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Early Days of the Tempest

How rope drive came into being

John Gunnell - June 26, 2014 03:25 PM

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There’s agreement among automotive historians that only one car has been named after an engine.

 

Pontiac’s first overhead-valve V-8 used from 1955-1957 was marketed as the Strato Streak V-8, but in 1958 it became the Tempest 395 V-8. The number indicated the torque rating. In 1959, there were Tempest 420-A and 420-E engines (420 was the top torque rating and the 420-E was an economy version with a special two-barrel carb and 390 lbs-ft of torque). For 1960, there were a variety of Tempest 425 V-8s. In 1961, big Pontiac engines became Trophy V-8s and the Tempest became a car. In 1961-1962, it was Pontiac’s innovative new small car. In 1963, it got bigger and slightly more conventional.

An increase in imported car sales spurred American automakers to develop economical, compact-size cars. At the forefront were the Chevy Corvair and the Pontiac Tempest.

New ideas of the Pontiac Tempest included its compact size, a base four-cylinder engine made by cutting a V-8 in half, an optional TempestTorque automatic transmission, a “rope drive” system instead of a normal rigid driveshaft, the use of a transaxle and an independent rear suspension.

With a 112-inch wheelbase and 189.3 inches in length, the ’61 Tempest was a bit larger than the average domestic compact. With more than 100,000 Tempests registered in the model’s first year on the market, it was far and away the best selling of the three new compacts General Motors introduced in 1961.

By model year 1962, the American car market was in a state of flux. The Pontiac Tempest grew by five inches in ’63. It retained its rear transaxle for another year, along with the base four-cylinder engine. The LeMans package – formerly an option – became a separate car line. A 326 cast-iron V-8 became a popular option used in 38 percent of the cars built.

A Buick-built aluminum V-8 weighing 350 pounds with a displacement of 215 cubic inches was a $216 option in Tempests, but 99 percent of the buyers preferred a version of the standard engine called the Indianapolis Four. This engine was essentially the Trophy V-8 with one bank cut off. The engine was designed and machined to be assembled on the same production line as the big-car V-8. The camshaft, crankshaft and distributor were all positioned exactly as they were on the 389.

Legendary Pontiac engine engineer Malcolm “Mac” McKellar was behind the new engine. He once described how he took a V-8 and drilled holes in the tops of two pistons from each bank to virtually turn it into a four and then bolted it in a big Pontiac and test drove the car over 90 mph!

The engine was tilted at a 45-degree angle to permit mounting the water pump to one side and shortening the length of the complete engine. With the block cut in half and angle-mounted, access to engine accessories was improved. Tempest owners could change the starter and generator without sliding under the car.

A two-speed manual all-synchromesh gearbox was standard in Tempests and required a clutch to be attached to the flywheel. The flywheel was retained when the automatic was ordered to mesh with the starter drive and to provide a torsional damping effect that controlled deflection of the flexible driveshaft.

Although it had the Powerglide name appearing on its case in 1961 and 1962, the TempestTorque automatic transmission was actually a Powerglide-based two-speed automatic with a torque converter that had a distinctive “split-torque” system. In high gear, 40 percent of the torque was applied in a direct mechanical manner and the other 60 percent went through the converter. This cut slippage and increased efficiency with better fuel economy as a result.

The Tempest’s drivetrain layout was unique for American iron and grabs peoples’ attention when they walk under a car while it’s on a lift. The round torque converter sits at the extreme rear of the vehicle. The ring and pinion, with its attendant differential, is ahead of it. The transmission sits in front of both these units. The tranny, pinion and converter all share the same centerline and the connecting shaft consists of tubes, one sitting inside the other.

When Bill Mawbey’s ’63 Tempest was leaking fluid after two expensive rebuilds, Larry Fisher of Fatsco Transmission Parts helped trace the source of the problem to an O-ring that was the wrong thickness. Fisher knew that the 1963 automatic had more robust internals. The TempestTorque logo was on it and the split-torque system disappeared.

TempestTorque was a $172.80 option in 1961. The standard transmission was a three-speed manual.

Pontiac’s first conception of the flexible or “rope” driveline introduced in the 1961 Tempest dates back to October 1956. Initial laboratory fatigue tests were conducted on this system at the GM Tech Center in Warren, Michigan. The first experimental car installation was done in August 1957. Between May 1959 and October 1960, prototype cars and production drivelines were tested at the GM Proving Ground and on the road racking up almost 1.3 million miles of use.

John Z. DeLorean is credited with the rope drive concept. Pontiac’s original patent application for a “curved” driveline was rejected as not being sufficiently novel since the feature had already been used in racing cars and industrial applications. The company had to take a modified ’57 Pontiac test chassis to Washington D.C. and show it to the Patent Office examiner before a patent was finally issued.

The main purpose of the flexible driveshaft was to lower driveshaft tunnel height. Instead of being built on top of the frame, the postwar cars sat down in it. Another benefit of a flexible shaft was it served as a torsional damper to make the engine feel smoother. The shaft was 0.65-inch diameter (0.75 with stick shift) over its entire length and had to rotate at high speed while it bowed out up to three inches. To accomplish this, the thin shaft was made of a forging of 8660 steel that was heat-treated and shot peened to prevent metal fatigue.

When you look under a Tempest, you do not see the flexible driveshaft because it is inside a long housing similar to a torque tube, but boxier, with what is actually a “hat” shape in sectional view. Three rubber-insulated ball bearing pillow blocks inside the housing hold the shaft within its flexing range. The attachment flanges for the engine and transaxle are angled five degrees to match the three-inch bow and mate with the ends of the flexible shaft.

An early digital computer was utilized to determine the specifications for critical design factors, then 1,286,000 miles of testing was done to verify that the system worked in the real world. In 1963, the diameter of the shaft was increased by ¾-inch and was standardized for all of the Tempest engine options.

The Tempest’s unique engine mounting system, with its thick rubber biscuits, was a departure from the traditional use of very soft rubber mounts to reduce torsional vibration. The sturdy driveshaft housing eliminated the need for U-joints and joined the engine and transaxle together as sturdily as the engine and transmission in a conventional car are mated together. Full swing axle shafts ran from the transaxle to the wheels and the standard axle ratio was 3.55:1.

The suspension components and the transaxle attached to a crossmember that was bolted to the unit body structure at four points. Corvair geometry was designed to induce understeer, but the Tempest’s rear suspension layout compensated for a “power-on oversteer tendency.”

In a 1978 interview, “Bunkie” Knudsen, who was in charge during the Tempest’s development, said Pontiac was never sued over its swing-axle suspension. He guessed that the front engine and rear transaxle arrangement gave the Tempest chassis better weight distribution than the Corvair, which had both the engine and transaxle at the rear.

A car as unique as the 1961-1963 Pontiac compact couldn’t go forever without catching the attention of collectors. The Pontiac Oakland Club International (POCI) Little Indians non-regional specialty chapter welcomes owners of these cars from all over the world. For information, visit www.littleindians.com 

 

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