The '76 Turbo prototype featured more smoothly integrated fenders than the Sebring GT. The GT's short, vertical rear spoiler, meanwhile, gave way to an integrated wing. The notchback design was filled in and became a "fastback" with a large, nonopening rear glass window. The prototype was painted a two-tone silver-over-gray, similar to Greenwood's No. 77 "Batmobile" racer. Taller, sport-style mirrors were developed to aid in over-the-fender rear vision.
A raised rear floor was required to accommodate the car's coilover suspension.
A few styling changes were adopted as the car neared production. The most significant arose out of the new glass hatchback of the '78 25th Anniversary model. The complications of the Sebring GT's fastback design were no longer an issue, so the rear wing was widened to fit flush with the extended fenders. It was also made thicker, with an upturned trailing edge to better manage airflow off of the new rear window.
The fenders were unchanged from the prototype and featured five vertical slots at the rear of the front panels to the new Turbo hood. A single NACA duct on the right side of the hood allowed additional cooling to the engine compartment.
The engine of choice was the L48 model due to its lower compression ratio. High boost pressures would cause detonation in the higher-compression L82, making the L48's stock 8.5:1 ratio a better choice. Still, it didn't resolve all the problems that arose when running a full 7 psi of boost. A water-injection system was devised to help alleviate the problem. Fed from the windshield-washer reservoir, this primitive charge-cooling setup could be considered the precursor to today's intercoolers.
BFGoodrich ads from the period featured the Turbo GT prototype.
Other pieces and systems were custom-made to overcome the problems associated with the blow-through setup. A right-side exhaust manifold was cast to hold both the Air Research T04 Turbo and a wastegate fitted with an integral muffler. A crossover pipe connected the right manifold to the left, and a custom-bent pipe connected the turbo to the stock exhaust. A cast-aluminum bonnet fed by the turbo sat sealed atop a Holley 650-cfm carburetor that was specially prepared to handle the boost pressure.
To ensure adequate fuel delivery, two electric pumps took over the duties of the original single mechanical unit. The TH350 transmission was also tweaked to handle the extra power. But despite all the fortifications, problems persisted. The pressurized approach forced a lot of fuel into the combustion chamber, sometimes causing a "wash down" effect.
The suspension-a Greenwood specialty-didn't go unnoticed, either. Coilover shocks and heavy-duty sway bars out back were a given. The rear suspension also utilized unique upper and lower A-arm configurations, not unlike those of the C5. This radical (at the time) setup virtually eliminated the dive and squat associated with hard braking and acceleration.
The location of the suspension pieces, did, however, necessitate raising the part of the rear floor where the battery and storage bins resided. The new floor was cast in fiberglass and proved to be quite an exercise in fitment. The rest of the interior remained stock, except for a boost gauge neatly integrated into the dash. The serial plaques on the ashtray door and doorjamb indicated which of the three Turbo GTs you wer e riding in.
For more information on the Greenwood street cars, check out www.greenwoodcorvettes.com.