Porsche is almost synonymous with flat-6 engine architecture, but V8 engines also come out of Stuttgart bearing the iconic crest. Carl Fausett and his 928 Motorsports specialize in Porsche V8 engines and can pull in the neighborhood of 900 horsepower with a Vortech supercharger, as evidenced in the above video.
“We have developed 930-horsepower Porsche engines for our Bonneville land speed efforts,” says Fausett. “But this engine is going road-racing (see racecar and tuning video below). So we dialed it back with emphasis on engine safety and reliability. We want to complete a 10-race season with no major maintenance. The finished engine came in at about 880 horsepower with a long, flat, useable torque curve that’s ideal for this application.”
Porsche offered the stylish front-engine/rear-drive 928 models for nearly 20 years starting in 1978. The original engines were 4.5-liter SOHC but the platform eventually graduated to 5.4-liter DOHC configuration. In addition, Porsche designed a new V8 found in the Cayenne and Panamera, a 5.0-liter version of which was approved for the old Grand Am Daytona Prototypes.
This engine started its life with a 100 x 78.9 mm bore and stroke. After ductile-iron liners are installed to handle the boost, Fausett opened the cylinders out to 105.3 mm and dropped in a 95.25 mm billet steel stroker crank, resulting in a final displacement of 6.6-liter. Completing the rotating assembly are Oliver parabolic I-beam E4340 steel connecting rods and coated Arias dished pistons providing a 8.5:1 compression ratio.
“There are two schools of thought on compression ratio,” explains Fausett. “You can build a 10.5:1 engine with less boost and attain 800 horsepower, or go to 8.5:1 and use more boost to attain the same power. The conventional wisdom is that the lower-compression motor with more boost has greater engine safety. If something should happen and the tune gets a little off mid-race–say resulting from a blown intake manifold gasket, for example–a low-boost engine will tolerate that while a high compression ratio motor will not.”
Developing the intake manifold
Lubrication upgrades include an extensive crank scraper system, windage screens adjusted to clear longer stroke, a Patterson oil separator and a Tilton pump to return the oil back to the motor. The oil system was also modified to divert more oil from the head back to the main and rod bearings.
The cylinder heads were CNC ported to Fausett’s specs, including increasing the intake valves from 36 to 39.5 mm and adding larger 33 mm exhaust valves. High-lift (.442-inch) cams were installed for Bonneville, but for road racing Fausett wanted less valve overlap and better throttle response, so the lift was set at .412-inch. The valvetrain also includes custom valve springs, titanium retainers and lightweight lifter buckets. The valve seats are beryllium-copper.
Considerable development went into designing a twin-plenum intake manifold and glass-filled, nylon-composite intake runners.
“That intake manifold was almost a year in the making. We flow-benched several prototypes before we knew which way we wanted to go,” recalls Fausett. “In the end I was able to print a set of high-temp, high-flow, tapered intake runners that delivered the characteristics we wanted.”
Before those glass-filled nylon intake runners were put on the car, they were bolted to a head, brought up to a temperature that was 20 percent beyond the expected operating temperature and pressure-cycled from 25 psi to zero and back again almost 30,000 times.
“We had to see if they were going to fatigue and crack under stress,” adds Fausett.
Providing the boost is a Vortech V-7 YSi supercharger that is routed through a large air-to-air intercooler manufactured by Bell Intercoolers. A custom cogged belt drive works off a fluid-filled damper.
Going NASA racing
The single throttle body is a monster 4-inch billet unit, feeding two 3-inch ram tubes going to each side of the split plenum.
“The plenum had to be split to reduce its volume to the size we wanted” says Fausett. “Otherwise the driver could snap out of the throttle yet keep accelerating because the plenum volume is just too great.”
Engine management chores were given to an Electromotive Tec GT ECU.
“We’ve had such good results with the TecGT, there was no reason to change. You go with what you know works,” adds Fausett.
At the season’s first race a camshaft chain in the head broke. The chain piled up and broke a drive tooth on one of the four camshafts.
“Here’s where I’m happy we opted for low compression when he developed this engine, as none of the valves or pistons were hurt,” concludes Fausett. “It’s always that way–the $30,000 motor fails for want of the $20 part. We have found and installed stronger cam chains. Our plan is to get to the Milwaukee Mile for testing later in May and rejoin the NASA GTS series at Mid-Ohio in the first week of June.”