Illustrated Corvette Series: No. 273
1968 XP-880: Frank Winchell’s Mid-Engine Corvette
Recently, we published a brief overview of the known 14 mid-engine experimental, prototype and racing Corvettes, from the 1960 CERV-I to the 2012 Daytona Prototype. They all fit into the Corvette family for a variety of reasons, but not all of them were built as possible production Corvettes. The CERV-I, CERV-II and GS-IIb were development race cars. The Monza GT and Astro-I were mid-engine Corvair-powered. The 1962 Monza GT was the forerunner to 1965 Mako Shark-II and the 1967 Astro-I clearly had Mako Shark-II influence. The V-8–powered, rear-engine XP-819 was experimental.
By the time the XP-880 Astro-II debuted at the New York Auto Show in 1968, the C3 had been in production and on the road for over six months. The Astro-II definitely looked like an extension of the C3. Common design elements included a pointed nose, Mako Shark-II–like front fender humps and the sugar scoop roofline. This was the first experimental mid-engine Corvette that looked like a real Corvette.
Zora Arkus-Duntov wasn’t the only Chevy man looking into the mid-engine platform. Only CERV-I and CERV-II were Duntov’s cars. Chevrolet R&D Chief Frank Winchell was behind the 1964 rear-engine XP-819, the 1966 GS-IIb and the 1968 XP-880. Winchell was a no-nonsense, low-profile man who, unlike Duntov, avoided attention. It is important to consider that Winchell’s R&D group had been assisting Jim Hall’s Chaparral racing effort. Winchell was up-to-speed with the latest racing technology that was moving from front-engine tube-frame cars to mid-engine monocoque chassis designs. The XP-880 was the next step in racing design that included the use of a steel center backbone to tie the front end with the mid-engine and rear suspension. This basic design became the foundation of the revolutionary C5.
The XP-880 was a beautiful effort for a prototype, but lacked real-car features such as bumpers, headlights and windshield wipers among other items. But it did have steel side-protection inner door beams. Like the 1968 McLaren, the XP-880’s 427/390 engine was orientated front-to-back and connected to a transaxle. While advanced for its days, the layout is simple. However, there was a critical element that ultimately kept the mid-engine Corvette a daydream for decades that I’ll get to later. For now, let’s look into the XP-880.
Here’s how the XP-880 came to be. Winchell knew that Ford wanted a car to compete with the Corvette and was working on their mid-engine Mach-II concept car. In 1967, Pete Estes, as the new Chevrolet general manager, completely reorganized Chevrolet, resulting in Duntov being made consultant and spokesperson for Corvette. Around the same time, Duntov was hospitalized for prostate surgery. With Duntov literally out of the picture, Winchell took the opportunity to design his own mid-engine Corvette.
Engineer Larry Nies created the basic layout while Styling created a functional shape, which did not have the typical Mitchell/Shinoda flair. In retrospect, the body was not over-designed. Winchell’s team had a mid-engine layout that was incomplete, as well as some extra running gear, axles and hub carriers from the rear-engine XP-819 project. Nies and Winchell’s chassis design featured a spot-welded steel center spine that was 12-inches tall and 6-inches wide. Inside the center spine was a 20-gallon fuel bladder. The rear of the frame was forked to hold the engine and rear suspension.
As the XP-880 was a prototype, production parts were used where possible. The front suspension used Camaro lower wishbones and spindles, with special upper suspension arms and coilover shocks that made the roll center at ground level. The front antisway bar was 0.75-inch thick. The rack-and-pinion was custom made and had very slow 23.6:1 gearing.
The engine and drivetrain are arguably the most interesting parts of the car. To keep the wheelbase as close to stock as possible, Winchell and his team took a stock L36 427/390 engine and turned it 180-degrees so that the engine accessories could hang over the top of the transaxle instead of between the engine and firewall. The XP-880’s radiator was placed all the way at the back, far away from the interior. The rear deck had a short dome for the engine with air intake grilles on each side and two radiator heat grille vents toward the back. To keep the 427 cool while the engine was running, a large fan under the sloped radiator pulled air into the radiator. Under the fan was a single transverse muffler.
The transaxle was an out-of-production two-speed automatic with a torque converter from a 1963 Pontiac Tempest. The halfshaft rear axles used the new sliding inner universal joints designed for the front-wheel-drive Toronado. The rear suspension used large boxed sections as an upper wishbone, sprung with a transverse Corvette spring and Corvette-like trailing arms. This setup created a roll center that was 2.75 inches above ground. Stock Corvette brake calipers and Camaro rotors were used.
Shinoda-designed snowflake, Can-Am–style aluminum multi-piece wheels measured 15×8 and were shod with F70x15 tires. Winchell’s team experimented with many tire sizes and was able to generate 1.0 g on the skidpad using H70 tires up front and L70 tires on the rear. These were considered “wide” street tires in 1968.
The XP-880 was close to the size of the 1968 Corvette. The length was 181 inches (1.5 inches shorter), width was 74 inches (5 inches wider), height was 43.7 inches (2 inches taller), the wheelbase was 100 inches (4.1 inches shorter) and the weight was 3,400 pounds (about the same as a big-block Corvette). There was a small storage area in the front. The entire back end was hinged at the rear to provide engine access.
The XP-880’s interior was functional with a basic array of gauges and no designed frills. Because of the car’s thick doors and 6-inch wide center spine, seating was described as snug, which designers didn’t mind because it helped keep the driver in place. Modern performance cars have seats with significant side bolsters to do the same thing.
The XP-880 was completed in February 1968 and sent to the Proving Grounds for testing. Everyone liked the car and were impressed with its handling. With as much grunt as the base 427 had, 60-percent of the car’s weight over the rear wheels and big tires, the design had potential. Most of all, XP-880 looked like a Corvette, even at a quick glance.
By mid-March, around 1,000 miles had been logged on to the XP-880. At the same time, Estes and Mitchell realized that they didn’t have a car to show against Ford’s Mach-II at the upcoming New York Auto Show that April. The XP-880 was quickly painted Firefrost Blue, detailed and renamed Astro-II. At the show, Chevy and Ford fans got to see both cars on display. Karl Ludvigsen of Motor Trend magazine wrote a comparison of the Astro-II and the Mach-II in the December 1969 issue. The two cars were similar but different, and both were prototypes. The Mach-II was a good-looking car, but nowhere near as sleek as the Astro-II.
So what happened? First, the new C3 was a big success. Sales went from 22,940 in 1967 to 28,566 in 1968 and 38,792 in 1969. Bean counters said, “We’re selling all the Corvettes we can make, why do we need a new one?” The second reason goes back to the transaxle. The 1963 Tempest two-speed automatic was out of production and was never designed for 400-plus horsepower. Also, the two-speed Powerglide was discontinued from the Corvette line after 1967. The 1953 and 1954 Corvettes took enough heat for their automatic transmission, Chevrolet wasn’t going to go backward. While transaxles were becoming common in racing, no one on the planet was making manual transaxles in volume production that GM could even buy.
The Astro-II made the show car rounds until it was replaced with a series of Corvette show cars and several more mid-engine concept cars. In retrospect, all were dead-ends because of the transaxle issue. Even the 1990 CERV-III had an unusual array of automatic transmission parts used to create its drivetrain. The introduction of front-wheel-drive cars in the early ’80s helped GM’s engineers get up to speed with transaxles. But it wasn’t until the Momentum Architecture was accepted as the foundation for the C5, with its all-new six-speed manual and four-speed automatic transaxle, that high-performance transaxles were perfected.
Today, the XP-880 Astro-II is a permanent part of the GM Heritage Center and looks as good as it did in April 1968 when it showed the Mach-II the door at the New York Auto Show. Vette
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