From 1961 to 1963, the “senior compact” Pontiac Tempest had a novel powertrain layout featuring a front engine and a rear transaxle, connected with an unusual flexible driveshaft known as the “rope drive.”
Pontiac Advanced Engineering
Along with their individual engineering departments, each of GM’s automotive divisions also had its own advanced engineering section, developing new concepts and new technologies not necessarily intended for any specific production model. The head of advanced engineering for Pontiac during this period was one John Z. DeLorean, who had arrived from Packard in September 1956.
One of the ideas DeLorean explored during the early part of his tenure in Pontiac advanced engineering was the front-engine/rear-transaxle layout. This was a rather trendy idea at the time; Chevrolet and Oldsmobile also studied the concept during this period, although their versions never made it to production.
The interest was probably sparked at least in part by the marvelous Lancia Aurelia. Introduced in 1950 to replace the Lancia Aprilia, the Aurelia was an exceptional car of a kind with no real parallel in the United States. It was by European standards a medium-size family sedan — although Americans would have regarded it as a compact, as it was 2 inches (51 mm) shorter than a 1950 Nash Rambler, albeit on a much longer wheelbase — with a pillarless four-door body (later joined by lovely two-door coupe and convertible variants) and an aluminum V-6 engine, mounted in front and driving a rear-mounted clutch and transaxle through a central driveshaft. Suspension was fully independent (though Lancia eventually switched to a De Dion axle), and static weight distribution with a full tank of fuel was almost precisely 50/50.
By the standards of its era, the Aurelia had superb handling and a firm but comfortable ride, abetted by a commendably robust monocoque structure. Despite modest engine displacement (initially just 1,754 cc (107.1 cu. in.)) and fairly hefty curb weight, the Aurelia also had respectable straight-line performance and decent fuel economy, though with list prices starting at about 1.8 million lire (equivalent to about $3,000 at early-fifties exchange rates, a lot of money in those days), it could hardly be called an economy car.
In any form, the Aurelia was an engineering tour de force, and undoubtedly an object of fascination for automotive designers and engineers around the world. We don’t know for certain that John DeLorean was among its admirers, but we would be very surprised if he were not.
As the Aurelia adeptly demonstrated, there are a variety of advantages to placing engine and transmission at opposite ends of a car. The most obvious is that it shifts a useful chunk of the car’s unladen mass off the front wheels (reducing steering effort and, at least in principle, the tendency to understeer) and onto the drive wheels (improving traction and allowing the rear brakes to do more of the work of stopping the car), without the extreme rear-heaviness that can make a rear-engined car disagreeably twitchy near the limits. Moving the transmission to the rear can also reduce the intrusion of the powertrain into occupants’ foot and knee room. Since the transmission must be fixed in place — which requires the use of independent rear suspension or a De Dion axle — rear unsprung weight is also reduced, improving ride quality and, potentially, handling.
There are also a number of downsides. While a rear-mounted transmission or transaxle may intrude less into cabin space than a conventional RWD drivetrain, it does reduce the rear volume that might otherwise be available for cargo. With a manual transmission, matching revs is more difficult, and the load on the synchronizers is greater because of the added inertia of the driveshaft. Maintaining the correct geometry between engine and transmission can also be challenging, not least during assembly. Additionally, this arrangement is more costly than a conventional live axle layout, and an ill-sorted rear suspension can result in treacherous handling behavior.
DeLorean was certainly aware of these pros and cons, many of which were discussed in a paper he wrote for the Society of Automotive Engineers in 1960. However, as the X-100 project unfolded, he realized that the front-engine/rear-transaxle layout offered something far more compelling: a way to make the Pontiac compact — eventually named “Tempest,” a name Pontiac had applied to its V-8 engines beginning in 1958 — something other than a badge-engineered Corvair.
On paper, the idea was quite simple: Pontiac could keep the Corvair body shell, transaxle, and other mechanical components, but mate them with a conventional front-mounted, water-cooled engine. In that way, the corporation’s desire for commonality would be satisfied, more or less, but a Pontiac salesman would still be able to demonstrate the difference between the Tempest and its Chevrolet cousin by simply popping the hood. All Pontiac would need to pull it off was an engine and a way to connect it to the Corvair transaxle.
From V-8 to Slant Four
At that point in the development process, Pontiac did not have an engine suitable for this application. The division had retired its elderly 239 cu. in. (3,920 cc) six at the end of the 1954 model year. The Pontiac V-8, which for 1959 was expanded to 389 cu. in. (6,372 cc) displacement, was far too much engine for a compact economy car — or for a transaxle designed for a 140 cu. in. (2,287 cc) flat six. There was the new aluminum V-8, but while Pontiac would reluctantly offer that engine as an option, it was too expensive to make standard equipment, something that would also become a problem for Oldsmobile and Buick. As we’ve previously discussed in our article on the 1961–1963 Buick Special/Skylark, Buick eventually solved that problem by creating a cast iron V-6 based on the architecture of the aluminum V-8, but that engine hadn’t even been conceived at this stage, and wouldn’t be available until the 1962 model year. The same was true of the inline six and four Chevrolet would offer for the Chevy II/Nova, a conventionally engineered compact that would supplement (and, as it happened, eventually supplant) the rear-engined Corvair.
Here again DeLorean had an answer: make a four-cylinder engine out of the Pontiac V-8. If this idea seemed somewhat ridiculous, early experiments demonstrated that it could work. Pontiac powertrain engineer Malcolm McKellar found that even with four cylinders disabled, the big V-8 had ample torque and adequate power. By amputating the left cylinder bank, Pontiac would have a 194.5 cu. in. (3,186 cc) slant four that could be built on the existing engine production lines.
It’s easy to second-guess this choice or to point to other alternatives that might have worked better, like a 90-degree V-4 or a de-bored, short-stroke 90-degree V-6. However, Pontiac’s overriding priority was to minimize the tooling investment, and the four-cylinder engine proved the most expedient way to do that. Although the four would need a new crankshaft (still with five main bearings), unique camshaft, and new intake and exhaust manifolds, Pontiac also wanted to share as many existing V-8 components as possible. The resulting “Trophy 4,” as Pontiac eventually dubbed the four-cylinder engine, was bigger and heavier than it needed to be, weighing almost as much as a Chevrolet V-8 of much greater displacement, but it was able to share about 120 parts with the V-8, and its development and tooling were a fraction of the usual cost of launching a new engine.
Cheap, the Trophy 4 was; smooth or refined, it most certainly was not. Any inline four-cylinder engine has a second-order vibration, caused by the unbalanced motion of the reciprocating masses. Even in a relatively small-displacement four, the result is a vertical shaking force of considerable magnitude, which becomes more severe as displacement increases. In a 90-degree V-8 engine, the second-order vibration of one cylinder bank can be balanced against the other through the use of a cross-plane crankshaft and counterweights, but that’s not possible with a slant four, and the bank angle means the second-order vibration has both vertical and horizontal components. With its large displacement and substantial reciprocating mass, the Trophy 4 shook like a paint mixer, and its torsional vibration was strong enough to shred the timing chain it had inherited from the V-8, a problem not fixed until after regular production had already begun.
Today, most passenger car fours displacing more than about 2.2 liters (134 cu. in.) deal with second-order vibration by adding twin counter-rotating balance shafts, but that technology was still underdeveloped in the late fifties and early sixties. (Frederick Lanchester patented the concept back in 1912, but modern balance shaft technology owes a great deal to Mitsubishi, which greatly advanced the state of the art with its “Silent Shaft” fours in the mid-seventies.) In any case, Pontiac didn’t consider balance shafts a possibility for the Tempest due to budget constraints. There was simply no extra money to develop balance shafts, or to modify the block to accommodate them.
Pontiac decided instead to focus instead on isolating the engine, and indeed the entire powertrain, from the body structure, resulting in the Tempest’s most unusual feature, and the one that would earn it its most lasting nickname.
The Rope Drive
Like the other Y-body compacts, the 1961 Pontiac Tempest would use unitized construction, but the suspension and powertrain were carried on detachable front and rear crossmembers. This appears to have been a matter of production convenience rather than an effort to isolate the body structure from noise, vibration, and harshness (NVH). While all suspension and powertrain loads were taken through the crossmembers, the crossmembers themselves were simply bolted to the body, relying on the rubber insulators in the suspension and the drivetrain mounts to limit NVH.
The Trophy 4 engine was supported on the front crossmember by two rubber-isolated engine mounts, one on either side of the block. It was mounted in approximately the same fore-aft position as the V-8 in bigger Pontiac models, albeit offset about 1.4 inches (36 mm) to the left. On cars with manual transmission, the clutch was mounted behind the engine in conventional fashion rather than with the transaxle, probably to simplify the clutch linkage. With automatic, the torque converter was rear-mounted at the back of the transaxle. (Because of this variation, manual transmission cars used shorter driveshafts than did cars with automatic, bolting to the end of a short clutch shaft rather than directly to the engine flywheel.) In back, the transaxle was carried on a differential support arm, attached to the rear crossmember through two more rubber-isolated mounts.
Between the engine and the transaxle was the driveshaft, which rode on two rubber-isolated ball bearing assemblies within a 76.2-inch (1,935-mm) stamped steel torque tube. The driveshaft itself was forged steel, machined, heat-treated, subjected to magnetic particle inspection, and shot-peened to ensure that it was as smooth and straight as possible; it also had a thin protective coating to prevent corrosion and surface wear. Because it only needed to transmit engine torque, not multiplied by the transmission gears (or the torque converter, on automatic transmission cars), the driveshaft could be quite thin. Diameter was 0.75 inches (19.1 mm) on cars with manual transmission, a mere 0.65 inches (16.5 mm) on cars with automatic. Being so thin, the shaft was also flexible, particularly in torsion, able to twist by up to about 30 degrees in either direction.
This flexibility was further demonstrated by the shaft’s unusual designed-in curvature. The driveshaft formed a gentle downward arc, which put the middle of the shaft around 3 inches (75 mm) lower than either end. Contrary to some early press coverage, this had nothing to do with the torque tube bearings. Rather, the curvature was created by the bending moment applied at both ends of the driveshaft by the shaft’s rigid connections to the engine and transaxle, both of which tilted downward by a total of about 11 degrees. This curvature led wags to dub the driveshaft the “rope shaft” or “rope drive.”
Curving the driveshaft in this way was another of DeLorean’s inspirations, an elegant if peculiar solution to a practical problem. Pontiac had found that with a straight driveshaft of such length and small diameter, the shaft would develop strong, destructive vibrations at a critical speed within the normal range of operating speeds. Bending the shaft into an arc shifted the resonant frequency out of the normal operating range, which eliminated the problem (although the driveshaft could still rattle on its bearings under certain conditions). A similar resonant frequency problem with the torque tube was addressed by varying the tube’s cross-sectional area, putting its critical speed below normal engine idle speed and giving the tube a constant bending stiffness throughout its length.
As a bonus, the driveshaft’s curvature also made it possible to minimize the height of the torque tube, reducing its intrusion into cabin space. To achieve a similar result, the Y-body Oldsmobile F-85 and Buick Special needed a costly two-piece driveshaft, with a constant velocity joint in the middle as well as a universal joint at each end.
The torque tube was something of an afterthought. It was not included on early test mules, and was initially added simply to help maintain the correct alignment of the driveshaft relative to the engine and transaxle. However, it emerged as an important part of the concept. Interconnecting the front and rear crossmembers proved to be an invaluable aid to production and assembly, and the torque tube also helped to mask the bad manners of the Trophy 4 engine.
By rigidly connecting the engine and transaxle, the torque tube limited the engine’s range of motion, preventing the big four from moving laterally or fore and aft. Some of the engine’s torsional vibration, meanwhile, was absorbed by the twisting of the driveshaft within the torque tube. That left only the vertical shaking forces caused by the engine’s secondary vibration and by torque reaction from the drive wheels, which were absorbed by the four soft rubber powertrain mounts. (The engine mounts were so soft that Pontiac had to add metal limiters to keep the engine from shifting too far out of its appointed space.) No one would ever accuse the Trophy 4 of silken refinement, but this arrangement proved remarkably effective. There was still a lot of commotion under the hood, but it was largely unnoticeable inside the car, at least so long as the engine and engine mounts were in reasonably good health.
Transaxles and TempesTorque
Although its transaxles had some commonality with the ones used on the Chevrolet Corvair, the manually shifted Tempest parted ways with the Corvair in several respects. In Corvairs with manual transmission, the clutch was mounted behind the differential, sending power forward with a clutch shaft that ran through the center of the hollow pinion shaft and main shaft to a clutch gear at the front of the gearbox. Power then flowed back through the transmission main shaft back to the differential pinion shaft. Since the Tempest mounted its clutch in front, this convoluted power flow was unnecessary; the trailing end of the propeller shaft was simply splined to the clutch gear of the three-speed gearbox, whose main shaft was splined in turn to the differential pinion shaft. The Tempest three-speed also had closer ratios than the Corvair three-speed (the same ratios as the Saginaw three-speed that would become standard on the 1962 Chevrolet II), although first gear was still unsynchronized. The all-synchro Corvair four-speed would be offered later, but wouldn’t be available at launch.
The automatic transaxle, which Pontiac dubbed “TempesTorque,” was more closely based on the Corvair Powerglide, even mounting its torque converter behind the differential, as on the automatic Corvair. In a Corvair Powerglide, two hollow shafts passed through the hollow differential pinion shaft: a central pump shaft that allowed the torque converter impeller to drive the transmission’s front oil pump and a hollow turbine shaft that allowed the torque converter turbine to drive the transmission’s input sun gear. TempesTorque added an internally splined coupling to the leading end of the pump shaft, allowing the flexible driveshaft to drive the pump shaft and thus the impeller of the torque converter. This was circuitous, but it allowed a transmission intended for a rear-mounted engine to take its power input from the opposite end with minimal design changes.
Pontiac used a different torque converter than the Corvair unit, providing a stall ratio of 2.0:1 rather than 2.7:1, and added a nuance not shared with the Corvair: a split-torque high gear. The clutch hub of the TempesTorque transmission’s direct drive clutch was splined to the pump shaft, and thus always rotated at engine speed, without any hydraulic slippage. When the clutch engaged with the 1–2 shift, the low sun gear would also turn at engine speed, while the input sun gear turned at turbine speed. This had the effect of “demultiplying” slippage in the torque converter by about 45 percent in high gear, a partial mechanical lockup that improved fuel economy and engine response in top-gear cruising. (You’ll find a further discussion of this principle in our article on split-torque transmissions and lock-up clutches.)
One unfortunate similarity between TempesTorque and the Corvair Powerglide was that it had no “Park” position, leaving the selector with only four positions: RNDL. This was a cost-saving measure, but it would later become a source of customer complaints, since the mechanical parking brake’s ability to hold the car on a steep grade left something to be desired. (A parking pawl was belatedly added to the Tempest automatic for 1963.)
While Chevrolet originally hoped to make Powerglide standard on the Corvair, we haven’t seen any indication that Pontiac contemplated a similar strategy for the Tempest. However, such a move would have made sense — the automatic imposed surprisingly little penalty in performance and fuel economy. The torque converter further ameliorated the effects of engine vibration, and also masked the twisting of the driveshaft, which was noticeable and occasionally disconcerting with a manual gearbox and conventional clutch.
The Y-Body Tempest
By mid-1959, GM corporate management had accepted the new direction for the Pontiac compact. The corporation also authorized Pontiac to adopt the longer Y-body shell. We assume that with Pontiac no longer sharing the Corvair mechanical package, the priority had shifted to recouping the tooling costs of the bigger body, since it was likely that the cheaper Corvair, then nearing the start of regular production, would handily outsell its B-O-P cousins (as indeed it did).
This meant that Pontiac’s focus now shifted from differentiating their compact from the Corvair to differentiating the Tempest from the Buick and Oldsmobile Y-bodies, the Buick Special and Oldsmobile F-85. Pontiac arrived quite late to that particular party, and had to contend with a stylistically inflexible unitized body shell whose basic dimensions had been largely dictated by Buick and whose styling had been tailored to suit Oldsmobile.
Fortunately, Pontiac had a readily identifiable styling feature that could be made to work within the limited tooling budget: the “hairpin” split grille, developed by previous Pontiac styling chief Joe Schemansky for the 1959 Pontiac line. Although the 1960 Pontiac line, mostly done when Humbert arrived, had gone a different direction, Humbert revived the theme for the 1961 models, correctly recognizing that it gave Pontiac an immediately recognizable “face.” The Tempest eventually adopted a variation on the 1959 theme, which reduced but could not wholly eliminate the inevitable resemblance to the Special and F-85.
To the likely dismay of the sales executives and accountants, the biggest tooling change Pontiac made to the Y-body shell was one most customers would never see: a new floorpan to accommodate the torque tube. This didn’t even provide a completely flat floor, which might have given the Tempest a unique selling point. It did at least lower the driveshaft hump to roughly ankle height, and the floor was flat in the area around the front passenger’s feet, which helped to justify subsequent advertising claims of six-passenger capacity — or would have, had Pontiac not decided that every Tempest with manual transmission should have a floor-mounted shifter. (As on the Corvair, the selector for the automatic was neatly integrated into the dashboard, again making us wonder if Pontiac originally intended to make TempesTorque standard equipment on the Tempest.)
Floor shifter or no, the Tempest’s potential sportiness was undercut by the decision to launch with only four-door sedan and four-door Safari wagon body styles — not exactly a pulse-quickening lineup — and bench seats. The Tempest wasn’t a bad-looking car, but as yet, there wasn’t much of the design pizzazz that would become a Pontiac hallmark throughout the sixties.