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

No better way to quadruple a two-valve Jeep engine's horsepower like 20 pounds of boost and water-meth injection.

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.

World’s most powerful Jeep 4.0, YouTube | The Horsepower Monster

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.

After some quick calibration on a dyno, the engine produced 861 horsepower and 881 pound-feet of torque, soaring past the 4.0’s record by 169 horse and 104 torque. And it did so stopping 700 rpm short of an earlier shakedown run, and below max boost, suggesting more power is still possible in the future.

The Horsepower Monster via YouTube

But if you hope to show up a Dodge Challenger SRT Super Stock in a YJ Wrangler, you’re out of luck for the time being, because Newcomer only built this engine for the sake of R&D. That means, however, that the knowledge gained from it will inform future fast Jeeps, and that a 10-second YJ may not be farfetched after all.

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