©1987, 1999 Hib Halverson. All rights reserved. No use without permission.

My girlfriend's kid plays the same game all little squids do. It's called "Pretend" and actually, I think big kids play this game just as well as the wee tots in the world. So let's try it.

First, we set the stage. 'Spose we could magically (poof!) axe CAFE and (poof!) slash gas prices. Then, 'spose we (poof!) raise the speed limit. And… (poof!) loose the smog laws. We'll (poof!) get rid of all the slimes that set rates in the insurance industry. Finally, just to cover all bases, let's (poof!) get rid of everyone and everything else that keeps us from having a good time with cars.

Hey, are ya having fun yet?

Okay, now imagine that all the poor guys at Chevy that have been workin' on all this fuel economy, smog and other unfun crap, suddenly have a lota free time. Next, pretend you're sittin' in Corvette Chief Engineer Dave McLellan's office and the General Manager, calls…

Ring, Ring…Dave picks up the phone and hears "Dave! Hey, what's happenin'? Beat any Testarosas lately…chuckle, chuckle?

"Okay, let's get serious for a sec; listen, we gotta have a Turbovette. It's gotta have 500 horses and be somethin' that even guys who like sex, beer and football more than cars will dig. It's gotta be so awesome that Porsche will give up and switch to building cat boxes. Can you handle it–and how soon?"

McLellan replies, naturally, "Gee boss, we just got scads of go-fast engineers around here with nothing to do but build the nastiest, fire-the-tires-into-the-next-county kinda Corvette you've ever seen.

The next model year starts and, low-and-behold, for-real, factory-fresh Turbovettes start popping out of Bowling Green. So, you and your significant other whip over to the Chevy store, plop down your cash and order a Turbovette just the way you like. "Let's see, dear, we want a convertible, the turbo engine (20lbs. boost and 500 horses at six grand, hurt me, hurt me!), Z51 suspension, Delco-Bose, yellow paint, 3.54 gears, a six-speed tranny, the big brakes and (what the hell, money's flowing like water here, right?) 17-inch Gatorbacks Dymags. Are we bitchin' or not?"

Finally, your very own Turbovette convertible arrives (this is Pretend so it comes already broken-in, okay?); you go to the local gas station, fill it with its first tank of 100 octane race gas (now we're really Pretending), put the top down and take her out for a maiden voyage. Strictly by coincidence (of course!) you end up at the local street race set's favorite venue. A Mustang pulls up--one of those with a water pipe roll cage, primer-grey paint, carbs sticking through a jagged hole in the hood, recapped N60s on rusted wheels, a rough idle and the words "Pro Street" lettered crudely on the side windows--a real dorkey lookin' ride, but the motor sounds pretty healthy.

The guy drivin' chucks an empty in the back seat, rolls down the window and yells, "Hey, you wanna run me with that high-dollar junk!? I got fifty bucks that says my Mustang can kick yer ass! Bet that fagey-looking car can't even do 14s! Oh, and you're ugly and your mother dresses you funny."

That does it!

Very cool–you drop your hand off the shifter and pop open the ashtray door exposing the boost knob. You firmly grasp it and turn counterclockwise, all the way to the stop--full boost--all five hundred seventy horsepower!

"This Ford jockey is gonna get some religion," you chuckle as you click off the radio, push the shifter into first and bring the R's up to 2500. Green light! The Mustang is off, rear axle hopping, N60s smoking and engine screaming.

Since Fords really bring out your bad attitude, you give the guy several car lengths before you pop the clutch and bury the gas. The turbos spool up to boost and, to keep the tires from smoking too much and the car straight; you back off a little. Hit the rev limiter and bang second gear, this time with the gas on the floor. The car's a little sideways as the turbos go on-boost again.

Right at the middle of third gear, you pass the Mustang. The pie-eyed, mouth-agape Ford freak can't believe the come-from-behind performance. Through third (you can really hear the turbos hiss with the top down), fourth (into three figures) and partway into fifth gear you're still hard on it. The Mustang recedes in your mirrors and turns off.

"Hit the road, Jack, and keep the money for a real car," you mutter. The turbocharger waste gates take a big gasp as you back off and, coasting down through a hundred, you yell to the wind, "Eeehaaa, we are having fun now!"

The bad thing about Pretend is that it has to end. No 570 horse, twin-turbo Corvette will ever come out of the plant at Bowling Green, though at one time Chevrolet did consider it. Chevy built just such a car–actually there were a number of them–as a prototype.

The one tested for this article and that inspired the previous "dream" carried the VIN plate: EX4289--It's an R and D effort. The "Rapid Roadster", as we named it, was the last of 15 turbocharged "experiments" (the only rag top of the bunch, too) that the Corvette Development Group started in the fall of 1984. At the time, they thought these cars would be not the test mules they're today; but for-real prototypes of a possible turbocharged, 400 horsepower option.

Marketing trends in Detroit change quicker'n' you can say "Corvette." Sure enough, half-way through the build of these prototype turbos, there was a change-of-heart in how they were going to do the "400 horsepower package." Although a hot item back in late '84, by mid-'85 they weren't in the Corvette's future. The car always, in the words of Development Manager, Doug Robinson, "…must incorporate leading edge technology." The fact is, turbos make tons of horsepower but, for marketing, driveability and durability reasons, are pretty low-tech items nowadays.

However, that didn't kill the "experiment. All 14 cars were built and were used for testing and development of powertrain and chassis components. Occasionally they were pressed into promotional duties. One of the cars, a blue coupe, received special engine modifications and hit the big time. Development Engineer, Jim Ingle, drove it 200 mph for a piece that was in the February '86 Road and Track.

What about EX4289 aka the Rapid Roadster? It was built in late '85/early '86 and was a combination of a roller that was one of the first Roadster prototypes and the powertrain from Ingle's 200 mph coupe. The Corvette Group was going to take the car to Indy to promote the Pace Car deal. It would give VIPs, race officials etc. a real thrill--as if a lap with Chuck Yeager in the stock, 150+mph Pace Car wasn't enough, right!

The week before the race those plans were nixed. The Corvette Group decided they'd have a credibility problem if it was on display along with the Pace Cars. Jim Ingle told me, "People were coming by all week before the race, looking at the cars and asking 'Are they turbocharged?' We kept saying 'No, they're not turbocharged. They're stock Corvettes.' We decided to leave the turbo convertible at Milford. All we'd need was to have a turbo sitting there while we were telling people Pace Cars didn't have turbos."

So, poor EX4289 spent Indy week, parked in a corner of the shop behind the Corvette Group's offices at GM's Milford Proving Ground. Late last year, the car was assigned to Ingle's '88-'89 powertrain validation program and trucked out to GM's Mesa, Arizona Desert Proving Ground. Last January, my friends at Corvette were cool enough to pull it out of its test program so CORVETTE Illustrated could drive this unique car.

The first time I saw it, the phrase "blatant nastiness" came to mind. The little clues--the hood vents and the intercooler intake--that something was not normal about this Corvette were there. If you didn't catch the little clues; there were a few big ones. It sat lower than any Roadster you'd ever see and fat S-Gatorbacks on black wheels made the car look downright evil; especially when the top was down.

The car had Z51 suspension and Goodyear Eagle VR50-S showroom stock race rubber and the evil wheels are Dymags. Made of dicast magnesium and imported from England, they are the hot fad on Corvettes in the ESCORT series. Other "little" iniquities were the twin-turbo 570 horsepower 350, a full-out Nash five-speed with overdrive (can you dig it; a six-speed!), an experimental 3.54 rear end and (stop to match the go, you know!) a set of the giant-sized front brakes the Corvette Group has been testing on showroom stock racers for the last 12 months.

With the S-tires and the suspension, Rapid Roaster handled better than other convertibles and It showed when I ran the car, top down and balls-out of course, over our mountain test course up in the Angeles National Forest. She was a blast to drive with less of the moderate understeer or slow steering I've felt in stock convertibles. About the only thing you had to watch was your right foot on exits. Lean on the gas too hard coming out of a turn, and the boost would have you sideways faster than you could say "Oversteer!" This is common with big-bore turbomotors and nothing an attentive driver couldn't handle.

To confirm my feelings about the car's ability to stick in turns, I ran it on the skid pad to the tune of .875g., close to what Z51 coupes get. The showroom stock brakes were a big step over what production Corvettes have. The bigger calipers, rotors and brake pads add up to lots of heat dissipation. The front brake fade I've felt in past Corvettes during repeated stops from high speed was gone. I abused the crap out of these brakes time and time again and they replied, "Hurt me more!"

What is the Rapid Roadster like in a straight line? I'll tell you what, I've driven race cars that can't perform like this! Zero to sixty in a tire smoking 4.4 seconds good enough for you? With most cars 0-100 times are boring; but not this time. I recalibrated our VC-200 Performance Computer, took the car way out into the desert and let her rip. Zero to 100? How about 9.5 seconds? That oughta get anyone's heart started. In the quarter mile, this bright yellow orgy of speed did a 12.46. Probably though, the figure that's most revealing about this car's abilities is the time it took to go from zero to 100 mph and back to zero. It was 14.3 seconds. The mix of turbopower, ABS and the big brakes is one hell of an experience!

The car weighs 3400 pounds and was hauling 500 lbs worth of two people, a tank of Trick and a bunch of camera gear. If that's not enough, the engine wasn't even running to its full potential. The rev limiter was malfunctioning during my tests. Ordinarily, it's at 6000 rpm. Sometime during the first week of my drive in the car, Murphy's Law came into effect and the limiter reset itself at 5200 rpm. Had we run the car with just a driver, half-a-tank of gas and buzzing the motor to six grand; we'd have probably knocked another seven tenths (high 11s--on street tires for crissakes!) off the quarter times, a half off the 0-100 and a couple more tenths off the 0-60. Top speed, with the bad rev limiter, short (by today's standards at least) gearing and "close ratio" 0.82 overdrive, unfortunately was a little tame at 139 mph. But, really, I wasn't as worried about how fast it would go as I was how quickly it got there and, it was damn quick!

The nice thing about this car was, since it was an offshoot of a prototype program, was its refinement. Nearly everything worked--the Delco-Bose, the air, all the digital instrumentation functioned properly. The only problems were little ones: the passenger side sport seat had an electrical problem, the right rear deck release solenoid stuck once in a while and the cigarette lighter wouldn't light. No big deal, hell the chassis was one of the first Roadster prototypes.

The car even got decent gas mileage! Average for the 1223 miles I had the car was 13.57 miles per gallon of Trick gas. That's quite an achievement for a 570 horse car. I did one short test where I purposely stayed out of the boost and the car got an amazing 18.7 miles per gallon. Considering the three-five gears and the short overdrive, that's pretty damn efficient!

Have these turbo Corvettes done more that supply a testbed for new drivetrain pieces and, on occasion, be the playtoys of us media types? The answer is, "Kinda yes." After the killing of the factory turbo program, the Corvette Group decided that it would be criminal to waste the expensive technology developed to date. They let it be known that if the right aftermarket guy came along, they might be willing to share the knowledge gained with the 14 turbos so any Corvette owner who wanted power like that could have it.

Along came a guy named Reeves Callaway. The rest is history. Chevrolet worked a deal with Callaway Engineering in Connecticut, that if the buyer specified option number B4Z, the car would be shipped from Bowling Green to Callaway where it would get a 345 horse, smog-legal, twin-turbo engine. From there the car would go to the customer's dealer. On these cars any Chevy-made parts are covered under GM warranty and any Callaway stuff is under their own warranty deal. So, even though the factory turbo idea went tits-up; we can get its offspring, the Callaway Corvette.

Our test car? It's now back at Mesa being abused daily in Ingle's '88-'89 powertrain validation test. After that? Oh…I guess it will go to the great test track in the sky. Since Chevy can't sell prototypes (Hate to say what I'd do to buy it if they did!) it will be "dismantled."

Maybe Jim Ingle will call me up after he ships it to the crusher. I'll open a cold beer and have a "memorial ceremony." I'll close my eyes and Pretend I'm spreading hate and discontent with the Rapid Roadster: the factory Turbovette that just mighta been.

The Techside of the Rapid Roadster

©1987, 1999 Hib Halverson. All rights reserved. No use without permission.

Our roadtest car was a promotion connected with the 1986 Indianapolis 500 and received the powertrain from the special 200 mile per hour car, one of the original 14 proof-of-concept cars. At present, not including the Rapid Roadster, ten of the "Chevrolet twin-turbo Corvette" coupes still exist. The others have been dismantled or returned to production specifications and sold as used cars. All powerplants for the turbocharged Corvette program were built by Specialized Vehicles Incorporated (SVI) of Troy, Michigan.

General Motors, medium-duty, 5.7 liter, truck short block assemblies were used for all 14 engines. They are composed of a heavy-duty cast iron block with four-bolt main bearing caps, a forged steel crankshaft, heavy-duty connecting rods, cast aluminum dished pistons and a double-moly ring set. Modifications included a 1985 Corvette camshaft, a big-block L88 oil pump and a double-row timing chain. The engines were blueprinted and used factory balancing.

The cylinder heads on our engine were standard 1985 cast iron Corvette units however, the other 13, because of lower turbocharger boost, were fitted with production aluminum heads. With the truck pistons, compression ratio was 8.0:1. The heads received moderate porting by Diamond Racing Products, mostly in the exhaust ports. Stock valves, valve train and 1969 Z/28 "service cam" valve springs were also used.

The induction system is unique to this engine. The others were fitted with modified Tuned Port Injection (TPI) manifolds. This engine, because of its use in the 200 mile-per-hour Corvette discussed in the road test, received a special unit composed of the baseplate from a Holley dual four-barrel intake manifold and a SVI-fabricated plenum chamber. The plenum was machined to accept a modified TPI throttle bore unit and the Holley baseplate was fitted with Rochester Products electronic injectors then port matched to the heads.

Additionally, the production electronic computer module (ECM) used to control fuel and ignition functions, received special software developed by Chevrolet and SVI that would meet the fuel flow requirements of a turbocharged engine. Lastly, two 70 millimeter Bosch hot wire air mass sensors were used instead of the stock Corvette's larger single unit and a stock Corvette ignition system was installed.

The turbochargers are Mitsubishi TD05 units. In order to have the underhood area visually appealing and to solve a packaging problem, Chevrolet had Mitsubishi manufacture 14 TD05's that were "mirror images" of the standard piece. This made for a "right" and a "left" turbocharger. According to SVI, when the Japanese were asked to make a limited run of "mirror image" turbocharger housings; they did exactly that. Even the letters "Chevrolet" cast into the compressor housing are reversed.

Between the turbochargers and the intake manifold is a 126 square inch charge air cooler. It is an air-to-air design manufactured by the Blackstone company. The turbochargers are fitted to two tubular stainless steel exhaust manifolds that were built by SVI and the car has a dual exhaust system, no catalytic converter and stock Corvette resonators.

SVI's horsepower and torque figures, derived with 11-12 pounds of boost, are: 440 horsepower at 6000 rpm and 570 pounds/foot of torque at 3000 rpm. However, this car was had cockpit-adjustable waste gates. Adjusted for maximum; 20 pounds boost is possible. Acceleration testing at the General Motors Desert Proving Ground at Mesa, Arizona by Development Engineer, Jim Ingle, showed at 20 lbs. boost; the engine was putting out 550-570 horsepower at 6000 rpm.

In the feature article, eeference is made to this engine as the centerpiece of a "…possible 400 horsepower package." Further investigation into this shows that, in 1984 within Chevrolet, it was more than just "possible." Corvette Development Manager, Doug Robinson, told me that the Corvette Group, "…wanted to look at a 400 horse package to study the idea and to see if we could get it through emissions and guzzler (CAFE). A single turbocharger would have drivability problems, throttle lag and so forth, so the Powertrain Group said, 'Let's try two small turbos.'"

When asked about the effort's demise, Robinson said, "The twin turbo program died as a result of the marketing people thinking it wasn't high tech."

Then Robinson discussed how Callaway Engineering came into the picture, "Almost a year's work was done. It seemed a shame not to get something out of it. If one of the aftermarket guys wanted to go with it, we were game. At that point, a strategy developed between us and Callaway Engineering. However, we made it clear to them, they were on their own without GM backing. All we would do is share technology."

This "strategy" developed into the B2K option which is a Callaway Engineering replacement of a stock 1986 or 1987 Corvette powerplant with a custom built, 345 horsepower, twin-turbocharged 5.7 liter engine. If the buyer specifies B2K the car is shipped from the Corvette plant to the Callaway facility in Connecticut for the conversion and thence to the customer's dealer. The Callaway pieces are covered under a separate warranty and the stock Chevrolet parts of the car are warranted by General Motors.

Although the engine in the "Rapid Roadster" certainly offers the driver spectacular performance, the drivetrain and chassis are also quite interesting. At this writing, the car is being used at Mesa, Arizona to validate systems to be used on the 1988 and 1989 Corvettes and some of the equipment on this car may give a clue to what we will be seeing in years to come.

The transmission is a Doug Nash Engineering and Equipment (DNE) Company manual five-speed transmission. It was fitted with a DNE 0.82:1 electric overdrive unit that could be controlled by the car's ECM or locked-out with a button on top of the gear shift lever. Effectively this made for a six-speed transmission. The rear axle was a production limited slip unit fitted a prototype 3.54:1 ratio ring and pinion.

Chevrolet/Pontiac/Chevrolet Canada (CPC) Engineering told us that the DNE five-speed/overdrive/3.54 axle system in this car was, "…a way to simulate future possible transmission/axle combinations." A six-speed transmission (probably three reduction gears, a one-to-one fourth gear and two overdrive ratios) is undergoing a validation program at Mesa that is not yet near term.

Although for our test period, the "Rapid Roadster" was fitted with P255/50VR16 Goodyear Eagle VR50-S tires and 16 inch Dymag wheels, that was not the way the car is equipped during its test work. The car had been using experimental Goodyear P275/40ZR17 tires.

Another interesting feature of the "Rapid Roadster" were its front brakes. The disc rotors, the calipers and the brake pads were substantially larger than the units used on 1984-1987 cars. The same hardware was seen on Corvettes at many of the 1986 SCCA/ESCORT showroom stock events. CPC Engineering told us that these brakes would indeed appear as a four-wheel system on the Corvette as a heavy-duty option, "…as soon as the validation program is complete." Commenting on the performance of these brakes CPC told us, "The new brakes have achieved 20 stops from 150 mph, each one mile apart with minimal fade."

We were told the brakes, "…were an outgrowth of the 400 horse package." The CORVETTE Illustrated staff wondered if this was the "old" 400 horse package for which the 14 turbo prototypes were built or was this a new concept? Additional investigative work led us to the existence of a program, known in-house, as "King of the Hill."

By 1985, it was obvious that other nameplates, the Porsche 928S4 and the Ferrari Testarossa for example, would by 1986, eclipse the Corvette as the world's best performing sports car. A source in CPC Engineering told us " The Corvette is a world class high performance sports car and will remain competitive in the world market. Chevrolet Marketing asked CPC to look at a package with the performance of a Testarossa."

This development effort got the nickname "King of the Hill" What form it will eventually take is a subject of some educated guesses we can make. On the basis of what we hear from CPC, a survey of existing information about future Corvettes and a close examination of the equipment on the Rapid Roadster; CORVETTE Illustrated feels that by 1989, the "King of the Hill" Corvette will be ready as an option. The price will be 50,000 to 60,000 dollars. The car will have an all aluminum, dual overhead camshaft engine of from 5.0-5.7 liters in displacement. It will be fuel injected but not turbocharged. The car will have a six-speed transmission, possibly with overdrive, and either the 3.31 or the 3.54 axle. It will have the larger brakes and the 17 inch wheels with the 275/40ZR17s, and perhaps even 315/35ZR17s will be used at the rear. We should note that the validation programs of the brakes, tires and wheels are planned to reach completion by the beginning of the 1988 model year and probably will appear on the option list then rather than 1989.

The King of the Hill engine would be capable of as close to 400 horsepower as possible and still meet Federal requirements. This is an important point that illustrates the challenge of this program to the Corvette development Group. None of the Corvette's competition meets CAFE. They are all eligible for the "gas guzzler tax" due to low fuel economy. The new car's suspension will be upgraded to a level commensurate with its speed and power.

A "King of the Hill" would equal or exceed by a small margin the performance of a Testarossa. An option such as this should certainly keep the Corvette as the technological leader in the very high performance sports car field.