Home / News / Tuner Pushes the Legendary Jeep 4.0-Liter Straight Six All the Way to 861 HP

Tuner Pushes the Legendary Jeep 4.0-Liter Straight Six All the Way to 861 HP

Owing to its simplistic cam-in-block design, Jeep’s 4.0-liter straight-six is known for two things: Unflappable reliability and a lack of power. Its maximum of 190 horsepower and 235 pound-feet of torque were enough to get every Jeep from the Wagoneer to the Wrangler TJ where they needed to go, just not in much of a hurry. But as tuners have since discovered, enough boost and forged internals can turn any under-stressed engine into a performance powerhouse, Jeep’s lazy 4.0 included, with one tuner recently wringing more power out of one than is produced by a Grand Cherokee Trackhawk, and setting a Jeep power record in the process.

The record-breaking build comes from Concord, North Carolina-based Newcomer Racing, which decided to vanquish the 4.0’s longstanding power record of 692 horsepower and 777 pound-feet of torque, or blow a no-expense-spared motor trying. Said motor was assembled from an all-star cast of aftermarket parts and modified AMC components, which started with the block from a WJ Grand Cherokee, chosen for its thicker cylinder walls and extra reinforcing ribs.

To account for a boring-out to four inches—beyond the 4.0’s generally accepted safe limit—Newcomer partially filled the block, leaving a small passage for coolant circulation. Lengthening the stroke is a pre-1980 AMC 258 (or 4.2-liter) crankshaft, which while almost twice as heavy as later iterations, is fully counterweighted and, more importantly for a big-boost build, stronger. In combination with the bore, it brings the 4.0 up to a mighty 4.8 liters.

Forged I-beam rods connect the crank to custom pistons, dished to reduce the compression ratio. Aiding in this is a cast-iron 1991 cylinder head, whose combustion chambers were enlarged some 20 percent, and whose 12 valves swelled about a tenth of an inch apiece. Due to a lack of favorable aftermarket options, these are actuated by a modestly ground cam.

A Holley ECU doses fuel through port injection to match the intake charge, which an On3 Performance 7675 turbo and an air-to-water intercooler can drive as high as 20 psi. Along the way, a water-methanol injection system kicks in, further cooling the intake charge and preventing knock.


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