Hennessey Grand Cherokee SRT600 Austin TX

BY DAVE VANDERWERP

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.

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