After the engine came back to GoodSpeed, Firpach bolted on a pair of GT2871 turbos to a custom stainless-steel exhaust manifold and 3-inch stainless downpipes. To get more air in, they modified the OEM intake manifold and also made two 3-inch ram-air intakes to feed the airbox. Firpach went with a liquid-to-air intercooler instead of an air-to-air unit that would've required cutting into the core support. Aside from a few stainless pipes, the engine bay looks next to OEM. The turbos are hidden discreetly down low, completely out of sight.

The engine uses a Proefi stand-alone ECU; an uprated, in-tank fuel pump and an Aeromotive fuel pressure regulator. Many of the hard-to-find OEM replacement parts were sourced through Saliya Ukuwela at Euro M Cars. The exhaust is a custom GoodSpeed fabrication utilizing E60 M5 cats and a Supersprint muffler and is completely emissions legal.

As mentioned earlier, I drove the car during the break-in period, so boost was set at 9.5 psi. Since then, they've turned it up to 16.5 psi and registered a corrected 682 crank horsepower on GoodSpeed's Maha dyno. This is with 91 octane pump gas. A high-boost race gas tune is in the works. Row at Metric Mechanic said he'd guarantee the engine to withstand up to 1,000 hp. Both the owner and GoodSpeed's Firpach say that with the boost at 16.5 psi, the power is now too much for words.

A Ground Control suspension setup keeps it planted to the ground, but because there was so much weight transfer to the rear, new, stiffer springs have replaced the ones I drove on. Powerflex bushings and Dinan sway bars replace the stock units. Eighteen-inch M Motorsport Parallel wheels from a 7 Series have replaced the Throwing Star five spokes. They're wrapped with Toyo Proxes RA1 tires, 245/40 in front, 275/35 in back.

One revealed weakness was that the Wilwood six-piston brake calipers simply weren't up to task. Although better than the stock setup, they started to fade after a few hard stops from high speeds during my brief drive. A pair of Brembo eight-piston calipers and carbon-composite discs from a Mercedes-McLaren SLR remedied the problem. A taller, Dinan 3.64 rear differential was also added, which Firpach says increases the load on the turbos and also lowers the revs for quieter highway cruising.

The owner has said he has yet to come up against a car that can run with him. To date, his list of victims includes a couple of E60 M5s, a Z06 Corvette and a GT3. Odds are, they thought he was easy prey and are probably still wondering what hit them. And it's that sleeper quality that appeals to him the most.

1991 BMW M5

Longitudinal front engine,rear-wheel drive

3.5-liter I6, dohc, 24-valve. 3.9-liter stroker conversion, custom GT2871 twin-turbo kit by GoodSpeed Performance Lab, liquid-to-air intercooler, custom ram-air intakes, custom exhaust manifold and 3-inch downpipe, custom GoodSpeed exhaust w/ Supersprint muffler, Proefi stand-alone ECU

Five-speed manual, Dinan 3.64 differential

Ground Control coilovers, Dinan antiroll bars, Powerflex bushings

Eight-piston Brembo calipers, carbon-ceramic rotors

Wheels and Tires
M Motorsport Parallel alloys, 18-inchToyo Proxes RA1, 245/40 (f), 275/35 (r)

Peak Power: 682 hp @ 6445 rpm*
Peak Torque: 616 lb-ft @ 5000 rpm
GoodSpeed Data

GoodSpeed Performance Lab
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