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Tuner Boosts Jeep 4.0L Inline-Six Past 1,000 HP—And It’s Not Done Yet

The engine now makes 900 hp more than it did when it left the factory.
Horsepower Monster via YouTube

Jeep’s 4.0-liter straight-six engine is iconic. Much like the Ford 300, it’s a down-and-dirty truck engine that stayed in production for decades. Also like the Ford 300, it can make a ton of power when you throw some boost at it. Take this one for example, which runs 27 psi and boasts a dyno sheet showing an unfathomable 1,082 horsepower and 1,017 pound-feet of torque.

It’s the work of Newcomer Racing, who has spent years working with this platform. The North Carolina shop previously built a Jeep inline-six making 861 hp thanks to 20 psi of boost, but now, they’ve cooked up something even crazier. What’s more, the previous build ran water-methanol and fuel injection—this one is carbureted and there’s no meth mixed in.

A recent video posted to the Horsepower Monster YouTube channel shows the build in-depth. The stock block has been bored out to 4.6 liters of displacement, though it retains the factory main caps. There’s an aftermarket cast crank that’s fully counterweighted spinning a set of custom JE forged pistons, and that’s pretty much it for the bottom end. Newcomer Racing did a lot of work to the top end, as you might imagine, with heavily ported aluminum Edelbrock heads, larger valves, and full roller-rockers for handling high revs and valve spring pressures. A 650-cfm Holley carburetor manages fueling and is hooked up to an old-school Clifford cast intake that has also been ported.

Newcomer Racing dyno tested this engine with several setups, including a naturally aspirated configuration. It made 394 hp at 6,100 rpm and 375 lb-ft of torque at 4,600 rpm without the turbo, which is mighty impressive considering the straight-six was set up for boost. From there, they threw on a 72-millimeter Precision turbo and made a super-solid 813 hp at 6,200 rpm and 716 lb-ft of torque at 5,800 rpm. Those boys know how to build a power plant.

As the numbers neared those of their previous record-setter, the crew went full-bore and installed a larger 77-millimeter Pulsar turbo. This was the key to success as they managed 989 hp on the first pull with that blower before running lean on 116-octane Sunoco MaxNOS race fuel. A simple jet change was all they needed to achieve their final four-figure power numbers.

The best result of 1,082 hp and 1,017 lb-ft of torque is believed to be a record for Jeep straight-sixes. It may not stand for long, though, as Newcomer Racing plans to swap this one over to fuel injection. It could handily beat those numbers while also being more controllable from a tuning perspective—a win-win.

It’s great to see platforms like this live on long after they stopped production. While the Jeep 4.0-liter enjoyed a lengthy run from 1987 to 2006, that means it hasn’t been manufactured for 18 years at this point. Still, a select few tuners like Newcomer Racing are keeping it fresh and taking it to new heights. I can’t wait to see where it goes next.

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