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, 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, consumes little power, and requires no lubrication. Better still, it’s considerably cheaper than either gears or chains.
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 still presented a problem. 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 or 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 1-inch (25 mm) wide 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 that engine’s crankshaft and connecting rods, but the Pontiac engine’s 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 those of Pontiac’s V8s. To save weight, the camshaft was actually mounted in the aluminum valve cover rather than in the head itself and 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.
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 most of 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.
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 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 (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, the SOHC engine 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.
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. 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 V8. DeLorean conceived it as an inexpensive sports car, a competitor for the new Ford Mustang.
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 in 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 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 worried 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.