Many Corvette aficionados car assume Chevy’s sports car has always been a global benchmark for performance, but a closer look at the model’s history tells a different story. Yes, there were racetrack success as far back as the mid-C1 era, but production offerings from that period were primitive, ill-handling beasts compared with their European competition. The C2 and C3 that followed had vague steering, marginal handing, lousy brakes and, in later years, soporific straight-line performance. And while the fourth-generation Vette was revolutionary in many ways, the first-year model was slow, unrefined and harsh-riding.
A more powerful, 150-mph model arrived in ’85, but the car was still hobbled by a fidgety manual transmission and middling build quality. Meanwhile, the new-for-’86 convertible’s flaccid structure prevented General Motors from offering the topless model with a true track-capable suspension. So while the C4’s performance was nearly there, it was a stretch to refer to the car, as Chevrolet did at the time, as truly “world class.”
The engineers and product managers who set Corvette’s course in the late-’80s knew that more power was a necessity if the car was to become a segment-defining product. The eventual result of their deliberations was the 1990-1995 C4 ZR-1, the first Corvette fully capable of competing alongside the world’s best. The road to the ZR-1 was long and winding one, and along the way there were multiple concepts, component cars, test mules, prototypes and development vehicles. In this issue and in coming months, we’ll be profiling three such cars, starting with VIN EX4289—a twin-turbocharged convertible I remember fondly as the “Rapid Roadster.”
From Six to Eight
In January of 1983, Corvette Chief Engineer Dave McLellan, Chevrolet Director of Engineering Paul King, Director of Powertrain Engineering Russ Gee and Market Planning chief Don Runkle launched a project known simply as the “400-horsepower package.” The first exploratory moves took place later in the year, when Specialty Vehicles, Inc. (SVI), a company that constructed prototypes for Chevrolet, fitted some C4 development vehicles with 4.3-liter V-6s featuring splayed-valve heads and a single turbocharger.
As it turned out, these engines didn’t have much more power than the coming L98, and their noise, vibration and harshness (NVH) characteristics were unacceptable for a production car. The project was summarily scrapped, and the team moved on to a more appropriate platform. As McLellan wrote in his excellent book, Corvette from the Inside, “So, the engine guys finally got the V-6 out of their system and we settled down to work with them to package a twin-turbo V-8.” From early 1984 to early 1985, the team focused the bulk of its attention on a twin-turbo (TT) V-8. SVI constructed 14 TT development vehicles using production C4 coupe chassis in late 1984, with testing beginning that summer.
In the car business, marketing trends change quicker than you can say “America’s Sports Car,” and so it was with the 400-hp Corvette. Performance-wise, the TT V-8 could meet the desired criteria. But GM Vice President Lloyd Reuss vetoed this approach, believing that a turbocharged, pushrod V-8 would be perceived as low-tech by potential buyers. His answer? A high-revving DOHC V-8 that pushed the limits of automotive design and engineering. In a late-1986 interview with this writer, Corvette Development Manager Doug Robinson said, “The car always must incorporate leading-edge technology. Fact is, turbos make lots of horsepower, but for marketing, drivability and durability reasons, [they] are pretty low-tech items nowadays.”
Twin-Turbos: Dead but Alive
The TT Corvette project was shelved, but the 14 already-assembled prototype cars were kept around to use in testing powertrain and chassis components destined for the “King of the Hill” ZR-1. Occasionally, they even did promotional duty. One of the last ones built was fitted with a more powerful engine pushing 560 hp, unheard of back then for a car you could drive on the street. A top-speed test published in the February 1986 issue of Road and Track saw Corvette Powertrain Manager Jim Ingle pilot the screaming blue coupe to an incredible 191 mph.
What about EX4289, the 15th and final twin-turbo and the only convertible? It was built in the spring of 1986 from one the first convertible prototypes and the powertrain from Ingle’s 191-mph coupe. Chevrolet planned to take it to the Indianapolis 500 as part of the ’86 convertible Indy Pace Car promotion, to give VIPs, race officials and press a thrill to remember.
But the week before the 500, Corvette engineers realized they would face a credibility problem if the Rapid Roadster were shown alongside a lineup of 230-hp production Pace Cars. Said Ingle, “People were coming by all week before the race, looking at the cars and asking, ‘Are they turbocharged?’ We kept saying, ‘No…they’re stock Corvettes.’ All we’d need was to have a turbo sitting there while we were telling people the [production] Pace Cars didn’t have turbos.”
In the summer of ’86, the TT ’vert was assigned to Ingle’s powertrain-validation program and shipped to GM’s now-defunct, Mesa, Arizona proving ground. In January 1987, I asked to test the car. Director of Chevrolet Public Relations Ralph Kramer agreed and had the car shipped to Los Angeles for a two-week road test.
Driving the Rapid Roadster
The car was clearly a “ringer,” with its two big cooling vents on the hood, a front intake for the intercooler and 50-series tires on powder-coated black wheels. The tires were a soft-compound “S” racing version of the Goodyear Eagle VR50 production rubber. The wheels were pricey Dymags made of die-cast magnesium and imported from the UK. Other notable items were the 560-hp twin-turbo engine and a real Nash five-speed with an add-on overdrive (not to be confused with the Nash 4+3 used in ’84-’88 C4s). The car also had an experimental 3.54 rear end and a set of larger front brakes that were being developed in conjunction with showroom-stock Corvette racers.
The first thing I did was attack the Angeles Forest Highway in the mountains north of LA. With the sticky tires and stiffer springs, bars and shocks, the car’s handling approached that of a Z51 coupe. The only thing you really had to watch for was the car getting twitchy near its limits. I attributed that to the effect the big springs and bars had on the C4 convertible’s weak structure. The other problem was holding back my right foot on corner exits. Lean on the gas too hard coming out of a turn, and boost would put the car sideways faster than you could say, “Loose!” There was no active handling or electronic limited slip to bail out an unwary driver.
To confirm my feelings about the car’s ability to stick in turns, I ran it on the skidpad and saw 0.875g, close to what production Z51 coupes could do back then. Zero to 60 took 4.4 seconds. I then took the Rapid Roadster deep into the desert and ripped a 0-to-100 run in about 9 seconds. In the quarter-mile, the bright-yellow orgy of acceleration went 11.76 at 122.4. But the most revealing figure was the 0-100-0 run, which took an astounding (for a production car back then) 14.7 seconds. The mix of a turbocharged engine, ABS and the big brakes made for a hell of a driving experience back in 1987. Top speed with the 6,000-rpm limiter, 3.54 gears and a 0.82 overdrive was a hair under 160 mph—plenty fast given the lack of a stabilizing deck spoiler.
Despite its hellish performance, the car proved a model of refinement. Nearly everything worked including the Delco-Bose sound system, the air conditioning and even the digital instrumentation. The only problems were a passenger-side seat with an electrical problem, an occasionally sticky rear deck-release solenoid and a nonfunctional cigarette-lighter socket.
The car even got decent gas mileage. My average over 1,223 miles was 13.57 mpg running 100-octane unleaded racing gas. That’s quite an achievement for a 560-hp car with only a .82 overdrive. I did one short test where I purposely stayed out of the boost, and the car achieved 18.7 mpg.
Callaway Picks up the Baton
After killing the factory twin-turbo program, Chevy decided it would be criminal to waste the technology it developed. Enter Reeves Callaway, who at the time had a modest business in Connecticut doing aftermarket turbo conversions. His most recent success had been a low-volume job for Alfa Romeo of America, the 230-hp GTV6.
Chevrolet worked a deal with Callaway Engineering under which new Corvettes would be shipped from Bowling Green to the firm’s facility in the town of Old Lyme, where they would receive a 50-state-legal twin-turbo engine good for 345 (later boosted to 382) horses. The option, which debuted in 1987, carried RPO B2K and an initial price tag of $19,995. As for the 1990 ZR-1 that effectively replaced the original TT, its J55 heavy-duty brakes, 3.54 rear end and manual transmission with additional forward gears were all inspired by those 14 original SVI prototypes.
Following my test drive the Rapid Roadster was sent back to Mesa, where it spent a year or so in Jim Ingle’s powertrain test fleet, partly in support of the B2K program. At the time I drove the car, only 10 of the original TT coupes still existed. Eventually all of them, including the convertible, went to the crusher. Chevy’s in-house turbo Vette might not have made it into production, but its spirit and influence lived on in the high-performance C4s that followed.
