File:1990 Chevrolet Corvette ZR1.jpg

en.wikipedia.org/wiki/Chevrolet_Corvette_(C4)#ZR-1_(1990%E2%80%931995)

General Motors acquired Group Lotus, a UK based engineering consultant and performance car manufacturing firm, during 1986. The Corvette division approached Lotus with the idea of developing the world's fastest production car, to be based on the C4 generation of the Corvette. With input from GM, Lotus designed a new engine to fit in place of the L98 V8 that was powering the standard C4. The result was what GM dubbed the LT5, an aluminum-block V8 with the same bore centers as the L98, but with four overhead camshafts and 32 valves. Lotus also designed a unique air management system for the engine to provide a wider power band by shutting off 8 of the 16 intake runners and fuel injectors when the engine was at part-throttle, while still giving the ZR-1 a power output of 375 hp (280 kW) when at wide open throttle. In addition to the engine, Lotus helped GM design the ZR-1's (which in prototype version was called "King of the Hill") upgraded braking and steering systems.[citation needed] The ZR-1 is fitted with Goodyear Eagle Gatorback tires having size of P315/ 35ZR-17 specially made for the car along with bigger ventilated disc brakes. Due to the heavier engine and body work along with wide tires, the ZR-1 is 200 lb (91 kg) heavier than the standard C4 Corvette. The ZR-1 came standard with the UJ6 Low-Tire-Pressure Warning System along with an ABS system manufactured by Bosch. The FX3 suspension system was engineered by Bilstein and was similar to the system used in the Porsche 959 albeit with modifications from the Lotus Formula 1 division. The system used a gas-over-oil shock absorber whose hollow center shaft came fitted with an adjustable orifice which controls the flow of oil in the shock absorber. The system allowed for six damping settings in each of the three driving modes namely Touring, Sport, and Performance and had 14 total steps. Servomotors coupled with a microprocessor governed the vehicle's speed and adjusted the suspension system accordingly.

The 5.7-litre 32-valve LT5 engine unique to the car had a central bore spacing of 4.40 inches. The distance was maintained by reducing the bore from 4.00 to 3.90 inches while the stroke was increased from 3.48 to 3.66 inches. The aluminum cylinder liners were Nikasil-coated and the engine block has a cast-aluminum oil sump. The crankcase has integral four- and six-bolt cast-iron main bearing caps which secure the forged steel crankshaft into place. The four camshafts of the engine are driven by a roller chain and actuate hydraulic lifters that eliminate valve lash adjustment. The four-valve combustion chambers feature centrally-located spark plugs which act in combination with dished aluminum pistons enabling for a compression ratio of 11.0:1. The engine held 12 quarts of oil, 7 more than the L98 engine. The LT5 also came with a unique two valve induction system along with 16 tuned-length intake runners and a specially designed intake manifold using three throttle bodies. The small primary throttle body was for responsive low speed operation while the two large secondary throttle bodies enabled for full-power usage. The engine used direct-fire ignition: Four coils ignite two spark plugs simultaneously, upon receiving their cue from a crankshaft sensor acting in combination with the ECM. Spark advance and retardation are electronically controlled by the ECM, which gets an additional information from a knock sensor. A distinctive cooling system incorporating a 15% larger radiator ensured that the operating temperature of the engine remained the same as the L98 despite the differences in construction and operation.

In order to transfer power efficiently to the rear wheels, a unique 6-speed manual transmission manufactured by ZF was installed with a code name of ML9. The transmission used Computer Aided Gear Selection (CAGS) which forced the driver to shift from first to fourth under low power urban driving conditions. The transmission has a tweaked ring and pinion ratio of 3.54:1 and a lower final drive ratio of 3.33:1.

Other exterior modifications include a hard coated acrylic roof panel, and seven additional exterior colors. The interior came standard with leather sports seats and a Delco/Bose sound system.

Tested performance figures by Road & Track magazine include a 0–60 mph (0–97 km/h) acceleration time of 4.9 seconds, a quarter mile time of 13.4 seconds, braking distance of 132 ft from 60 mph and 233 ft from 80 mph along with skidpad acceleration of 0.94 g. The car's tested top speed by the magazine amounted to 179 mph (288 km/h).

GM found that the engine required special assembly, and that neither the Corvette plant in Bowling Green, Kentucky nor any of their normal production facilities could handle the workload, so Mercury Marine corporation of Stillwater, Oklahoma was contracted to assemble the engines under their MerCruiser division, due to their experience in working with aluminum, and ship them to the Corvette factory in Bowling Green where the ZR-1s were being assembled. The engine assembly involved 95% drilling and boring completed at the Mercury Marine plant. The engine was largely assembled by hand and was Dyno tested before being sent to Chevrolet. Mercury Marine secured two LT5 V8 engines for itself. These engines were used in the ZR-1 owned by the company president and a custom made speedboat called the "Wette Vette". The engine was modified to be used in the boat and had an increased output of 420 hp (313 kW).

The ZR-1 was introduced at the 1989 Geneva Motor Show and went on sale later that year and was available only as a fastback coupé bodystyle with a removable roof panel. It was distinguishable from other Corvette coupes by its wider tail section, 11-inch wide rear wheels and its new convex rear fascia with four square shaped taillights along with a special red ZR-1 badge in between. 1992 Corvette ZR1

In 1990, the 1991 ZR-1 and 1991 base model received updates to bodywork, interior, and wheels. The rear convex fascia that set the 1990 ZR-1 apart from the base model found its way to all 1991 models, making the high-priced ZR-1 even less distinguishable. Further changes were made the following year in 1991, including extra ZR-1 badges on the fenders and the introduction of Acceleration Slip Regulation (ASR) or traction control. For model year 1993, modifications which were designed by Lotus were made to the cylinder heads, exhaust system and valvetrain of the LT5 bringing power output up from 375 to 405 hp (280 to 302 kW) at 5,800 rpm and 385 lb⋅ft (522 N⋅m) of torque at 5,200 rpm. In addition, a new exhaust gas recirculation system improved emissions control. The model remained nearly unchanged into the 1995 model year, after which the ZR-1 was discontinued as the result of waning interest, development of the LS series engines, manufacturing cost and the forthcoming introduction of the C5 generation. A total of 6,939 ZR-1 models were manufactured over the six-year period. Not until the debut of the C5 based Z06 in 2001 would Chevrolet have another production Corvette capable of matching the ZR-1's performance.

Although the ZR-1 was extremely quick for its time (0-60 mph (97 km/h) in 4.4 seconds, and onto over 180 mph (290 km/h)), the huge performance of the LT5 engine was matched by its robustness. As evidence of this, a stock ZR-1 set seven international and world records at a test track in Fort Stockton, Texas on March 1, 1990, verified by the FIA (Fédération Internationale de l'Automobile) for the group II, class 11 category:

   100 miles (160 km) at 175.600 mph (282.601 km/h)
   500 miles (800 km) at 175.503 mph (282.445 km/h)
   1,000 miles (1,600 km) at 174.428 mph (280.715 km/h)
   5,000 km (3,100 mi) at 175.710 mph (282.778 km/h) (World Record)
   5,000 miles (8,000 km) at 173.791 mph (279.690 km/h) (World Record)
   12 Hours Endurance at 175.523 mph (282.477 km/h)
   24 Hours Endurance at 175.885 mph (283.059 km/h) for 4,221.256 miles (6,793.453 km) (World Record)