It's faster than a Porsche Cayenne Turbo. It trounces a BMW X5. It's even quicker to 60 mph than a new Mustang GT. Jeep's raucous Grand Cherokee SRT8 cooks, thanks to its big 6.1-liter Hemi V-8. Pumping out 420 horsepower and 420 pound-feet of torque, the SUV family hauler delivers a throaty roar as it covers the quarter mile in 13.5 seconds. That's musclecar territory.

However, Jeep's Grand Cherokee SRT8 still doesn't match the thrust of the Dodge Viper SRT10 Coupe, and that's why Hennessey Performance created its new SRT600 package. Long known for its Viper tuning, Hennessey now offers a performance package that allows a Grand Cherokee SRT8 to outaccelerate the ferocious Dodge V-10 Viper in zero to 60 and the quarter mile. In fact, the Hennessey's SRT600-fortified Jeep outpaces a Viper to 60 mph by half a second and crosses the quarter-mile finish line two-tenths sooner than a much lower and lighter Viper.

Where does the Jeep's added Saturn V-like thrust come from? Rather than a solid rocket booster, a big Garrett turbocharger provides boost to a reworked 6.1-liter V-8. Hennessey starts by removing the stock SRT8 V-8 and tearing it down to the cast-iron cylinder block. Then, heavy-duty bearings are added before the installation of aftermarket forged-steel connecting rods, forged-aluminum 9.5:1 pistons, and high-performance piston rings. The cylinder heads also receive a total rework that includes full porting on the intake and exhaust runners along with polished combustion chambers. On an airflow test bench, the ported heads flow 360 cfm on each intake port, about 60 cfm more than stock.

To further increase airflow to the cylinders, a single Garrett T4 turbocharger delivers seven psi of boost. Custom-made stainless-steel headers deliver exhaust to the turbo, then boost is fed through 3.5-inch polished stainless-steel tubing, a big front-mount air-to-air aluminum intercooler, and custom inlet tubing replete with twin billet-aluminum blow-off valves. The single 44mm wastegate keeps the turbo's boost in check. A 180-degree thermostat helps the Hemi keep its cool.

A high-volume fuel pump delivers a steady diet of petrol, as do oversized fuel injectors and a special fuel-pressure regulator monitored by a custom-programed engine computer. The result is a 6.1-liter V-8 that delivers 600 horsepower (180 horses over stock) at 5200 rpm backed by 650 pound-feet of torque at 4000 revs. Such monstrous torque requires a lot of engine airflow that eventually needs to leave, so Hennessey also fits a dual three-inch-diameter stainless-steel exhaust system, high-flow mufflers, and polished four-inch diameter exhaust tips. At idle the SRT600 is docile, but flat-foot the throttle and the force-fed Hemi roar thunders out the exhaust.