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Dream Garage: 1957 Corvette SS XP-64

1957 Corvette XP 64 leadWhen the Corvette debuted in 1953 it offered sensational visual presence and lust-worthy sports car proportions, but its performance was simply not on par with its competitive set. With sales already on the decline by 1954, General Motors briefly considered putting the Corvette out to pasture, but newfound competition from Ford with the upcoming Thunderbird two-seater gave Chevrolet some additional incentive to see the project through.

That year would also mark two pivotal events that would have a profound effect on the Corvette’s trajectory – the introduction of Chevrolet’s new 265 cubic-inch small block V8, and the addition of GM engineer Zora Arkus-Duntov to the Corvette team.

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Although the Corvette was an initial sensation when it debuted in 1953, widespread reports of quality control problems paired with its underwhelming performance almost spelled the premature demise of the American sports car icon. But in just a few years’ time the model was comprehensively transformed, due in no small part to the efforts of Zora Arkus-Duntov, who has been nicknamed the “Father of the Corvette”. He also had some help from the introduction of the Chevy small block V8, and by 1957, Chevrolet was touting the one-horsepower-per-cubic-inch capability of the 283ci mill found under the hood of the ’57 “Fuelie” Corvette seen here.

Over the next few years with Duntov leading much of the charge, the Corvette would be transformed from a head-turning runabout into a hot-blooded sports car with legitimate performance credentials. But Duntov knew that if the Corvette was going to establish itself as a world class performance machine it would need to compete, and win, on the world stage of motorsport.

Despite the significant strides that he and his team had made in terms of adding performance prowess to the production car, Duntov was aware that they would need to go more than a step or two beyond off-the-shelf components if the Corvette was going to be competitive with the likes of Ferrari and Maserati in professional endurance racing.

After convincing General Motors head Harvey Earl that a race car based on the production Corvette wouldn’t be able to hold its own against the strongest entries from Europe, a clandestine effort dubbed Project XP-64 got underway in the summer of 1956 to build a racer that could bring home a win for Chevrolet at the 12 Hours of Sebring in March of 1957.

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While its body bares some resemblance to the ’57 production car, the mechanicals of the Corvette SS XP-64 shared very little with the road-going sports car. Under its magnesium body was a tube chassis, coilover front suspension, a de Dion live rear axle and suspension setup, and inboard-mounted aluminum drum brakes. All told, the car weighed in at just 1850 pounds. With a 307 horsepower V8 providing the thrust, its no surprise this thing really moved – that’s a power-to-weight ratio roughly on par with a C7 Z06.

Although fate would ultimately not be on the side of the Corvette SS (Super Spyder) XP-64, Duntov’s team succeeded in building a car that had the potential to put the rest of the world on notice.

Project XP-64 Development

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Duntov’s team had just six months to take the XP-64 from pen and paper to a working race car that could compete on the world stage.

Using a Mercedes 300SL as their template, Duntov and his team set about designing and building a light weight race car to bring to Sebring and, if all went well at that race, the 1957 24 Hours of Le Mans. Though its magnesium body echoed some of the aesthetic of the road-going 1957 Corvette, the underpinnings were another matter entirely.

Under the skin, the XP-64 consisted of a tubular steel space frame which featured coil-over-shock front suspension, a de Dion rear axle and inboard-mounted aluminum drum brakes. Motivation was provided by a race-spec version of Chevy’s 283ci iron block V8, which featured aluminum cylinder heads and Ramjet mechanical fuel injection.

At full wail the motor generated 307 horsepower at 6400 rpm, which was plenty considering that the XP-64 weighed nearly half a ton less than a production Corvette at 1850 pounds.

Although the team built the Corvette SS XP-64 with a plastic canopy as per FIA race regulations in 1957, it wasn’t equipped during the race at Sebring. Images: WheelsAge.org

The Corvette SS XP-64 Heads To Sebring

Two chassis were built – one considered the test mule and the other the actual race car. The mule was brought to Sebring for testing and tuning during practice sessions, where Duntov and racers John Fitch, Stirling Moss and Juan Manuel Fangio got a sense of what the XP-64 would be capable of at speed.

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Duntov sits at the helm of one of the first motorsport iterations of the Corvette that he would help develop over the years.

The initial results were promising – Fangio beat his previous year’s lap record with the car during one of those sessions, and he didn’t even drive for the Corvette team. But the Corvette SS campaign faced a constrained time frame leading up to Sebring, and it would prove to have a profound effect on the car’s success at the race.

The race itself was nearly canceled just before the event was to take place, as the Fédération Internationale de l’Automobile (FIA) issued an edict at the last minute which mandated that during the first tire change, teams would have to use a spare tire that was carried on board in the car.

This last minute rule change actually worked in the Corvette’s favor and against teams Ferrari and Maserati, as those Italian teams used different sized wheels for the fronts versus the rears while the Corvette team did not.

Representatives from those European automakers along with the event organizer met with FIA officials to protest the rule change, citing that the implementation of these new regulations violated the FIA’s own rules concerning the adoption of such changes, as the rulebook stated that rules had to be unanimously approved by all the competitors or they would not be implemented.

Although no official press release was ever offered by the FIA, it’s safe to say that the automakers’ protest rightfully overturned the ruling, allowing the race to move forward as originally planned.

The 1957 12 Hours of Sebring had developed into a massive event, with a field of 86 cars registered for the race. Among these, 66 cars would arrive for the practice sessions, qualify, and start the race. Of particular interest to the media was the story circulating that the Americans were going to bring the heat to the much-favored European teams with a wild new Corvette entry.

In the days leading up to the race, the visually unrefined XP-64 test mule was the car seen in and around the track. Despite its relatively shabby appearance, word got around the paddock that this car was indeed very quick, which is what had prompted Fangio – who normally drove for Maserati and would go on to win at Sebring for them – to request some seat time in the car, which resulted in the aforementioned lap record.

While the test mule chassis was a roughly put together machine, the car that went on to actually run the race at Sebring was a sight to behold. Under the hood was a 283ci Chevy small block V8 that dished out 307 horsepower at 6400 rpm. In the days leading up to the Sebring race, word got around of just how fast this new Corvette was, prompted Maserati team racer Juan Manuel Fangio to ask Duntov for some seat time in the car. He promptly broke the track record with it. Although its pace was undeniably quick, reliability issues plagued the car during its debut race. Images: WheelsAge.org

But the mule wasn’t the car that was going to compete – the beautiful, magnesium-bodied race car was. This presented some serious problems as the team prepped for the race, as Fitch recalled in an interview documented in the book Corvette Racing Thunder.

“That car had so much potential, but we never had time to get it race ready,” Fitch explained. “We didn’t get the car to the track until the very last minute and so we transferred the parts that worked – and everything we had refined on the mule – to the racing car when it arrived. The brakes were absolutely terrible and they never got that resolved. For some reason, Zora Duntov made the decision not to use disc brakes on the car and it was a bad one…We had absolutely no time to sort that car out. If we had, we might have made a real impact on that race.”

Despite its fast pace, the lack of preparation time would prove detrimental to the XP-64’s reliability. Endurance races are never won on the first lap, and the 12 Hours of Sebring proved no exception.

Drivers Piero Taruffi and John Fitch were hampered with numerous mechanical issues shortly after the race got underway, with “electrical gremlins, bad brakes, and finally, terminal rear end problems” forcing the team to retire after just 23 laps.

A Stunted Racing Career Saved By Reinvention

Although its race performance was disappointing, the Corvette SS XP-64’s pace proved impressive (as well as its top speed, which was later recorded at 183 mph), and it was enough to keep General Motors interested in further developing the project. When Daytona Speedway was opened with its high speed banked corners, Duntov reportedly recorded an average speed of 155 mph in the SS during a practice session.

Running just 23 laps of a 24 hour race is an abject failure by any race team's standards. However, the pace that the XP-64 had established during practice was enough to keep GM brass interested in the project. Development likely would have progressed throughout 1957 and the car might've run in that season's event at Le Mans, but the AMA racing ban issued later that year promptly ended all factory-backed racing efforts, including the XP-64 project.

Yet for all of its potential, the project could not escape the AMA ban of 1957. That summer, a consortium known as the Automobile Manufacturers Association issued a group-wide ban on all factory-backed racing efforts as a reaction to the tragic events of the 1955 24 Hours of Le Mans, where a Mercedes 300SL racer spun off the track and exploded, killing the driver and numerous spectators in the process.

Some speculate that the ban may have been a strategic measure by the AMA to prevent government regulators from stepping in and altering the legality of factory-backed motorsport on some level. Though the ban was loosely adhered to at best, with numerous privateer teams seeing factory support on some level throughout the duration of the ban, the XP-64 project was scrapped before the 1957 24 Hours of Le Mans race.

However, the story of the Corvette SS XP-64 doesn’t quite end there. In 1959, young GM engineer and designer Peter Brock, along with Bill Mitchell, GM’s Vice President of styling, and Corvette designer Larry Shinoda took the XP-64 chassis and repurposed it as the underpinnings for Corvette Stingray Racer Concept Car.

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The 1959 Corvette XP-87 Stingray Racer, seen here on display at the Peterson Automotive Museum, was really a re-bodied XP-64. Underneath the Stingray body was the chassis of an XP-64 which had been repurposed by GM for this design exercise. It proved to be worth the effort, as the XP-87 would inspire the overall design of the second generation Corvette that would debut in 1963. Image: Wiki Commons

Although the company was prevented from campaigning the new car, internally known as the XP-87 Stingray, its value in Corvette history cannot be overstated. Not only would the XP-87 serve as a test bed for a number of new components that would be introduced on subsequent Corvette models, its curvaceous body would inspire the lines of the Mako Shark concept car, which in turn would heavily influence the final design of the second generation Stingray Corvette.

About the author

Bradley Iger

Lover of noisy cars, noisy music, and noisy bulldogs, Brad can often be found flogging something expensive along the twisting tarmac of the Angeles Forest.
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