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1973 4-Rotor Corvette Prototype: “The ‘Almost’ Mid-Engine Production Corvette”

Trend Setting – Part 11: A look back at Chevrolet’s experimental, prototype, concept car, and show car Corvettes.

Scott Teeters Aug 5, 2015
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From 1960 to 1973 there were at least 10 functional mid-engine Corvette prototypes. The 4-Rotor didn’t have a dedicated “XP” number because it was a variation of the XP-882. The Wankel engine initiative was a huge distraction and the 4-Rotor and its magnificent body design fell victim to a road that should not have been traveled.

Duntov DID NOT want to do this project. With three years left before retirement, the window of opportunity for a mid-engine Corvette was narrowing. Familiar with the Wankel engine since 1955, he knew that the basic design was inefficient because of the surface-to-volume ratio in the combustion chamber. The Vega was scheduled to be the first GM Wankel-powered car and Zora didn’t want the Corvette to be powered by a Vega engine. But Ed Cole was hot on the Wankel and tactically said, “yes” to the mid-engine Corvette, but with a Wankel in the middle. Duntov had no choice.

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Zora told his engine man, Gib Hufstader, “Make me a fast car!” Gib’s solution later won a U.S. patent. The layout consisted of two separate Wankel engines, one on each side of a shaft that ran back to the bevels at the transmission output. Each engine was 90-degrees out of phase to smooth out the performance. A toothed and grooved cog belt ran the ignition, alternator, and fuel pump, while a V-belt controlled the air conditioning, power steering, and water pump. The combined size of the two engines was 585 cid and was rated at 350 to 370 hp. Hufstader said with some development the setup could make 480 hp. Gib pulled it all together in just two months. In July 1972, Ed Cole, with Duntov, took the completed bodiless car out on the 1-mile GM Tech Center check road, hit 148 mph and was still accelerating when they had to slow down. The sound was described as an “incredible shreek!”

A full year went by when in July 1973, Duntov cornered Mitchell and said, “Let’s put a deadline on this. The Paris Show.” Mitchell agreed and assigned the project to Hank Haga and Jerry Palmer. Mitchell wanted to see a break from the Kammback design, something pointed and streamlined, similar in flavor to the Mercedes-Benz record-breakers of the 1930s. Palmer said, “It was a very hard car to design. After my second shot Mitchell said, ‘Hey, look, this is like baseball, Kid – three strikes and you’re out.’”

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Haga explained, “Mitchell wanted a teardrop shape. The design problem was to put together a piece of equipment that didn’t have a tail 40 feet long. It started with a long nose and tail … it looked like a record car rather than a Corvette. So we did several overlays and got a fair balance between the nose and tail. We kept shortening it and that’s how it evolved.”

With the basic shape set, everyone in the design group fell in love with the car. With a 72-degree windshield slope, the wheelwell flares and vents behind the rear glass, the coefficient of drag was 0.325 – astonishing for its day. Other details included side cooling intakes for the engine, bifold gullwing doors, and a deep, V-angled front windshield. Chevrolet Interiors created the cabin with plush leather and suede seats, a digital smoked-black instrument display that pivoted along with the telescopic and tilt steering wheel. The center console had more digital gauges, warning lights, radio, climate controls, transmission selector, and handbrake.

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Both the 2-Rotor and 4-Rotor Corvettes were shown at the Paris Salon Show in October 1973, with mixed reviews, but magazines went crazy with speculation. Regardless, the project stalled on September 25, 1973, when Ed Cole postponed the Wankel project and four days later, retired. The 4-Rotor was never developed and didn’t run well. Full out, it was a bear, anything else, not so good. Designers tried to take the body design and make it a producible car, but every production issue solved detracted from the beauty of the original. “Death by a thousand cuts.” said Jerry Palmer.

Late in 1975, Bill Mitchell had the 4-Rotor sent to the Design Center to be retrofitted with the all-aluminum XP-895’s drivetrain and a 400-cid small-block. The new “Aerovette” was relegated to show car duty. Chief Engineer Dave McLellan said, “Showing the Aerovette was a sign of what wouldn’t be produced.” If not for the Wankel distraction, this “might” have been the first production mid-engine Corvette. Duntov later said that the car was equal with the 1957 Corvette SS as his favorite one-off Corvette. When Zora retired, he was given a detailed model of the 4-Rotor. Current V.P. of Global Design, Ed Welburn said that he and his pals used to look at the car on their lunch breaks. “I knew every inch of it, and I didn’t even work on it.” Now that’s inspiration!

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