Specialty File
We can testify that John Hennessey's highly modified cars are never hurting for horsepower, as we've been the ballerina behind the wheel of his 800-hp twin-turbo Viper and Ram SRT10, trying hard to coax the best 0-to-60 times without the rear tires going up in smoke. With that kind of juice, the difference between hero and stooge is infinitesimal throttle control.
The problem is traction. And that's why we were so eager to test the Houston, Texas, company's 620-hp turbocharged Jeep Grand Cherokee SRT600 and somehow keep its all-wheel drive intact.
To the SRT8's already formidable 420-hp V-8, Hennessey Performance Engineering adds an even 200 as part of the $27,900 SRT600 package, which is available for any 6.1- or 5.7-liter Hemi (5.7s make 550 horsepower) and comes with a two-year engine warranty. The extra power comes courtesy of a Garrett turbo — mounted underneath the transmission — pushing six pounds of boost, an air-to-air intercooler, and an upgraded fuel-pump-and-pressure regulator. Rods and pistons are replaced, the engine is bored out slightly, adding 59cc, and the compression ratio is reduced from 10.3:1 to 9.5. Flow through the ported-and-polished cylinder heads increases by a claimed 20 percent. The factory engine computer is reprogrammed to up the fuel delivery accordingly and allow the throttle to open all the way. Stock SRT8s electronically limit the throttle to 75 percent in first gear, and Hennessey claims the throttle mod alone, available by itself for $895, is worth 0.1 second in acceleration times.
Most tuners can't resist squandering thousands on wheels, body kits, and interior upgrades, but everything else on this Jeep is left stock. Then again, not much else needed changing. It already has supportive seats and direct steering, and it doesn't understeer like most SUVs. This one pulled a sports-car-like 0.90 g on the skidpad, although we were skeptical of the stock drivetrain's survival with the added power. Hennessey wasn't worried, citing more than 200 trouble-free standing-start launches during the development process.
The world's quickest SUV feels relatively tame as long as you only step into the first two or so millimeters of throttle. More than that, and the terrific rumble at idle turns into a hissing, whistling whoosh as a wave of torque awakens. Stab the throttle, and the Jeep rears up so dramatically that it seems as though the front wheels might lift off the ground. They don't, but the steering does get light enough to make a driver nervous of the beast's trajectory.
Even with all-wheel drive and an automatic transmission, launching this brute isn't a no-brainer: It takes just the right amount of brake torque and throttle-and-brake tap dance. When you get it right, there's some rear wheelspin before it hooks up, then it's a face-distorting explosion to 60 mph in 3.7 seconds, followed by an astounding 12.2-second quarter-mile time. Gratifying, concussive blasts accompany each upshift, as in other SRT8s.
The SRT600 is 0.8 second quicker to 60 mph and a second quicker through the quarter than the 420-hp Jeep. Even more impressive is that it's 8.4 seconds quicker to 130 mph.
But let's talk quarter-mile times: The stock Jeep SRT8 is already the quickest SUV on the market, so it's no wonder Hennessey's box of fury destroys the Mercedes ML63 AMG and Porsche Cayenne Turbo S by at least a second. In fact, there's not enough space here to list all the high-performing cars it humiliates, but here's a glimpse: It's quicker than every BMW M product as well as all Mercedes AMGs other than the 604-hp SL65. Bye-bye, Audi S6, Chevy Corvette, Porsche 911 S, and Lamborghini Gallardo. The 4831-pound Jeep is just a half-second slower than the two paragons of production-car performance, the 911 Turbo and the Corvette Z06.
The bad news is that after we recorded our test numbers, Hennessey's driver went out and did 10 successive launches in an attempt to get slightly better times. He didn't, but that abuse fried the stock torque converter, even though Hennessey assures us the $1200 upgrade he now offers would survive.
Still, all this performance can be had for less than 70 grand. No wonder DaimlerChrysler's SRT8 products make up 30 percent of Hennessey's business, even though he has only sold a dozen fire-breathing SRT600s. If you want one, plan on six weeks of thumb twiddling in such distinguished company as country star Tim McGraw, who was brave enough to order a rear-drive Charger SRT600. Careful, McGraw, and better hope Faith likes smoky burnouts.
Hennessey Performance Engineering, 11513 North Petropark Drive, Houston, Texas 77041; 713-466-3420; www.hennesseyperformance.com.
