We began with the GM 572/620 HP in its out-of-the-crate configuration. To that we added 2-
We hot rodders love numbers. Whether it's e.t., miles per hour, torque, or horsepower, in our world of extreme automotive performance, everything is measured or interpreted with sexy figures (uh, the mathematical kind). As gearheads know, there's no substitute for big cubic inches, so when Chevy recently unleashed its largest and most powerful street engine, the ZZ572/620, the hot rodding world took notice. Based on a new Gen VI tall-deck Bow Tie block, the ZZ572 is fitted with a forged-steel crankshaft, a 4.375-inch stroke, and forged-aluminum pistons with full-floating wristpins. With redesigned aluminum rectangular-port cylinder heads, 2.25-inch intake valves, and a big hydraulic-roller camshaft yielding 0.632-inch lift and 254/264 degrees of duration at 0.050, the giant Rat easily surpasses the factory-rated 620 hp at 5,500 rpm and 650 lb-ft of torque at 4,500 rpm. But even with such a hefty stroke, the ZZ572's long-duration camshaft skews the mammoth power to the upper-rpm range leaving it with a ratty idle quality. If you've been reading CHP for some time, you know our Performance Q&A guy Kevin McClelland loves a tuning challenge. When McClelland and the crew at Flowmaster recently set out to dyno and wrench on a Burt Chevrolet-supplied GM ZZ572/620 to improve the motor's performance manners, we couldn't pass up the chance to help. The performance mission was to refine the ZZ572's power curve to deliver more usable torque with a small selection of existing GM parts, a good street intake manifold, and a healthy Flowmaster 3-inch exhaust system fitted with new Super 40 mufflers.
Our crowd of gearheads included Flowmaster's dyno man John Wilson, Burt Chevrolet GM Performance Parts Manager Ken Casey, and yours truly. The mission was clear: We didn't want to lose any of the ZZ572's incredible performance, but we wanted to increase and raise the torque curve across the rpm band. As performance folks understand, big, wide torque curves produce impressive acceleration.