Ford currently offers two of the most powerful engines ever built for Detroit iron: the 550-horsepower, supercharged V-8 for the Ford GT super car (below) and the 500-horsepower, supercharged V-8 that powers the Shelby GT500.
They’re not the same engine, but they’re awfully close, and the 50-horsepower difference in their outputs comes from a host of detail differences. Their power and torque curves are similar but not overlapping. The GT makes its power at 6500 rpm, while the GT500 peaks at 6250 rpm.
Both engines are 5.4 liters in displacement. The GT engine uses a lighter, stronger, and more expensive aluminum cylinder block, while the GT500 uses a cast nodular iron cylinder block. Both use the same cylinder heads and crankshafts, but beyond that, according to Ford engine engineer Curt Hill, different part numbers abound. Both engines use forged-steel connecting rods and forged-aluminum pistons that are similar, but not the same, with 8.4:1 static compression ratios.
One major difference lies in the lubrication systems. The GT, one of the lowest cars ever offered to the American street-racing public, uses a very shallow oil pan and a high-pressure, dry-sump lubrication system with two scavenge stages and one pressure stage. There is very little internal windage in the GT engine because of this setup, but, Hill says, there is an offset because the external dry-sump pump takes some horsepower to run. The GT500 engine uses a conventional deep-wet-sump oil pump and an internal mechanical oil pump with a windage tray inside the pan of each engine to keep oil from slowing the crankshaft as it rotates.
On the intake side of the engine equation, there are big differences. The Ford GT V-8 uses twin thirty-two-pound-per-hour injectors mounted side-by-side in the intake ports, while the Shelby GT500 engine uses a single forty-seven-pound-per-hour injector in each intake port. Upstream of the injectors, the engines breathe through different throttle bodies for architectural reasons. The GT uses a mechanical throttle linkage to operate twin seventy-millimeter throttles; the GT500 uses drive-by-wire technology and twin sixty-five-millimeter throttles. Both engines use intake-charge intercoolers mounted down in the valley of the engine between the cylinder heads.
Camshaft profiles are about the same from engine to engine, with slightly heavier valve lift on the GT—11.2 millimeters on the intake side and 11.5 millimeters on the exhaust side. On the Shelby GT500, its intake lift is 10.01 millimeters and exhaust 10.09. Both use 1.88:1 rocker-arm ratios.
Both engines use coil-on-plug computer-controlled ignition systems and front-inlet water pumps that can pump between 75 and 100 gallons of coolant per minute. While the radiators are completely different because of the engine packaging, both have water-to-air intercoolers.
Another major difference between the two is their superchargers. The Ford GT V-8 uses a very expensive Lysholm screw-type supercharger designed in Sweden, a design that is a positive-displacement pump with very little loss in its pumping power. The GT500 engine uses a more conventional Eaton Roots-type supercharger with counter-rotating impellers that don’t operate as efficiently as the Lysholm design. Hill says the Lysholm design delivers a more linear output relative to engine speed than the Roots blower with very little boost drop-off at high rpm. The GT500 operates at boost levels as high as nine pounds per square inch at rated peak power while the GT engine operates at closer to 13.5 pounds per square inch at rated peak power.
On the exhaust side, the mid-engined Ford GT uses a much shorter overall path from its cast-iron headers to the tips of the tailpipes because the engine is behind the driver. The pipes are three inches in diameter and the mufflers are very free-flowing, shall we say (others may say “loud”). The exhaust system on the Mustang is obviously much longer from manifold outlet to tailpipe tips and uses smaller, 2.5-inch tailpipes. Hill says, “The engine presence in the cabin of the GT is greater by design.”