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Car of the Week: 1988 Chevrolet Corvette ZR1 prototype

The ZR1 Corvette destined for the crusher somehow escaped the abysmal fate and was resurrected to GM glory.
Car of the Week 2020
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The lives of automobile manufacturers’ prototypes are often short-lived. After coming to life as a test bed for design or engineering ideas (or sometimes both), the prettiest and luckiest among prototypes usually stand tall on a display stand at an auto show. After the concepts that these prototypes test become obsolete, they are almost always destroyed to prevent them from becoming corporate liabilities. Only a few have survived, and they are cherished.

Many readers are familiar with General Motors’ practice of crushing many of its prototype vehicles because they are not necessarily tested to ensure they meet the requirements to be safely operated on U.S. roads. Furthermore, these prototypes are not always built to the standards required for a reasonably long service life. Such was the case with the 1988 Corvette ZR1 prototype known by its GM code of EX-5023. EX-5023 is one of two prototype ZR1s that was to be sent to the crusher with the rest of the ZR1 prototypes, but miraculously survived.

Corvette EX-5023 is equipped with 17x9.5-inch front and 17x11-inch rear aluminum ZR1 prototype wheels fitted with extremely rare experimental Goodyear Gatorback tires with the words “not for sale” molded on the inner sidewalls.

Corvette EX-5023 is equipped with 17x9.5-inch front and 17x11-inch rear aluminum ZR1 prototype wheels fitted with extremely rare experimental Goodyear Gatorback tires with the words “not for sale” molded on the inner sidewalls.

Developing the ‘King of the Hill’

When the Corvette ZR1 program was initiated, the car was officially labeled “King of the Road,” or simply “KOH.” The “ZR1” moniker came later for the production version of the car.

The roots of the King of the Hill Corvette began with the introduction of the fourth-generation Corvette released for the 1984 model year. The next link in the chain of events leading to the ZR1 took place in 1986, when GM purchased Group Lotus, an engineering consulting and performance car manufacturing company headquartered in the United Kingdom. Soon afterwards, representatives from Chevrolet’s Corvette section began discussions with their counterparts at Lotus’ engineering facility in Hethel, Norfolk, regarding the development of a special Corvette engineered to become the fastest production car in the world. Lotus agreed to the proposal, and in collaboration with GM’s engineers, a new V-8 was crafted with the same bore centers as the Corvette’s standard 350 L98, but with extensive modifications including an aluminum block, four overhead cams, 32 valves and a special air management system that would shut off eight of the 16 intake runners and fuel injectors when at a part-throttle state. This engine, labeled as the LT5, was connected to a ZF six-speed manual transmission. Both the LT5 and the ZF transmission entered production for the 1990 model year. The LT5 installed in the feature prototype was the 43rd such engine built.

The original camouflaging cover for the tail panel, a unique feature used to distinguish the original production ZR1, was long lost, so Brett had it replicated.

The new engine, though, required special assembly techniques not possible at the Corvette’s Bowling Green, Ky., assembly plant, nor was allocating the work to other GM facilities an option. The solution to the problem arrived in an interesting form — via Mercury Marine Corp. in Stillwater, Okla., roughly 60 miles northeast of Oklahoma City. Once the ZR1 was green-lighted by GM management, Mercury Marine was contracted to produce the engines and send them to the Bowling Green plant.

Of course, more than a high-performance powertrain was needed to make this Corvette live up to its “King of the Hill” label. While Lotus designed the LT5 engine and subsequently tested the ZR1 package. another British company — Hawtal-Whiting Ltd., of Leamington Spa, Warwickshire — was given responsibility to develop the body and chassis modifications needed for the new engine. Hawtal-Whiting Ltd. already had a long-term relationship with GM and, at the time, was working on the development of the 1988 Reatta for Buick. GM sent to them 1986 Corvettes without engines to design wiring, hydraulics and instrumentation interfaces for the new engine. Attention was also given to braking and steering. The revised systems resulted in some changes to the standard ZX3 active suspension system. With these revisions, this very high-performance Corvette could not only go fast, but also handle well enough to take advantage of all that power on the racetrack.

This prototype of the Corvette ZR1, EX-5023, built in July 1987, was one of 25 “wide body” cars used in the development of the production version. It is one of only two to still exist. EX-5023 was restored in its original finish of Medium Blue Metallic. It still had the factory build sheet when owner and restorer Brett Henderson acquired the car. He also obtained the hand-written engine engineering book from Lotus.

This prototype of the Corvette ZR1, EX-5023, built in July 1987, was one of 25 “wide body” cars used in the development of the production version. It is one of only two to still exist. EX-5023 was restored in its original finish of Medium Blue Metallic. It still had the factory build sheet when owner and restorer Brett Henderson acquired the car. He also obtained the hand-written engine engineering book from Lotus.

After test-driving trials were completed at GM’s Milford, Mich., proving grounds, this prototype, along with the others, was shipped to Lotus for additional work. That helps explain its foreign license tag on the back.

Other than the special features previously mentioned, experimental components on this King of the Hill prototype include an 8,000-rpm tach and 17x9.5-inch-front and 17x11-inch-rear aluminum ZR1 prototype wheels fitted with Goodyear Gatorbacks with “not for sale” molded onto the inner sidewalls. The car was also fitted with a camouflage covering to hide the ZR1 tail panel. Those special tires are as rare as rare gets, but to make the car true to its original form, owner Brett Henderson had to try to find a set.

The car received an all-new bright-blue leather interior in 2017, though the original console plate, power key, and the 8,000rpm tachometer that was not available on production cars was retained.

The car received an all-new bright-blue leather interior in 2017, though the original console plate, power key, and the 8,000rpm tachometer that was not available on production cars was retained.

“The car does have prototype rear Goodyear 315s. They are from the run of the 25 wide bodies built along with EX5023,” he said. “The dated rear wheels are from the same batch of 25 cars.” (The phrase “wide bodies” is in reference to the wider width of the rear body, one of the distinguishing features of the production ZR1. In all, 25 prototypes were built during July of 1987 for testing.) The original camo tail panel covering was gone, thus it had to be replicated.

Back from the grave

Henderson’s ZR1 prototype was one of what Corvette fans refer to as the “UK graveyard cars.” The ZR1 prototypes were destroyed in the U.K., or were supposed to be fully destroyed, per a GM edict issued in 1990. To the vast majority of people, Henderson’s test car seemed destroyed as per the GM requirement. Indeed, it was badly flattened through the use of a front-end loader, but the frame, suspension and most of the body tub remained largely intact. The majority of the fiberglass bodywork, as well as the windshield frame, were destroyed. However, the chassis and door geometry were not affected. At the time, it was sufficiently damaged to pass for destroyed. After all, no one would try to piece it back together, because it was cost prohibitive. Right? Wrong! As time passed, interest in the C4 generation of Corvettes began to increase. Though still not among the most coveted of Corvettes, there are some serious fans of the design. Furthermore, to some collectors, owning a prototype ZR1 has strong appeal, including to Henderson. He bought the car from Geoff Jeal, who was the chief calibration engineer for Lotus at the time the KOH project was in progress.

The dash came from a 1987 Corvette with the bright blue exterior and interior color scheme.

The dash came from a 1987 Corvette with the bright blue exterior and interior color scheme.

The other surviving prototype, EX-4607, was used to test all of the new-for-1988 features. This car was also driven extensively at the Milford Proving Grounds to test all the improvements to the C4 suspension, steering and brakes. Later, it was among the batch of Corvettes sent to England to assist in the development of the King of the Hill prototype. It was registered EX-4607 at the local vehicle licensing office as “E282 LAC” in January 1988. After its U.K. engineering assignment, it should have been returned to the United States or crushed in the United Kingdom, but it was eventually sold to a farmer in Essex in the early 1990s.

Henderson found many of the extremely rare replacement parts needed to bring his car back from the dead. Among them were two NOS pieces — the rear upper surround panel and the luggage compartment tub — plus three empty “birdcages” that were used by General Tire, of Marion, Ind., for panel fitment. (General Tire made panels for GM.) The three birdcage-frames were not stamped with a vehicle identification number. A number of pre-production ZR1 components were used to rebuild the car’s body.

The door panels of EX-5023 are like those of an early fourth-generation Corvette, thus differ from those used by 1990-1995 Corvettes and ZR1s.

The door panels of EX-5023 are like those of an early fourth-generation Corvette, thus differ from those used by 1990-1995 Corvettes and ZR1s.

EX-5023 has a PhaseII engine “with Simplex chains and different castings. The castings are fairly rough and have no Corvette identification. There are very few production LT5 pieces that will interchange with it.

EX-5023 has a PhaseII engine “with Simplex chains and different castings. The castings are fairly rough and have no Corvette identification. There are very few production LT5 pieces that will interchange with it.

Henderson purchased a 1987 Corvette with the blue exterior and interior color scheme to get the interior and everything else needed such as trim, the blue-tinted glass top, instrument panel and small under-hood components.

“The top fit like a glove on the first try,” he said. “The geometry of the tub was not compromised when they tried to crush it with the front end loader.”

Other facts regarding this car that Henderson noted is that the build sheet for it refers to it as “KOH” and it has a PhaseII engine “with Simplex chains and different castings. The castings are fairly rough and have no Corvette identification. There are very few production LT5 pieces that will interchange with it… it is not an engine you ever want to run for very long — certainly no more than 4,000 miles due to its delicate internals; they are very expensive to rebuild.”

Here is how EX-5023 appeared with the damaged body panels removed and ready to go to the frame shop.

Here is how EX-5023 appeared with the damaged body panels removed and ready to go to the frame shop.

Henderson left no doubt he is not going to be driving this Corvette, “No driving on the open road for this PhaseII LT5... Just on and off the enclosed trailer… just one big conversation piece,” he said. EX-5023 is currently residing at the Lingenfelter Collection where it was photographed for this story.

The Corvette ZR1 went into production for 1990, but only as a coupe. Characteristics which distinguished it from other Corvette coupes included its wider tail section, 11-inch-wide rear wheels, its exclusive convex rear fascia with four square-shaped tail lamps and a center high-mounted stop lamp installed at the top of the hatch glass instead of between the tail lamps.

At this point EX-5023 was well on its way to being a functioning car again.

At this point EX-5023 was well on its way to being a functioning car again.

With the body ready, the engine and transmission installation was the next step.

With the body ready, the engine and transmission installation was the next step.

Shown here is the original console tray insert with “KOH” hand-written on it.

Shown here is the original console tray insert with “KOH” hand-written on it.

The ZR1 showed it had tremendous acceleration and handling characteristics, but carried a very high price; MSRP for the ZR1 in 1990 was $58,995, nearly twice the price of a base-model Corvette. By 1995, the final year for the ZR1, the price had increased to $66,278. This pricing placed the ZR1 in the same price class as the Porsche 964. Fewer than 7,000 ZR1s were produced during the span of its production run. Today, ZR1 Corvettes sell for as little as $20,000 in driver-quality condition while extremely low-mileage examples are in the $40,000 to $50,000 range. Prototype ZR1s that have escaped the crusher may be historically priceless, but one of the two survivors sold for $75,000 in December 2020.

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