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Horsepower - Hemi Jeep Grand Cherokee

Marco Martinez 6.1L Hemi, 518HP Cherokee

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Marcos Martinez / Downey, CA
While it may not sound like much these days, know that this engine’s 518 hp was calculated at the rear wheels of Marcos’ Hemi Jeep Grand Cherokee. Accounting for drivetrain losses, this Hemi is making close to 650 hp at the crank. Even tastier is the torque curve, which peaks at 609 lb-ft at the wheels but is broad and flat on either side of that. There are a few ways to get power numbers like this out of a stock Hemi, but Marcos chose to go turbo and sent his Jeep to Greg Monroe, owner of Racers Edge Tuning in Downey, California. Greg built a custom turbo kit that added a conservative 160 hp and 263 lb-ft to the engine’s power numbers. Uncertain about the limits of the Hemi’s internal components, Greg limited the boost to a pump-gas-friendly 7 psi. Greg figures horsepower numbers could be close to 800 on a fully forged bottom end. Another noteworthy feature about this engine is that it is one of just a handful of turbocharged SRT8 street engines in the country. Marcos hadn’t even driven his car home yet when he said he wanted more power. Maybe there will be a new rotating assembly in his near future.

A. Turbocharger
Greg selected one of Precision Turbo & Engine’s 76mm turbochargers with a billet compressor wheel. A TiAL wastegate limits boost to 7psi so Marcos doesn’t blow the engine up on his way home. On the induction side, Greg chose a TiAL vent-to-atmosphere blow-off valve to prevent surging in the intake when the throttle is closed abruptly. This particular valve is one of TiAL’s “noisy” valves, meaning it chuffs like a squirrel when you let off the gas. “It appeals to the ricer in all of us,” Greg quips.

B. Fuel
Greg swapped the stock fuel injectors on his Hemi Jeep Grand Cherokee with a set of 80 lb/hr ones. Though Greg installed a Kenne Bell Boost-A-Pump, he hadn’t wired it in yet. So far, the stock fuel pump is keeping up with the added demand.

C. Tuning
Greg tuned the Jeep’s stock ECM with DiabloSport software. A cool feature of the Hemi Jeep Grand Cherokee is that the mechanism that drives the front wheels can be disabled, allowing the car to run in dyno mode, sending all the power to the rear wheels.

D. Long-Block
This 6.1L Hemi is totally showroom stock and hasn’t even had the valve covers removed. With a 4.055-inch bore and 3.58-inch stroke, and a compression ratio of 10.3:1, these engines made 425 hp and 420 lb-ft in SRT8 vehicles. They came with forged crankshafts but hypereutectic pistons and powdered-metal connecting rods. On Greg’s DTS chassis dyno, Marcos’ Jeep rolled baseline numbers of 362 hp and 346 lb-ft.

Hemi Jeep Grand Cherokee: Plumbing
Greg plumbed all the tubing by eyeballing the routing and cutting lengths of 304 stainless mandrel-bent tubing as needed to route exhaust from the stock manifolds to the turbocharger mounted behind the passenger-side headlight. The downpipe connects to the stock cat-back exhaust system. On the inlet side, compressed fresh air runs through an air-to-air intercooler mounted behind the bumper cover, then up into the stock throttle-body. All the joints are made with V-band clamps for easy removal and leak-free fitments. “The plumbing wasn’t too difficult. I didn’t have to reroute anything,” Greg says.

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