Cammer: The Pontiac OHC Six

THE TIMING BELT

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, as well as 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, 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 internal damage. (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, beginning with a Cadillac V8 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 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 strong enough for a torquey big-bore six presented a bigger challenge. Furthermore, McKellar didn’t consider the Glas engine’s frequent belt changes acceptable; he wanted a belt that would last the useful life of the engine, with little to no maintenance.

Developing the belt itself was the biggest single challenge of the design process. Simple rubber belts were not strong enough or durable enough. Steel-reinforced belts had adequate strength, but tended to corrode. Stainless steel eliminated the corrosion, but was too expensive, and showed signs of fatigue at high mileage. The eventual solution, developed in partnership with Richard Case of Uniroyal, was a one-inch (25 mm), neoprene-impregnated fabric belt, reinforced with fiberglass. It was strong and durable, and testing showed minimal wear at high mileage. Unlike some later timing belts, it was not overly sensitive to dirt and oil, although Pontiac ultimately decided to keep it covered, protecting it from snow and road spray.

The rest of the engine was a study in compromise. The block was loosely based on that of Chevrolet’s 1962-vintage OHV six, sharing its crankshaft and connecting rods, but the skirt was extended about 2.4 inches (60 mm) below the crank center line, for greater rigidity. The oil pump, fuel pump, and distributor were driven by an accessory drive shaft mounted parallel to the crank. The heads used wedge-shaped combustion chambers with side-by-side valves, like Pontiac’s V8s. To save weight, the camshaft was actually mounted in the aluminum valve cover, rather than in the head itself, and it had unusually wide lobes, in an effort to reduce wear. The valves, shared with Pontiac’s V8s, 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.

Pontiac OHC Six in a 1967 Firebird Sprint
The “cammer” had an iron block and heads, but the cam cover (where the camshaft is actually mounted) and the belt cover are aluminum. 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)

One of the bugbears of many early overhead cam engines was the need for periodic valve lash adjustment. That, too, was unacceptable to Pontiac; division policy required all fully warrantied engines to have hydraulic valve lifters, which prevented over-revving and needed no adjustment. Hydraulic lifters had never been seen as practical for OHC engines, but Pontiac engineers developed a clever solution. Although the camshaft was mounted almost directly above the valves, it actuated them through finger-type cam followers — essentially small rocker arms. The pivot point for each cam follower was a small hydraulic sphere, similar to a hydraulic lifter on a pushrod engine. The hydraulic sphere maintained a constant zero valve lash, without adding to the reciprocating mass of the valvegear. These hydraulic valve lash adjusters reduced mechanical noise, and they eliminated the need for routine valve adjustments.

Despite those novel features, the Pontiac engine was notably less racy than its European contemporaries. The basic version, with a single-throat carburetor, 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 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.

OHC EIGHTS

Prototypes of the 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. 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 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 Chevy Mk. 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 (6,372 cc) engine, 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 dual overhead cams with four valves per cylinder, a cross-ram intake manifold, and sequential fuel injection. Although the DOHC V8 initially used chain-driven cams, it was later converted to a more robust version of the six’s timing belt with similar hydraulic valve-lash adjusters. Pontiac never released power figures for the DOHC engine, but it was probably 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 V8s on an experimental basis. Toward the end of 1963, McKellar developed a simpler OHC 421, with one belt-driven cam per bank and two valves per cylinder; with Tri-Power carburetion, it was capable of some 620 hp (462 kW). This was followed in 1965 by a SOHC 428 (7,008 cc) V8 with a rear-mounted cam drive and three valves per cylinder.

McKellar showed off the experimental engines to Hot Rod editor Eric Dahlquist in 1968, but none of the V8s 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 very powerful engines. A 500-horsepower (373 kW) SOHC V8 would have been a provocative gesture as far as Washington was concerned, and the GM brass was in no mood for provocative gestures.

THE BANSHEE

While it originated in DeLorean’s Advanced group, the OHC six, unlike the V8s, was always intended as a production engine. Its prospects for production improved significantly in November 1961 when John 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 Banshee.

The Banshee project, known internally by its styling code, XP-833, began in August 1963. Designed by Roger Hughet and Ned Nickles, it was a compact fastback coupe, looking something like a miniature Corvette Sting Ray. It 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 V8. DeLorean conceived it as an inexpensive sports car, a competitor for the new Ford Mustang.

1964 Pontiac Banshee roadster side
There were two running Banshee prototypes (not counting earlier non-running mock-ups), 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) V8, which probably would have been optional if the Banshee had made it to production. (Photo © 2009 EvThoMcC; used with permission)

GM management was unenthusiastic about the Banshee, preferring Pontiac to 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. DeLorean assigned Advanced Engineering chief Bill Collins to oversee the project, which proceeded with great secrecy.

In the summer of 1965, DeLorean was promoted to general manager of Pontiac. Seeing his opportunity, DeLorean had Bill Collins show off the two fully finished XP-833 prototypes to senior management. Collins made a thorough presentation, describing the Banshee’s expected market position, tooling costs (a modest $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 it would help to bolster Pontiac’s sporty image.

Unfortunately, GM chairman Jim Roche and president Frederic Donner were not interested. They thought the XP-833′s lack of rear seats would limit its sales potential and said 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 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.

14 Comments

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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. Grandpa was a Pontiac man for years – I was carsick numerous times as a young boy in the back seat of his 1966 Tempest OHC-6/Powerglide four-door.

    Years later, the car ended up in my hands, but the top end of the six had already died – I pulled the engine and replaced it with a Chevy 350 and THM350. Always loved that car – the dash was jewel-like with its deep-set gauges, and I always marveled at the “Wondertouch” power steering and brakes.

    The car is long gone, but I still have the OHC valve cover up in the attic somewhere – always thought it was a true piece of automotive art.

    1. Top-end oiling was a persistent problem with these engines when they were new — inadequate flow to the cam covers, particularly when the oil was dirty. I’m told that with modern oil and regular changes, it’s not a big deal, but it killed a lot of cammers when they were new(ish).

  3. 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.)

  4. 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.

  5. Can anyone help with a diagram of the timing marks for a 68 Pont Firebird 6 ohc engine. It would be greatly apprecceiated. Thanks.

  6. 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

  7. 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…

  8. I need a starter for a 1967 firebird,4.1 liter overhead cam sprint with a two speed power glide trans. or a gm part number, picture anything thanks in advance Tony

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