He eventually got one for himself and took it straight to his garage in Old Lyme, Connecticut. Determined to squeeze more power out of the venerable M10 four-cylinder, he did what BMW engineers had done with the 2002 Turbo and developed a turbocharger system for it.
The tuned 320i was driven by Car and Driver journalist Don Sherman who was blown away by its performance and featured it in a one-page article. This made Callaway a household name among American BMW owners who started visiting his garage to gain some more horses for their beloved rides.
By 1977, Callaway Cars was established and, in the years that followed, the company started offering turbocharger kits for BMW, Volkswagen, Porsche, Audi, and Mercedes-Benz models. In the 1980s, the former driving instructor also developed the HH IndyCar V8 engine and later, a twin-turbo kit for the Alfa Romeo GTV6.
Searching for a way to make the sports car more appealing to performance enthusiasts, GM struck a deal with Callaway in 1987, enabling those who wanted to buy a rabbit ‘Vette to order one equipped with a factory-sanctioned performance option . Called Regular Production Option (RPO) B2K, this kit added a twin-turbo system among other upgrades, giving the engine a boost of about 150 hp.
To promote the option and showcase what he could do with a GM V8, Callaway decided to build the fastest, most outrageous street-legal Corvette in the world.
Under the hood, it hid a 5.7-liter that was hand-built around a blueprinted Chevy block. It featured align-bored journals, four-bolt splay-design bearing caps, a Cosworth crankshaft, forged pistons, a dry-sump lubrication system, an MSD high-performance ignition system, and a Zytek engine management unit.
The cherry on top was, of course, a pair of Turbonetics TO4B intercooled turbochargers that provided up to 22 psi (1.51 bar) of boost. The engine exhaled through a 321-stainless-steel exhaust system with four SuperTrapp adjustable diffuser disc mufflers that stuck out from under the rear bumper.
The purpose of the project was to take the beefed-up Corvette to the track and attempt to break the speed record for a street-legal car. To do that, Callaway connected the engine to a custom Doug Nash-developed gearbox, but that unit was replaced by a ZF six-speed manual after the record attempt.
Furthermore, the SledgeHammer needed extensive body modifications to cope with the targeted speed. Automotive designer Paul Deutschman was tasked with creating the AeroBody kit that improved aerodynamics and added several functional vents for optimum engine and brake cooling.
Inside, the car remained in its factory configuration. It had two leather-upholstered power-adjustable seats, power windows, a Delco Bose stereo, and even an automatic climate control system. The only modifications included a Callaway boost gauge, a four-point, leather-padded roll cage, a fire-suppression system, and five-point safety harnesses.
Finished in an OEM Silver Metallic (13U) paint and sitting on Dymag one-piece magnesium wheels clad in bespoke Goodyear tires, the audacious machine was taken to the 7.5-mile (12 km) oval at Transportation Research Center in East Liberty, Ohio.
Although it seems to be forgotten, especially now, with everyone talking about the Z06 C8, this amazing vehicle remains an engineering masterclass that earned its place in the history books and made Callaway famous around the world.
Initially, the company wanted to continue the program and develop even faster versions. Sadly, this never happened and with its original front fascia swapped for a stock AeroBody component, as well as standard Goodyear Eagle tires in place of the custom set, the SledgeHammer was eventually sold.
This summer, it was up for grabs through the Bring a Trailer platform, with only 2,000 miles (3,218 km) on the odometer. The highest bid reached $500,000, but the owner's reserve was not met. A month after the auction concluded, the seller reported that the iconic Corvette was eventually sold to a “respected collector for more than the owner’s reserve.”
If you want to know more about Project SledgeHammer, I highly recommend the documentary below, posted on YouTube by Callaway Cars Inc.