With all the furor surrounding Ford and Chevrolet’s new 300+ horsepower V6 Mustang and Camaro, you would think hot six-cylinder engines were a new idea, at least in America. Not so — in 1965, about a decade after the demise of the Hudson Hornet and its “Twin H-Power” straight six, Pontiac introduced a sophisticated new overhead cam six-cylinder engine that promised V8 power and six-cylinder economy. This week, we look at the short life of the 1966-1969 Pontiac OHC six, Pontiac Firebird Sprint, and Tempest Le Mans Sprint.
(Photo © 2006 Robert Nichols; used with permission)
JOHN DELOREAN AT PONTIAC
John Zachary DeLorean was born in Detroit in 1925. Like many automotive executives of his era, he was a second-generation automobile man; his father, an immigrant from Alsace-Lorraine, had worked for a time as a millwright for Ford. As a teenager, DeLorean earned a scholarship to Lawrence Institute of Technology (now Lawrence Technological University) and, after a brief stint as an insurance salesman, took a job with the Chrysler Corporation. In 1952, he joined the Packard Motor Car Company, working with Forest McFarland, Packard’s chief R&D engineer, on projects like the second-generation Ultramatic transmission.
Packard was quite small by the standards of domestic automakers, with a deeply ingrained culture of unhurried Old World craftsmanship. Largely unencumbered by bureaucracy and nurtured by the ever-patient McFarland, DeLorean thrived, enjoying a level of autonomy rare in a conservative industry. When McFarland departed to join Buick in 1956, DeLorean was promoted to replace him as head of R&D.
If the Studebaker-Packard Corporation had been healthier, DeLorean might have enjoyed a fine career there. Unfortunately, by 1956, the company was staggering toward collapse. That summer, the Studebaker-Packard board decided to eliminate Packard’s own design and manufacturing facilities, consolidating development and production at the Studebaker plant in South Bend, Indiana. DeLorean started looking for other job opportunities.
He was soon contacted by Oliver K. Kelley, then the head of GM’s corporate Transmission Development Group (and one of the principal architects of the original Hydra-Matic and Dynaflow transmissions). Kelley made a concerted effort to recruit DeLorean and arranged a series of interviews for him, including a meeting with new Pontiac general manager Semon E. “Bunkie” Knudsen.
Although DeLorean was wary of GM’s top-heavy corporate culture and put off by Pontiac’s stodgy reputation, Knudsen convinced him that they could reinvent Pontiac and offered him a lucrative salary (around $14,000 a year, a handsome sum in the mid-fifties) to become the head of Pontiac’s new advanced engineering section.
DeLorean arrived at Pontiac on September 1, reporting to new chief engineer E.M. (Pete) Estes, whom Knudsen had recently recruited from Oldsmobile. DeLorean’s role was to develop new engineering concepts that might eventually find their way into production Pontiacs. As at Packard, he was given a free hand to explore novel and sometimes radical ideas, ranging from a rear transaxle with an unusual flexible driveshaft (later used for the 1961 Pontiac Tempest) to an experimental six with an unusual combination of air- and water-cooling.
Unlike most American compacts of its era, the 1961–1963 Pontiac Tempest did not use a six-cylinder engine. Most 1961-1962 Tempests were powered by a 196 cu. in. (3,186 cc) slant-four engine — essentially Pontiac’s 389 cu. in. (6,372 cc) V8 shorn of one cylinder bank. Buick’s 215 cu. in. (3,528 cc) V8 was optional in 1961-1962, but rarely ordered; fewer than 5% of buyers selected it. (Photo © 2009 Norm Stephens; used with permission)
By 1959, DeLorean had embarked on a new project: an advanced six-cylinder engine with a single belt-driven overhead camshaft.
SIDEBAR: CAMSHAFTS, OVERHEAD AND OTHERWISE
We should pause here to explain a little bit about camshafts for the benefit of our less technically inclined readers. As you probably know, internal combustion engines produce power by burning both fuel and air. A four-stroke reciprocating piston engine — the type used by the large majority of cars and trucks — draws air and fuel into the cylinders, compresses it, ignites and burns it (either via an electrical spark or the heat of combustion), and then expels the burnt exhaust gases.
Reciprocating engines generally use spring-loaded poppet valves to admit air into the cylinders and expel the exhaust. In a four-stroke engine, each valve must open and close once for every two rotations of the engine’s crankshaft. When the valves open (timing), how far they open (lift), and how long they stay open (duration) all have a dramatic effect on how the engine performs.
Naturally, a reciprocating engine needs some mechanism to open and close the valves at appropriate times. This is generally accomplished with a camshaft, a metal shaft with a series of lobes that actuate the valves as the shaft rotates. The shape and position of those lobes (the cam profile) determine the valve timing, lift, and duration.
A typical mid-sixties automotive camshaft. This one is from a 1966 Chevrolet Corvair.
Figuring out where to put the camshaft presents a number of challenges for engine designers. The camshaft must be driven by the crankshaft and turn at one-half the crankshaft speed. The simplest way to achieve that is to mount the camshaft in the engine block, just above the crankshaft, and drive it with gears or a short metal timing chain. Until well into the 1970s, the vast majority of engines were cam-in-block designs.
Mounting the cam in the block is reasonably convenient for L-head (flathead) engines, where the valves are also in the block, but it poses some challenges for overhead valve (OHV) engines, which became predominant after World War II. As the name implies, an OHV engine mounts the valves in the cylinder head, which improves breathing and thermal efficiency. The problem is that it puts the valves some distance away from the crankshaft. Therefore, if the camshaft is in the block, it must actuate the valves remotely via pushrods and rocker arms. That, in turn, increases the inertia the camshaft must overcome each time it opens or closes the valves; the camshaft lobe must act on the mass of the pushrods and rockers, as well as the valve itself. That extra mass (and any slack in the linkage) limits how high and how quickly the engine can rev. At very high engine speeds, the valvetrain can develop more inertia than the camshaft can overcome, leading to a condition called valve float.
Diagram of a typical pushrod/rocker-arm layout of an overhead-valve engine. (Illustration: “Pushrod2” © 2007 IJB TA at English language Wikipedia (Ian Brockhoff?); resized, changed file format, and used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)
These problems can be mitigated somewhat by minimizing the mass of the valvegear and using stiffer valve springs, but a simpler alternative is to mount the camshaft in the head rather than in the block. An overhead camshaft (OHC) engine needs no pushrods; depending on the position of the cam in the head, it can potentially eliminate rocker arms as well, greatly reducing the mass and inertia of the valvegear. The reduction in valvetrain mass not only enables the engine to rev higher, it increases the acceleration and deceleration rate of the valves. That allows the valves to be open longer (longer duration), which improves power, while minimizing the time the intake and exhaust valves are open simultaneously (overlap), which makes the engine smoother at idle and at low speeds than a pushrod engine with the same cam profile.
Inevitably, there are trade-offs. First, OHC engines tend to be taller than comparable pushrod engines, which can complicate packaging. Second, overhead-cam engines (and particularly engines with dual overhead cams) are usually somewhat heavier than pushrod engines and have a higher center of gravity. Third, an OHC V6, V8, or V12 requires two camshafts — four with dual overhead cams — while a pushrod engine can get by with one. Fourth, the camshaft still needs to be driven by the crankshaft, which becomes more complicated the further the camshaft is from the crank. OHC engines may use a long timing chain, a rubber belt, gears, or shaft drive to run the camshafts, any of which adds complexity and cost.
Those trade-offs made OHC engines quite rare in American-made cars until the 1980s. There were exceptions going as far back as 1904, but most were either competition engines or cost-no-object luxury cars like the DOHC Duesenberg Model J. The closest any American OHC engine came to mass production was the Wills Sainte Claire of the twenties, which accounted for fewer than 14,000 sales in an eight-year run. Other than the Pontiac OHC six, the only production overhead-cam engines in America between 1945 and 1970 were the short-lived Crosley CoBra and CIBA fours and the Willys/Kaiser Tornado engine, an OHC conversion of an older 226 cu. in. (3,622 cc) flathead six. The Tornado six was short-lived in America — Kaiser Jeep dropped it after 1966 except for certain military trucks — but it was used by Kaiser’s Argentine subsidiary, IKA, into the early eighties.
Alfa Romeo was one of the few automakers of the fifties to adopt dual overhead camshafts; one cam operates the intake valves, one cam the exhausts. DOHC engines are more complex and more expensive than single overhead cam (SOHC) engines, but minimize the reciprocating weight of the valvegear and allow more efficient placement of both the valves and the spark plug.
European automakers were quicker to adopt overhead camshafts, although they did not become common for mass-market cars until the sixties. They eventually became nearly universal on European and Japanese engines as a way of extracting more power from relatively small displacements.
ORIGINS OF THE PONTIAC OHC SIX
In the early sixties, six-cylinder engines were enjoying a modest resurgence in the American market. A decade earlier, buyers had shown a marked preference for the new breed of OHV V8s, leading some mid-priced automakers to abandon sixes entirely. Pontiac had dropped its venerable flathead six at the end of the 1954 model year and didn’t offer another six-cylinder engine until 1964. The sharp recession that began in 1957 sent the pendulum swinging the other way, leading to a new generation of six-cylinder compacts. Pontiac had bucked that trend with the four-cylinder Tempest, but it was clear that the division would need a new six eventually. That also presented an attractive opportunity to explore new ideas.
Both DeLorean and motor engineer Malcolm McKellar were intrigued with OHC engines both for their practical advantages (see the sidebar above) and for their rather racy connotations. Although overhead camshafts were very rare for American production cars, they were almost de rigueur for European racing engines and DOHC Offenhauser racing engines had been extremely successful at the Indianapolis 500 for many years.
Jaguar was another firm adherent of overhead cams; its XK six (pictured here in a 1963 Jaguar E-Type fixed-head coupé) had dual overhead cams while Jaguar’s later V12 was SOHC. This engine had an enviable pedigree: In competition trim, it won the 24 Hours of Le Mans five times.
The 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.