Weak Won't Survive
Anyone modifying an '05 GT for more performance should be aware of the engine's limits. After all, it's not Ford's responsibility to provide an engine for racing, so consequently, the three-valve 4.6 was designed for 300 hp, plus a stress safety margin. Steeda says the engine will live with 425 hp as long as timing and fuel is managed to prevent detonation. Anything more than 425 hp and "You're just waiting to buy a new engine," says Dario Orlando.

The weak links are the piston ring lands and powdered rods. For emissions, the '05 has new pistons with ring lands very close to the top of the piston. "One of the ways to control emissions is to reduce the unscavenged area around the piston," Orlando explains. "If you move the ring land up as high as possible, you have less unburnt fuel. So Ford moved the ring land up as high as they could while maintaining minimum strength. But the whole purpose of it is alien to performance because they break easily."

The powdered rods--which have been used in all modular engines except the supercharged four-valve 4.6 in '03-'04 Cobras--also have their limit. "A powdered rod is simply powdered metal compressed at extremely high pressures where it literally bonds together to make a rod," Orlando says. "And on top of that, the rod cap itself is broken off. So you take off one of the caps and it's literally jagged-edged. It's inexpensive to produce, yet it provides more than adequate strength for a stock engine. But under a performance scenario, you're really stress limited."