suspect some aspects of its design were influenced by Pontiacs development work, can you verify or deny this?.


    1. Roger,

      I honestly don’t know — I haven’t looked closely at Vauxhall’s behind-the-scenes history in that era. If I find anything out in that regard, I’ll comment here.

      1. If you haven’t already you should check out Vauxpedia that has lots of useful information on what Vauxhall was up to in terms of development.

  7. Can a base 67 Firebird 6 cylinder ohc engine be modified to be a sprint engine? If so, what reference is available to complete the conversion?

    1. I’m not qualified to advise anyone on modifying engines — sorry!

    2. I would imagine higher comp pistons and sufficient flowing manifolds along with a hipo carb setup.

  8. I just picked up the base overhead cam 6 engine from a 68 Firebird. My Dad worked at the Pontiac dealership when these cars were new. He said the only one that gave cam trouble were the ones the older people had. He any younger people that drove them kept them revved up high enough I guess to keep the cam lubed.

  9. It’s a damn shame that GM is such a bullheaded company when it comes to innovation.

  10. My first car was a Pontiac LeMans bought from my grandfather. It had the OHC six 2-barrel. I enjoyed the car for a year and then sold it to my parents. They had to rebuild the top end twice and finally junked it at 67,000 miles. The cam design was definitely flawed. Interesting to read all the knowledgeable comments about this engine.

  11. I worked, as a mechanic, at a Pontiac dealer and was excited when these cars stated coming in to be sold. They drove well and were economical. A few months later the only excitement was trying to keep them from eating their overhead cams. I could not believe that GM would release such junk.

  12. hi, i have 66 tempest custom still has the original engine ZD CODE that came factory in it with the optional four speed, it has 98.000 miles on it, at 80.000 miles had to put cam assembly on the engine, but she run’s just fine, and the only rust spot’s are at rear glass and above back bumper, floor’s solid, and i have rare 67 firbird ohc 6 sprint that came factory with transistorized ignition, the amp box mount above the heater, and it has the factory wiring, do you have something like this, tell me about it thank’s k.t.

  13. Can anyone confirm an attempt to extend the Pontiac OHC engine lifespan in Australia powering Holdens that soon fell apart once it was revealed to be close in power to Holden’s own V8 during that period?

    A pity it never lived on particularly in Australia along similar lines to the Australian versions of the Ford Straight-6 that eventually became the Ford Barra engine (topped with 320-420 hp 4.0 Turbo variants).

    1. I’ve never heard anything about that, although it does sound reasonably plausible. (I assume such a thing would not have been the Pontiac OHC six per se, but rather a similar OHC conversion of the existing Holden six, so as to preserve as much of the original tooling as possible.) There’s certainly a lot of precedent for that kind of thing. Of course, given how much GM-Holden probably spent tooling for the locally built V-8, I can see how they would ultimately have decided not to also go forward with an extensive revamp of the existing six, especially if it produced similar power.

  14. Loved your article on the OHC6.
    I’m still driving my 1967 LeMans, 2-door hardtop. Tahiti blue, OHC6,1-bbl Rochester carb, with the 2-speed power glide transmission.
    I bought her new in December, 1966, in Austin, TX. I drive it monthly with more than 178.000 on her. She’s 100% original with the same hub caps, gas cap, car keys, interior, etc.
    I overhauled the engine in 1979. The timing belt, with 142,000 miles on it, actually looked good. She still has the same cam and lifters.
    The main reason I still have this car is it’s beautiful design, at any angle, as well as such an easy driver.
    I just wanted to talk about Becky Blue (her name) since she’s at the New Braunfels Classic Car Restoration shop for a complete redo. It’ll be her 50th birthday coming due soon.

  15. I have always liked these Pontiac OHC sixes. I never owned one, but I did come close to obtaining a very old and beaten up 67 Lemans Sprint back in the 80s.

    These engines are very cleverly designed, and I still do not understand how they are more expensive to make that the corresponding Chevy stove bolt 6.

    For instance, the block is very simple: There is no machining for a cam, oil pump, lifter bores and lifter galleries, fuel pump, and a distributor.

    The oil pump, distributor, and fuel pump are mounted on a die cast aluminum assembly (somewhat analogous to the die cast front ends on Cadillac and Buick V8s). All of this was driven by an auxiliary shaft that was, in turn, driven by a pulley that was also used as the cam belt tensioner. This whole assembly moved up and down on a pad, machined on the lower right side of the block and was held to the block by four bolts working in slots in the die cast assembly. The whole assembly could be moved up and down against the block to achieve the proper cam belt tension. It was prevented from tilting out of alignment by a slot milled into the pad on the block and corresponding key in the assembly. Like the four mounting slots, inlet and output oil galleries in the block matched with slotted passages in the assembly.

    It seems to me that casting and machining a die cast part is less expensive than performing similar operations to make cast iron parts.

    There is more expense in making and setting up gears to drive the cam (as in the Chevy 6) than in making pulleys and a timing belt — this is even cheaper than making sprockets and a silent chain, as some other 6s used.

    The cam is held in another die casting, again, easier to machine than a cam in a cast iron pushrod block.

    I see this engine as being very adaptable to all sorts of uses, from a heavy duty truck engine (longer strokes could more easily be accommodated in a higher block that used the same tooling as the car block) and head design would be practically unlimited, with later aluminum technology, a cross flow head, and even a DOHC head. Performance, in other words, could be easily manipulated more cheaply than in the normal pushrod design.

    Unfortunately, Detroit’s lack of attention to engineering a reliable design was capitalized on by companies like Toyota and Honda, who found cheaper ways to make better parts and, much to their customers’ delight, didn’t expect their customers to do their trouble shooting for them.

    A word about the “Y Block” design of the block (and Y Blocks, as well!). Deep skirted blocks, so despised by the Cfhevy-Synchophant car rags, was a design used by GM, as well, such as the Small Block and Nail Head Buicks, in Chrysler’s B and RB V8s, and AMC’s old 287/327 V8s, besides Ford’s Y Block, Lincoln Y Block, FE, MEL, and SD truck motors.

    The purpose of the deep skirt is not to provide a means of using cross bolted main caps to increase the strength of the main caps, as those aforementioned car rag writers would have us believe.

    First of all, the purpose of cross bolted mains wasn’t to reinforce the main caps. It was to keep the main caps from “walking” on their seats and consequently allowing the mains to spin. This was a problem when slamming a two-ton stock car into a high speed corner at nearly 200 mph and then letting off of the throttle. The forces in the block are tremendous in this case, and anchoring the main caps was the problem. Ford FEs and Mopar 426 Hemis accomplished this by tying the main caps to the deep skirt with cross bolts.

    Pontiac, back in the early 60s, accomplished the same thing by using four bolt main caps on their skirtless V8 blocks in the early 60s — something Chevy later copied to solve similar problems.

    Furthermore, in a V8, it can be argued that the “Y Block” deep skirt does directly support the crankshaft partially, as it is clear (contrary to the silly arguments made by the most famous car rag) that in a V8 engine, the crank isn’t being pushed out the oil pan opening, it is being pushed at a 45* angle to perpendicular. This, however, is immaterial in an inline six.

    A famous inline six made by GM that used the deep skirted “Y Block” was the old Detroit Diesel, such as the common 6-71. The deep skirt is used to strengthen the engine longitudinally, which is why this design is quite common in many engines today. Regarding the Pontiac OHC 6, it would give the engine the durability needed in its high performance garb to push relatively heavy intermediates and pony cars around with surprising performance.

    Regarding the question posed by one of the posters in this thread, the Sprint engine differed from the base six, not only in having higher compression and a Rochester 4v Quadrajet, but in having a more radical cam and different, stronger rods. The block also has room to accept the long-stroke Chevy 292 truck inline six crankshaft.

    John Z DeLorean had admirable skill as an engineer, and this Pontiac engine is part of his legacy, along with other automotive designs you have related to us on this sight. It’s a shame that his business ethics didn’t match his skill, but the Pontiac OHC 6 is a design I’ll always admire.

  16. Have you ever heard of, or do you have any information about, an over head cam engine based on the Corvair horizontally opposed six cylinder engine? This engine was proposed for the GM Astro I show car in the mid 1960’s. The over head cams, one on each cylinder head, were belt driven, similar to the Pontiac’s OHC 6. This engine would have been in development around the time of the development of the Pontiac engine.

    1. The Astro I is mentioned briefly in the Corvair article and I think in the Opel GT story as well. Detailed information about its engine is surprisingly sparse. As far as I could gather, the show car didn’t actually have a running engine (not atypical for concept cars) and it’s not clear if there was a running version of the engine; it does not appear to have been a serious production prospect. It was notionally based on the existing Corvair engine, but taken out a bit in bore (which probably would have meant new cylinder barrels) for a displacement of 176 cubic inches. I have no information on the belt drive.

      1. Aaron, thanks for the info. I had an occasion to sit (or rather, repose, because of its laid back seating position) in the Astro I, but it was almost 4 decades ago, and although I knew of the significance of the car at the time, I didn’t memorize all details about it. I did notice it had sloppy spot welds in the engine compartment. The car did not run in our presence, so I can’t verify if whatever engine was in it was runnable. I understand that it now has a common 140HP (four Rochester H carbs) engine, mainly because somebody got tired of pushing it all over the place. I’m working with someone who is working with someone who insists there are six or seven operational Cammer engine prototypes in the wild (outside of GM), and he insists that he absolutely must have one. As you indicate, there is very sparse information about this engine. Someone claims to have the blueprints for this engine, but I’ll bet he has the blueprints for the experimental Rochester fuel injection Corvair engine. I have seen those, as well as a box of parts (but not assembled on an engine). I was hoping you might have had some insight or leads as to a direction of information about the Cammer engine, but you are reinforcing the concept that there is simply nothing out there to be had. I thank you for your info and response.

        1. If there were any operating prototypes, I’ve never heard of any — which doesn’t mean they don’t exist, necessarily. It’s certainly conceivable that Chevrolet engineering (or the corporate Engineering Staff) toyed with the idea of rigging up an OHC Corvair engine, but whether it got beyond paper plans and mockups, I don’t know. (The divisions in those days had “Advanced” engineering budgets for R&D projects not necessarily intended for production.) By the time the Astro I was built, the likelihood that Chevrolet would have seriously considered a more expensive high-performance version of the Corvair engine was pretty remote.

  17. Who has blueprints on the experimental cross-flow DOHC Pontiac-6? Those could be 3-D’d via sintered metallic powders & triple-lazers in a metal-substrate 3-D replicator. The greatest cost would be the alumno-titanium sintered powders. Is there a Pontiac Museum where a example may lie? A portible scanner could obtain enough info to replicate a DOHC alloy-head, and I would expect the patents have long expired.

  18. I got to own a 69 Firebird Sprint in the late 70’s early 80’s. Swapped 3 speed manual to M 4 speed and changed rear gears, not sure of #’s. Not the best off the line but it was sweet from 15 to 110. One of the funnest highway cars I have owned. Had to sell do to way to many tickets but I still smile thinking of that sweet ride.

  19. I owned a 1966 Tempest Sprint option, 3spd, 3.55, standard steering, brakes, etc. Went like crazy on the highway. I put 69,000 miles on it with no problems whatsoever. Drag raced it quite a bit and used nothing but Kendall GT-1 racing oil. I then purchased a 1968 Tempest Sprint with the same set-up, plus 15/1 steering ratio, heavy duty rear axle and traction-lock differential. It was a 250 cube and was a much better engine for off the line. I managed a record run with it at 15.29 at 86 miles per hour. With a fours speed and 3.90’s I could have easily got down into upper 14’s. Put 68,000 miles on it and also had no problem with the engine.

  20. Well get this… Mac McKeller was my cousins husband… he was a wonderful person to know and ride with in those new GTO take home cars… yep at 17 I had the pleasure of riding with him every night in a new car for a week while I visited with my relatives… God bless Mac and the rest of my family.

  21. Enjoyed reading all the responses here.I bought my first new car in July of 68, a Lemans Sprint optioned 3 speed, metalflake Verdoro green with y paunchet white interrior.First week I owned it , it developed an severe oil leak at the external oil pump housing – apparently not sealed correctly at the factory.I never had another problem with the car – drove it daily for 7 years and put 105, 000 mileson it.w What a really nice and fun car to own A totally reliable car and a lot fun to drive. I will own anot one some day.

  22. I have ridden more than a few miles in Pontiac OHC 6 powered cars. One was a Tempest with the base engine and automatic transmission. The other was a Firebird with the 215hp version with 4spd. The Tempest acted almost like a comparable Chevelle with 6 and auto, but in the upper rpm ranges a little peppier. The Firebird on the other hand sounded like a 6 but went like a V8. One night while speed shifting my friend left the rear end in pieces over a 150ft stretch of road which forced us walk 7 miles in the dark starting a 12:30am. Never heard of anyone else blowing a rear end with a six cylinder engine.

    In my opinion GM really screwed themselves royal by letting this engine die. It worked fairly well and most of the teething problems had been worked out by the time they let it die. When they needed newer and different engines due to gas crisis and other events they could have used this as a blueprint for a 4 cylinder OHC engine and refined it for use in the larger downsized cars without having to reinvent the wheel. The original Pontiac Tempest with a 4 cyl engine had the engine created by loping off one bank of the V8 engine Instead they drunkenly lurched from one disastrous engine to another and in the process with all the other missteps destroyed the worlds largest and most profitable car company. One engine alone wouldn’t saved GM but it would have greatly helped as not to have to reinvent the wheel.

  23. Loco Mikado
    August 24, 2016 at 7:30 pm

    One night while speed shifting my friend left the rear end in pieces over a 150ft stretch of road which forced us walk 7 miles in the dark starting a 12:30am. Never heard of anyone else blowing a rear end with a six cylinder engine.

    WELL…we scattered the AMC 15 rear end with a stroker 4.9 based on that OHV engine…still running about in millions and millions of Jeeps! Cured it by installing the Mustang 8.8.

    The sixes are, as Clifford Engineering’s Logo says: “6=8”

  24. Aaron, What you said;

    ” direct inspiration for Pontiac’s OHC engines was the contemporary Mercedes big six, a 183 cu. in. (2,996 cc) engine found in the Mercedes 300 sedans and coupes and, in somewhat more highly tuned form, the 300SL sports cars. With its iron block and single overhead camshaft, the Mercedes engine was not as exotic as the twin-cam engines from Jaguar and Alfa Romeo, but it had an impressive competition pedigree and offered a fair compromise between power, fuel economy, and complexity. It became the conceptual starting point for Pontiac’s design work.”

    The engine that comes even closer to the Mercedes is all of the L- series Nissan engines. L1300-L2000 4 cyl and the L2400- L2600 and L2800 six cylinder. We even use the same special valve adjusting tool for both engines!

  25. Our second new car after we were married was a 1966 Tempest wagon with the OHC six and standard transmission. I really enjoyed that car and there was just something nice about the one piece chrome plated shift lever! But the oil rings failed after 44,000 miles and oil was spraying out of the breather-filler all over the engine compartment. Rather than spend money on an out-of-warranty engine rebuild, I traded it for 1968 Chevy BelAir wagon. Memories!

  26. I bought a ’68 Firebird Sprint OHC 4 barrel new in 1968. It was a disaster. It *drank* oil through the valve guide seals. When it got down to 400 miles/qt (making it essentially a 2-stroke) the dealer would replace the valve guide seals. I did this 4 times under the warranty, and then got rid of the car.

    Excellent idea, terrible execution.

  27. I bought a new ’66 Tempest Sprint, 3-speed after driving a co-workers base mode Tempest. To my everlasting regret I traded in a ’57 Chevy 2-dooe wagon with a built 283. (I worked in an automotive machine shop at the time and the Chevy was built for the ’56 Corvette I owned before acquiring a wife and child)

    The Sprint was fun and ran with the Mustangs of the time but fuel mileage was awful, although we normally didn’t worry about it back them.

    A friend bought what must have been a one-of-a-kind, special-ordered ’66 Chevy wagon with a 427 with full synchro 3-speed column shifter. It was insanely fast and the ultimate sleeper. GTO Pontiacs were its favorite food. I followed him from Phoenix to Durango CO once for a camping trip. He was loaded with camping gear, a wife and two Newfoundland dogs. I had a wife, infant kid and a suitcase. In the mountains I had a tough time keeping up with him and when we filled up, he got better mileage than I did. He probably weighed nearly twice as much as I, had twice the engine displacement and essentially the same carb.

    Memory fails to recall the exact engine problem that caused me to dispose of the car; a valve problem of some sort.

  28. Great article, learned everything about my (former) ’68 Sprint I wanted to and much more! If only I could get her back, sigh.

  29. Given what Ford of Australia did with the Straight-6 / Barra engines, GM missed an opportunity to improve on the engine by having an updated OHC layout filter down to the related Chevrolet Straight-6 (plus 153 4-cylinder) while spawning more potent Twin-Cam variants to replace the Pontiac OHC-6.

    1. They certainly did. At the time, GM corporate saw the OHC six as an unnecessarily expensive elaboration of an economy engine nobody really wanted anyway, but the picture would have looked quite a bit different five years later!

      A lot of the divisions played with OHC and DOHC concepts during this period, although it appears most of them were conceived as ultra-performance engines of a kind that wasn’t necessarily compatible with early emissions controls (at least with carburetors). However, even when that was addressed through electronic injection and feedback response systems, GM remained weirdly resistant to offering engines of that kind for a surprisingly long time. Low-revving pushrod V-8s and big sixes matched to automatic transmission seemed more their comfort zone.

      1. OHC mainstream cars didn’t really catch on in Europe until the 1970’s.
        The BMC E series engine in Austin Maxi’s and allegro’s, Fords into sohc offered in 1.6 and 2.0 liter sizes in Cortina’s and Capri’s joined the GM Vauxhall in the market all around 1970, Renault an Fiat also had OHC engines available. But many cars soldiered on with old fashioned ohv engines into the 1980’s and even beyond in a few cases.
        Most push rod engines could keep up with ohc engines for power and economy for everyday cars, it wasn’t until the European emissions regulations came int force in the 1990’s that manufacturers were essentially forced to adopt an ohc, and later, dohc layouts with the extra costs involved to meet the new requirements. One offshoot of this is modern engines with proper maintenance will outlast the older engines by a massive margin. Quarter million mile engines are far from uncommon, back in the day 100k miles was a fair life.


      2. Strange GM were reluctant to properly develop a long running ubiquitous engine, especially from the 3rd generation onwards. The same could be said of Chrysler as well with the Slant-6 despite them actually looking as properly developing the Slant-6.

        Given what the likes of Austin did with their copy of the 2nd generation Straight-6 (plus distantly related downsized engines), it could be argued GM actually had an almost perfect engine in the Straight-6 from which to develop and spawn related engines in a myriad of ways at relatively little cost.

        1. Well, yes and no. The big Chevrolet six was a fairly bulky, heavy engine to begin with and the Pontiac OHC version was more so. Even with the OHC, the 250 was not an especially free-revving engine, nor was it very economical by the standards of the ’70s (although for panicky buyers circa 1974, it would probably have seemed pretty good). The born-again Buick V-6 was lighter, a good deal more compact, and more easily adaptable to transverse FWD applications.

          1. The Striaght-6 as well as the related 4-cylinder would obviously be restricted to RWD models even with OHC/DOHC.

            That said GM could develop downscaled 4/6-cylinder versions of the Chevrolet 4/6-cylinder engines (or downscaled even further), roughly envisioned as being essentially a GM equivalent of the BMC B-Series (plus O/M/T-Series or Nissan J engine) without the involvement of Harry Weslake.

            Agree the Buick V6 is more adaptable to transverse FWD applications, yet a pity the closest GM ever got to a 60-degree V6 for passenger cars prior to the 1979 60-degree X V6 was a theoretical 60-degree V6 derived from the shelved 1960s Cadillac V12 prototype.

          2. Scaling down an engine is not a trivial matter, though, and is not necessarily desirable in a number of respects. There are three frequent results: a modest reduction in the weight or dimensions of an existing engine that doesn’t really justify the expense (e.g., the version of the BMC C engine used in the MGC and Austin 3-Litre); a lighter but weaker version of an existing design (e.g., the Pontiac “301”); or else creating a miniature version of an old engine that can’t share much if any of its progenitor’s tooling.

            The issue for companies the size of GM and Ford was always tooling, which is almost always the answer to the question, “Why didn’t they just reuse the design for X?” Designing engines or cylinder heads wasn’t a big deal for GM — they had engineers doing that all the time. It always came down to, “Where are we going to build this in the quantities we need and how much is that going to cost to set up?”

          3. The C-Series was actually completely unrelated to the B-Series because it was a pre-merger Morris design, with possible roots to the post-war SV Morris Six MS and OHC Wolseley 6/80.

            A 2-litre 6-cylinder version of the 1.2 4-cylinder Austin A40 engine (which formed the basis of the B-Series) was considered, with the intention to use as many common parts as possible to keep manufacturing costs as low as was consistent with reliability and servicing costs of the units. However it was not pursued due to the Austin 16 hp / Austin A70 being powered by a 2.2 4-cylinder, which was derived from Austin’s copy of the 2nd generation Chevrolet Straight-6 known as the Austin “D-Series” (making it a sort of 2nd gen-based 153 4-cylinder that spawned dieselized variants in taxis and powered the original Big Austin-Healey).

            Though a UK developed 6-cylinder B-Series never happened. The Australians later developed a 80-84 hp 2.4 6-cylinder B-Series known as the Blue Streak engine, while the Japanese built a 109 hp 2.0 6-cylinder B-Series copy called Nissan J engine.

            Interestingly during development of the MGC it was established the 2.4 Blue Streak 6-cylinder B-Series was not only significantly lighter and more compact compared to the Morris developed C-Series, but also capable of putting out 115 hp up to a maximum of 128 hp on the dyno.

            Could see GM initially developing a downscaled 6-cylinder at the lower-end of the North American market (followed by a 4-cylinder) possibly in place of the Chevrolet 153 4-cylinder, with Envoy models and even Opel (pre-CiH) utilizing both 4/6-cylinder engines. Such engines could have also evolved along similar lines to the B-OHC prototype and O/M/T-Series petrols as well as the Perkins and L/G-Series diesels with a very long production life.

          4. I wasn’t implying that the C-Series six was related to the B engine; my point was that it illustrates the practical limitations of trying to scale down an existing design while preserving the tooling.

            Again, there’s a clear temptation with these things to look at in engineering terms (or performance engineering terms) what are predominantly manufacturing problems. The primary reason GM or, to Roger’s point, Ford of England/Ford of Europe clung to old designs was that it was the path of least resistance from a production standpoint, not that they weren’t able to come up with better designs. For instance, GM’s North American divisions were not building any Opel engines (other than the handful of 327s Chevrolet built for Opel for the Diplomat 5,4) and were not equipped to do so. Therefore, using Opel engines would have meant either buying them from Germany or extensively reengineering the design for North American manufacturing and setting up a completely new line, at which point there was little incentive to use the existing design rather than something homegrown. It had very little to do with the superiority of the design.

            Also, I have to underscore again that until the late seventies and early eighties, it was very rarely a matter of “GM” developing something and more a question of whether an individual automotive division considered something worthwhile enough to put some resources into developing AND was able to convince corporate management to fund it. There was not a single unified engineering arm (or even North American engineering arm) doling out engine designs and deciding what the corporation as a whole might need. It was very different from Ford, Chrysler, or most European or Japanese manufacturers in that respect.

          5. Given the C-Series likely roots, an earlier 1.5 4-cylinder version was developed via the SV Morris Oxford MO and Wolseley 4/50 (with a 1.1 OHC was developed though not produced), additionally 4-cylinder OHV versions were developed though not pursued in 1750cc and 2-litre forms for the mk2 Morris Oxford and MGB (with some alleging the prototype engines were comparable to the Volvo B18 / B20 units).

            Had the merger with Austin never happened, came about later or under more equal circumstances (where Morris was not the weaker partner by investing like Austin did beforehand), it is likely a downscaled “A-Series” equivalent would have been developed to replace their own pre-war SV / OHV engines (that were essentially copies of the Ford Sidevalve engine). The post-war Morris Minor was originally intended to be powered by the 918cc Wolseley Eight OHV and would have received a 960-1000cc version had the merger not happened, with Morris engineers claiming it was better then the A-Series that replaced it.

            While understanding where you are coming from with each GM marque / subdivision having their own engineering arm and relative autonomy, the point is using downscaled engines derived from the Chevy Straight-6 / Pontiac OHC-6 (and related 153 4-cylinder would have negated the need for importing Opel engines and placed GM in a better position by the time the 1970s fuel crises hit.

            They would have not have to use Isuzu engines nor potentially even the Vega or Iron Duke engines at the lower-end of the range, as all could have been adequately taken over by a range of small 4-cylinder petrol and diesel engines (with distant scope for a 5-cylinder variant). The small 6-cylinder likely being pensioned off to South America and markets due to GM adopting the Buick V6 and other V6 engines.

          6. Here’s the thing: Chevrolet only offered the 153 four in the U.S. very grudgingly, and I don’t think production was ever very great. It was one of those things that was useful to have in the catalog, but that dealers didn’t want to order except maybe to have a bottom-of-the-barrel price leader for newspaper ads (“Prices as low as …”), and that sold in tiny numbers. Under those circumstances, having it be a derivative of the Nova six, sharing a lot of off-the-shelf parts, made sense. It was a minimal-investment option, and any significant variations from the engine on which it was based would have defeated the purpose.

            Had there been greater utilization of it, that picture would have started to change. Producing several different engines on the same line requires less capital investment, but it can really tie up production. At some point, it forces a decision on whether to kill the less-popular/less-profitable option or set up a second production line.

            If Chevrolet had decided they really needed a small four (which by sixties standards would have meant “2.0 to 2.5 liters”) in sufficient numbers to justify setting up a unique line, there would have been no particular advantage to maintaining the architecture of the 153/230 engines and various reasons why it wouldn’t have been desirable — too much weight and too much bulk for the sort of applications in which it would have been useful. And indeed that’s sort of what happened with the Vega 2300 engine, although that wasn’t conceived by Chevrolet. Chevrolet could undoubtedly have come up with something better, or at least less troublesome, but the old 153/230/250 family architecture would not have been either terribly useful in terms of addressing the logistical questions or in terms of performance.

            Broader use of the OHC version of the 215/230/250 engine — not scaled down, just a tidied-up version of the Pontiac six — would have made sense in the ’70s, but as a more economical alternative to low-end V-8s in X-body, A-body, and low-end B-body cars, not as a substitute for a decent, modern four or small six for cars like the Vega or the H-body compacts.

          7. The 153 was fitted to South African versions of the Vauxhall Viva HC called the Chevrolet Firenza including a 2-litre version (and opens up the possibility the engine could easily fit into similarly sized T-Car Chevette).

            Which makes one wonder if the 153 engine could have been fitted into larger Victor FB / FC / FD models, slotting both the Chevy II / Nova and Corvair either as an entry-level Chevrolet (if built in the US) via some early form of TASC or as more unique Envoy models.

            Agree with broader use of a properly developed Pontiac OHC-6 though would include smaller 181, 194 and 215 variants as well as equivalent Pontiac OHC-4 engines (particularly in 2-litre OHC-4 form), with the Australians embracing such an engine in place of the old Holden Six and later updating it to feature DOHCs.

  30. Must have been 7th or 8th grade when I first came into contact with an older kid who had a beautiful 66or 67 Le Mans Sprint that could outrun V-8’s. I have been a fan ever since but have not owned one. It seems these engines would be unusual replacements for cars that originally had Chevy 230’s and 250’s. Sad that many were tossed and replaced by V-8’s. I would like to replace my Impala’s 250 with an OHC-6 Sprint or even a base one. I have owned a number of the Chevy sixes, an Opel GT, a slant six Volare, 3.8 Transport, and two of the Vortec sixes in my GMC Envoys. I guess I am a “six” guy! (Only 2 V8’s in my lifetime, 283 sbc and an Olds 350) This was a fascinating string of comments and replies, thank you.

  31. Aaron, rereading your fine article again, may I observe that, while your description of the OHC design states that inertial factors make it generally superior to the pushrod OHV design, there’s another factor to take into account: especially in 50s to 70s Detroit wedge combustion chamber configurations, the pushrods form a “picket fence” through which the ports must pass, or, at least in the Detroit V8 crossflow head configuration, the intake ports must pass. This also forced constrains on the size and configuration of ports, which related to port flow and power production potential.

    Perhaps the best solution to this problem was Ford’s use of the “tunnel port”, which ran the intake port directly at the valve with the intake valve’s pushrod running through a tube passing through the port. While this resulted in a nice power increase for Ford FE 427 NASCAR motors, for some reason it was a failure for Trans-Am 302s, and Ford went to the “Cleveland” style splayed valve Boss 302 head design for that engine. Pontiac, somewhat later, whet to the tunnel port design for its Ram Air V engine.

    My point here is not that pushrod layouts absolutely prevented optimum port designs, but that in Detroit 6 and V8 engine designs with wedge combustion chambers, where the valves were in line across the head, they posed a limitation in head port flows. In Chevy and Mopar wedge V8s, two intake ports were squeezed side by side through this “picket fence”, where Ford attempted to lay ports out evenly, mandating longer runners to the end cylinders and resultant distribution challenges.

    1. That’s true, and has been a challenge with pushrod engines pretty much as long as OHV engines have existed. (I confess I’m not familiar with the porting arrangement on GM’s LS1 and later modern OHV V-8s, although I imagine if nothing else, they have the advantage of sophisticated computer modeling that would have made engine designers of the sixties weep with envy.) That said, it’s hard to make any absolute statements insofar as it is possible to design a pushrod head so that the pushrods don’t present any great impediment to porting, although there are costs for that in other areas, as Chrysler’s old FirePower and 426 Hemi engines demonstrated. The reduction in inertia is something that applies even if the porting isn’t necessarily that different.

      An interesting case study in this regard is Toyota’s 1.6-liter 2T engine of the seventies. The standard 2T was pushrod, but it had hemispherical combustion chambers, so its breathing was already quite good. The 2T-G had a Yamaha-designed DOHC head, also with hemispherical combustion chambers. In gross output, the dual-carb twin-cam engine had an advantage of 10 PS JIS or 12 hp SAE, or just about 10 percent, over the dual-carb pushrod 2T-B. The gross power peak was the same for both engines, but the 2T-G had a bit more torque and a higher (gross) torque peak, which makes me think the cam profile was about the same. Some of the difference was that the twin-cam engine had a higher compression ratio (0.3 or 0.4 points, due I think to a thinner head gasket), some was due to the 2T-G’s bigger valves, and some was the reduced inertia. I can’t quantify it a lot more than that, but it gives some idea of how these things stacked up in terms of output.

      1. Aaron, examination of the GM LS engines is somewhat interesting, since they remind me mostly of the Ford FE series. The ports are evenly distributed along the manifold face, and are tall and narrow, like an FE. GM has apparently abandoned trying to sneak paired ports through the pushrods. Your observation regarding computer analysis is interesting, given this choice and its divergence from past practice. Perhaps in the days of carburetion, Chevy and Pontiac engineers didn’t like the fuel distribution they got with an evenly spaced layout, but now, with dry intakes and injection, find it more suitable.

        Using splayed valves, as the Hemis (early, 2nd Gen, and modern) and Mopar polyspheres used, along with Ford 335 (Cleveland and 385(Lima and Boss), permitted a wider window of opportunity for porting. Big Block Chevys had splayed valves, but still stuck with paired, rather than evenly spaced ports, which allowed large ports but gave two “bad” ports and two “good” ports in each head. But wedge chambered heads with the valves in line, along with American 6s, had the picket fence issue.

        Your example of the Toyota engines isn’t familiar to me, but I’d think that splaying the valves in the pushrod hemi version would show pretty close performance, given the way that flow rate technology was applied at the time. Something a little closer to my knowledge would be the Ford FE SOHC and Mopar 2nd Gen Hemi — both were had configured combustion chambers, and both had similar intake flow rates in stock form.

        As I think about that OHC == pushrod example, it occurs to me that designers may have had more latitude with OHC designs when it came to using cams of larger base circle than pushrod engines. Larger cams allow a faster opening valves. The Ford SOHC, however, had rollers on the cam end of their rockers, which is a “cheating” way of getting around a small cam, like radiused lifters.

        Anyway, you are right: there are ways of getting around a “picket fence”, at the cost of other issues. The inline valve wedge heads of the 50s and 60s were retained (or adopted, in the case of Chrysler) because they offered something serviceable at lower cost, like the flatheads before.

        1. Regarding fuel distribution, the other consideration that I imagine was significant in the sixties was that most cylinder head designs had to be compatible with an assortment of different carburetion and injection setups. Of course, some engines were only offered (at least from the factory) in one specific configuration, but GM divisions were generally very keen on offering 2V, 4V, and even 6V or 8V versions of many of their engines, which likely entailed certain compromises in basic design. (The divisions did come up with exotic variations intended only for performance, like the Boss 429, but the applications for those were narrow and senior management was pretty frequently saying, “Look, boys, could you please develop some engines we can actually sell to Mom and Pop? Remember, we’re not in racing.”) Modern engines all have port injection, direct injection, or a combination of the two, so fuel distribution is now a very different picture.

          Another consideration may be that many if not most modern engines now have technologies like cam-phasing and variable-length intake runners. A port design that might be less than ideal where valve timing/duration/lift and intake runner length/shape/diameter are constants might work just fine where the latter parameters can be dynamically altered.

          Old JDM Toyota engines are an interesting study because there are a bewildering number of variations on the same basic engines. The T-system four, for instance, had low- and high-compression versions, several different carburetion options (supplemented by the late seventies with electronic injection), OHV and DOHC versions, and even for a while a lean-burn version using (under license) Honda CVCC technology. Many of these were offered side-by-side at different times, so, they’re a good reference point if you want to get a sense of how changing a given parameter affects engine output.

  32. Traditional Detroit engines were mildly tuned with low specific power, getting their output from cubic inches rather than RPMs. The high-revving capabilities of OHC would have been wasted on them.

    1. For most Detroit passenger car applications, that was definitely true. The exemplar in that regard was the contemporary Oldsmobile L66 Turnpike Cruiser engine, which was ruthlessly optimized for the 1,000-to-3,000-rpm regime. The fact that it was essentially breathing through a straw, even by the standards of sixties wedge-head pushrod engines, was a feature, not a bug; its design parameters didn’t call for more than two carburetor venturi, much less overhead cams. Of course, that’s a strategy that works better if you have more than 6.5 liters of displacement and no punitive displacement-based taxation rules!

  33. Aaron, I really enjoy your writing and your responses to comments. It is staggering to realize that GM was a collection of engine manufacturers that styled common bodies. It is a pity that GM couldn’t figure out before 1970 to rationalize engine development. So many engines, so very similar. I must admit I don’t really see why OHC engines were so slow to be adopted with cost usually being named as the reason. Considering the Japanese model of several blocks with a whole lot of heads allowing a lot of variety, it seems that variety is possible in an economical manner. Is it that the American cost controls were more rigid than other countries auto industries? It seems that an OHC engine only has a longer cam drive. How expensive is that? On the same topic of cost control, inline valves are in both the Pontiac OHC 6 and almost all car engines of that era. Given that all V engines are cross flow and almost all inline engines were reverse flow, why? Was reverse flow, before EFI, substantially easier to start and warm up? Does it have much loss in volumetric capacity? Oh, so many questions, related to manufacturing and costs. Thank you!

    1. GM was unusually resistant to OHC engines for a remarkably long time (into the ’90s!), so I think there ended up being a degree of organizational cultural inertia on top of anything else. Mostly, though, it came down to cost. With corporations on the scale of General Motors, one has to keep in mind that they’re accustomed to thinking of production on an enormous scale, and on an enormous scale, changes whose cost is pretty nominal on a unit basis can start to add up to real money. This also creates strong incentive for accounting, finance, and product management people to cut corners and pinch pennies; there are various infamous cases of ill-advised product decisions coming down to a desire to save a few dollars per car.

      Beyond that, the value of overhead cams in a sixties American context was not as clear as it was in Europe or Japan. The U.S. has never had the kind of barriers to big-displacement engines that many other markets do, and so the question engineers and their bosses constantly ended up asking when weighing the pros and cons of overhead cams and the like has been, “Why don’t we just bore and stroke a bit and get the same results without all the extra cost and fuss?” This is how by the sixties, Americans had become accustomed to thinking of 4- and 5-liter engines as “little” economy engines, where even a 3-liter six was a costly indulgence in markets like France or Japan.

      Also, during this period, the general direction of GM engine design philosophy was toward slow-revving engines emphasizing off-idle torque and big throttle openings. The corporation (discounting some enthusiastic individuals within the various divisions) had never been keen toward high-revving engines, both out of concern for low-end response and worries about accessory drive problems like tossing the A/C compressor belt at high RPM. The average passenger car engine very rarely saw engine speeds higher than about 4,000 rpm and was hoarse and unhappy at such speeds. Given that, overhead cams were perhaps understandably seen as unnecessary added complexity. (For that matter, only a handful of American sixes of this period even offered more than a single carburetor venturi as a factory option.)

      Vee engines, it should be said, are not all cross-flow. A typical American V-8 of the sixties had wedge combustion combustion chambers, usually with more or less inline valves. American automakers were generally resistant to cross-flow layouts (with various obvious exceptions) because of the additional valvegear complexity. Again, there were undoubtedly certain technical arguments, but mostly, it came down to penny-pinching.

      1. Aaron, thank you for your replies. I thought of you today as I was looking at the New York Times and their generally poor articles about cars. I thought you would be able to write a much better article just as fast as those hacks.

        I was struck by your last paragraph above. If you had to describe engines just by breathing arrangements using a word followed by flow, how would you describe a common American OHV V8 of 1970. Reverse flow, U flow, parallel flow? I think we agree that the inline American OHVs of the 1960’s and 1970’s are one or all of reverse, U or parallel flow. To describe an OHV V8 lends itself to significant confusion. I used cross flow because it describes the flows as being across the cylinder and even more across the plane of cylinders. I am certain I have read of inline valve and opposed valve layouts. The Chrysler Hemi being an opposed valve OHV and the Datsun 240-280Z being inline valve OHC.

        Is your usage of nomenclature that all inline valve engines are not cross flow, and only opposed valve engines are, standard in the literature? I know by your bibliography that you read a lot of car books.

        Oh, Aaron, I should reassure you that I have been reading American car magazines since Car Life folded. Not much else to do in Vermont.

        So, the need for better breathing and concomitant valve gear only occurs with the pressures for smaller cars that are easier to park while the demands of greater performance to handle the widespread construction of the National Defense highways. Detroits resistance to small cars because of the low profit margin meant they generally just ceded that part of the market with periodic attempts to reclaim it generally of limited success. Once the Clean Air Act of 1970 (gotta love Nixon) was signed and EFI began to appear, it must have been pretty clear to the engineering staff that inline valves on a 4 or small 6 cylinder had a horizon that was being defined. With the gas crisis of 1973, the sun started to set on that valve train in those engines.

        The arrogance of GM and it’s willingness to just produce bad vehicles is astounding. Hardly alone, with Ford and Chrysler right with them, but GM was rich enough to do better.

        Thank you again for your writing and responsiveness. Do you write professionally on vehicles, something your fans can read? If not, I hope you do find such a position soon.

        1. I admit that my use of the term reverse flow (not a term I customarily use except with regard to cooling systems) is not the most precise from an engineering standpoint — combustion chamber design is one of those subject areas where I reveal that I’m not an engineer! I’m looking at this less in terms of gas flow and more in terms of the distinction between (more or less) inline and opposed valve layouts (the latter exemplified by hemispherical combustion chamber engines like the Chrysler 426 Hemi or Toyota’s various DOHC -G engines), and even in that sense, there are obviously examples that don’t quite fit either model (such as the splayed valves of a big block Chevrolet V-8). Part of the reason I tend to think of it in that way is that there were in some cases opposed-valve variants, some of which didn’t necessarily reach production (e.g., experimental DOHC versions of the Pontiac V-8) and some that eventually did (e.g., the crossflow version of the big Ford six that was eventually introduced in Australia).

          My main point, at any rate, was that one should not underestimate the American corporate resistance to any kind of added complexity that would drive up unit costs — heaven forfend a division bear the extra expense of having two camshafts (or even two rocker shafts) where one might do! (Granted, the Chrysler hemispherical combustion chamber engines did incur some significant penalties in weight and bulk for their head design in addition to the added cost in dollars, but given that most American engines of this period were big ol’ hunks of iron in any case, I think the latter was the more compelling argument so far as automakers were concerned.)

          Trying to quantify the impact of the seventies (including both the Clean Air Act and CAFE) in engine design is complex because the mad scramble ended up greatly prolonging the lives, or at least the lineages, of some fairly venerable engine designs (the small block Chevrolet, the Buick V-6, Ford’s Cologne V-6, the Chrysler LA-series) that could be tweaked and prodded into emissions compliance with acceptable performance and economy, and those engines often outlived (and certainly outshone) some newer designs that were conceived with emissions, unleaded fuel, and fuel economy in mind.

          It would be tempting to say that the changes of the seventies browbeat American automakers into getting in line with contemporary trends in Europe and Japan (with the move toward smaller-displacement, more powerful four- and six-cylinder engines of greater sophistication), but it really took a remarkably long time for that to happen. Consider, for example, that the 1986–1995 Ford Taurus, arguably the most successful American attempt at a modern “import fighter,” relied primarily on a 3-liter pushrod V-6, where its key Japanese rivals extracted similar performance from 2.2-liter OHC fours. (The Yamaha-designed DOHC engine in the Taurus SHO was strictly a specialty item, and in any event was still based on the Vulcan V-6.) What’s particularly interesting in that respect is that the latter were variants primarily intended for the American market; you could get a 2.2-liter Accord in Japan, but it was expensive to own (as a 3-number car) without the prestige of a six.

          I think in a broad sense the legacy of American automakers is that they have epitomized a central characteristic of postwar capitalism: a determination to dictate what the market should be for maximum profit rather than responding to the existing needs of the market. There’s often been a clear difference between cars that are in a U.S. manufacturer’s sweet spot (that is, vehicles they want to be building and consider commercially worthwhile) and ones that clearly aren’t (that they’re only building because their franchise holders have been screaming at them and that aren’t deemed sufficiently profitable to bother with), which has cost them in many segments over the years. Even where they’ve done something well, there are often areas of frustrating cheapness and opportunities missed, which on an individual level is regrettable and taken en masse starts to take on the shape of a Greek tragedy.

        2. To your question, I am a professional writer, just not exclusively an automotive one, although at points I have published some articles in Autoweek (which may or may not still be available online — I haven’t checked in ages). I originally started Ate Up With Motor as a showcase of sorts, since it’s difficult to get automotive writing work without some kind of portfolio, but a lot of automotive publications have collapsed in the interim, and there are no longer a lot of outlets other than simply publishing something yourself.

    2. In terms of consolidation, there were a number of considerations that are not necessarily obvious today, but that were considered compelling prior to the eighties. First, GM’s divisional structure made each individual division responsible for its own balance sheet. The reason each division generally developed and manufactured its own engines, even where those engines were quite similar, was that it was cheaper for that division than to purchase engines from outside, even from another GM division. The principal exception was where the expected volume was too low to justify the investment in plant capacity and tooling. (I think it’s safe to presume that each division had a very precise idea about how many units they would need to sell for producing something within the division to be financially worthwhile.) Of course, this only made sense if each division had quite substantial volume, but in the sixties and seventies, that was definitely the case. When Buick, Oldsmobile, and Pontiac each sold as many cars as some rivals sold across all brands and divisions of their entire company, it was not an unreasonable proposition for them to have their own engines, and buyers expected that they would; indeed, some GM divisions got sued when they started using engines from other divisions.

      The way GM was organized, consolidating engine production would have entailed a substantial and costly corporate reorganization that would have involved taking considerable resources (and indeed entire plants) away from existing divisions and either giving those resources to a new centralized division or else making a series of command decisions that certain divisions would henceforth only make certain engines that would be available to everyone, which would have amounted to the same thing. Of course, this is essentially what ended up happening in the eighties, which was an enormous and costly mess that did substantial damage to GM’s financial position and the reputation of the individual divisions — not exactly a ringing recommendation!

      By that point, GM’s hand had been forced to some extent by emissions standards and CAFE, which significantly raised the cost of developing and maintaining distinct engine families and brought about much more rapid shift in the corporate product mix than the divisions’ traditional ROI/cost amortization model was set up for. (The divisions could handle evolutionary changes on their own, but the sudden scramble to trade V-8 engines for the Buick V-6 was quite another matter.) Taking on such a change a few years earlier, when its necessity was far less evident and GM divisions were on top of the world in their financial performance, would have seemed like madness. I wouldn’t be terribly surprised if there had been corporate study groups that had looked at such possibilities well before the Roger Smith era (study groups were where unpopular ideas went to die, essentially), but if so, nothing came of them. Before the Clean Air Act and the OPEC embargo, the cost-benefit analysis would likely have been pretty grim.

  34. I was trying to think if a US manufacturer ever made a DOHC straight six when I realized my father has one in his Magic Trailblazer EXT, so called because the step-monster keeled over and died getting out of it. It’s a smooth, responsive engine, too bad it’s in a tank and extinct.

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