Up until 1970, powertrain engineers assigned to the Corvette could focus nearly all of their attention on finding ways to extract ever more power from Chevrolet’s engines—but with the passage of the Clean Air Act that year, those same engineers had to shift their focus to reducing emissions. Then the OPEC oil crisis of 1973 made gasoline consumption a national issue in the U.S. and the Energy Conservation Act of 1975 made reducing fuel consumption law. As a result, Corvette engine output dropped precipitously during this period.
Ever mindful of the decisive role that engine power plays in the Corvette’s appeal, and thus sales, Chevrolet pursued several technologies simultaneously in order to meet government-imposed emissions and economy requirements as well as customer expectations through the ’70s and ’80s. Those technologies included electronic fuel injection, turbocharging, multi-valve cylinder heads and multiple camshafts. When management eventually decided to bring a multi-valve head/multi-cam engine to production in the form of the C4 ZR-1’s LT5, it made a rather unusual decision concerning GM’s turbo research. Instead of simply putting it aside, as typically happens when a competing path wins out, GM instead struck a deal with Callaway Cars. The arrangement, orchestrated by Chevrolet Market Planning chief Don Runkle, entailed Chevrolet providing technological, logistical and financial support so that the Old Lyme, Connecticut specialty builder could keep moving the technology forward, with an eye on production viability.
Callaway was not the first outside company to help GM develop a turbocharged Corvette. In May 1983, Chevrolet contracted Troy, Michigan-based Specialized Vehicles Incorporated (SVI) to produce 16 Corvettes with prototype turbocharged engines. Because of packaging concerns, the plans initially called for SVI to produce a twin-turbo V6. Corvette Chief Engineer Dave McLellan and others with influence at Chevrolet, including the marketing department, were not particularly enthusiastic about installing a V6 into the Corvette, regardless of how much horsepower could be squeezed out of it. So, after Chevy learned that a twin-turbo V8 could fit in the C4’s engine compartment, SVI was asked to abandon the V6 and concentrate instead on a V8. It delivered a functional and reasonably well-sorted twin-turbo V8-powered Corvette to Chevrolet early in 1985. By August 1985, SVI fulfilled its contract with GM by delivering the last of 16 twin-turbo Corvettes for further testing.
The twin-turbo Corvettes built by SVI were intended to eventually lead to production, but their primary purpose was to evaluate the feasibility of packaging a twin-turbo V8 in the existing Corvette and test its effects on the chassis. By contrast, Callaway’s expressly stated mandate was to develop a complete twin-turbo package that would pass all of GM’s validation tests while not impacting its existing emissions systems in any significant way.
The knowledge gained from Chevrolet’s collaboration with SVI, as well as one of the twin-turbo Corvettes the company created, was turned over to Callaway Cars in June 1985. Callaway was chosen to make the turbo package production-ready because the company had already demonstrated its ability to successfully design, fabricate and sell turbocharging systems—and, equally important, get through the onerous certification process required to bring low-volume production turbocharged cars to market. In 1983, Callaway had done so for Alfa Romeo, creating a twin-turbo V6 for the carmaker’s GTV coupe.
Even after all the work SVI and Chevrolet had done, Callaway still had to overcome several very difficult problems, including developing a viable fuel-injection system, packaging all of the extra hardware in the Corvette’s already cramped engine compartment and simultaneously meeting power targets and emissions requirements. Callaway quickly abandoned Chevrolet’s plans for a scaled fuel-injection system designed to deliver the extra fuel the turbochargers demanded while meeting the EPA’s stringent emission standards, and instead solved the riddle by leaving the regular production fuel-injection system untouched and adding a pair of additional Bosch fuel injectors ahead of the throttle. These two auxiliary injectors operated independently of the stock engine-management computer; they were directed by their own control system, using Callaway’s proprietary Microfueler II computer module. The Microfueler II delivered the necessary extra fuel when the turbo boost kicked in, but did not interfere with the stock fuel-injection system’s functions when the car was driven without boost.
As far as packaging was concerned, Callaway took advantage of every bit of what little room there was under the Corvette’s hood. A compact IHI RHB52 turbo was mounted on either side of the engine toward the bottom, while twin air-to-air intercoolers were prominently located on top of the engine and fed cooling air by two large NACA ducts in the hood. It was an elegant solution to the packaging conundrum.