1991 Chevrolet Corvette C4

The 1991 Chevrolet Corvette was part of the C4 / fourth-generation produced from 1983 through 1996, highlighted by the return of the convertible, higher performance engines, and the introduction of the 375 horsepower LT5 engine found in the ZR-1. They had a completely new chassis, numerous improvements throughout, and modern styling that was completely different from the Zora Arkus-Duntov-designed C3. The C4 was the work of designer Dave McLellan and his team who did away with the fiberglass panels and body-on-frame construction of its forebearers, replaced by what GM termed a 'uniframe' consisting of a traditional perimeter frame, with the windshield frame, doorposts, and rear portion of the floor pan integrated into one welded assembly. Although similar to a unibody setup, the exterior body panels were not structural members.

With the goal of developing the world's fastest production car, General Motors acquired a UK-based engineering consultant and performance car manufacturing firm named Group Lotus in 1986. Together they designed a new engine named the LT5 to replace the L98 V8 powering the standard C4. It used a lightweight aluminum block, the same bore centers as the L98, but with overhead camshafts and 32 valves. The engine block had a cast-aluminum oil sump and the aluminum cylinder liners were Nikasil-coated. The four camshafts were operated by a roller chain and actuate hydraulic lifters that eliminate valve lash adjustment. The forged steel crankshaft was secured in place by the crankcase's integral four- and six-bolt cast-iron main bearing caps. The centrally-located spark plugs worked in tandem with dished aluminum pistons with a compression ratio of 11.0:1. It used direct-fire ignition with four coils igniting two spark plugs simultaneously, operated by a crankshaft sensor acting in combination with the ECM. The ECM electronically controlled both the spark advance and retardation, receiving additional information from a knock sensor.

The engine used a unique air management system designed by Lotus that offered a wider power band by disabling 8 of the 16 intake runners and fuel injectors when the engine throttle was not fully engaged, while providing the full 375 horsepower when prompted. Additionally, Lotus aided GM in designing the braking and steering systems for the ZR-1. Standard features of the ZR1 included the ABS system manufactured by Bosch, UJ6 Low-Tire-Pressure Warning System, ventilated disc brakes, and Goodyear Eagle Gatorback tires measuring P315/ 35ZR-17. Bilstein engineered the FX3 suspension system similar to the setup used in the Porsche 959, complete with a gas-over-oil shock absorber whose hollow center shaft came fitted with an adjustable orifice that controls the flow of oil in the shock absorber. Three driving modes (Touring, Sport, and Performance) adjusted the suspension system accordingly with the help of servomotors and microprocessors based on the vehicle's speed.

After its debut at the 1989 Geneva Motor Show, production of the ZR-1 soon commenced, available only as a fastback coupe body style with a removable roof panel. The wider tail section and 11-inch rear wheels gave it a unique appearance from its Corvette siblings, along with special red ZR-1 badges, and the new convex rear fascia with four square-shaped taillights.

For the 1991 model year, the Corvette received styling refinements. The ZR-1 performance package was further segregated from the standard Corvette when the high-mounted stop lamp was repositioned into the rear fascia. New wrap-around front parking and cornering lamps were added as well as new side-panel louvers and a ZR-1 style convex rear fascia on all models. The ZR-1 models received additional badges on the fenders and the introduction of Acceleration Slip Regulation (ASR) or traction control.

The ZR-1 was capable of going from zero-to-sixty mph in just over four seconds and had a top speed of 175 mph. The engine, developed by Lotus and GM, was built at a Mercury Marine plant in Stillwater, Oklahoma, a plant known for its precision aluminum machining capabilities. Pricing for the ZR-1 was initially $27,016 on top of the Corvette Coupe's $32,479 base price, setting a new record for a GM passenger car. Production of the ZR-1 lasted five years before it was phased out in 1995. In March of 1990, the ZR-1 set a new record for the highest 24 hour-5,000 mile land-speed by going over 175 mph (282 km/h).

The production total for the 1991 model year was 20,639. 14,967 were coupes including 2044 ZR1s and 5672 were convertibles. Approximately 2,044 examples of the ZR-1 were built in 1991 with a base price of $64,138.

The RPO B2K package, a $33,000 Callaway Twin-Turbo engine package, was still offered through specific Chevrolet dealers. When buyers selected this package, the Corvettes were sent from Bowling Green to the Callaway factory in Old Lyme, Connecticut for engine modification and performance upgrades.

by Daniel Vaughan | Apr 2003