Video: Building The World’s Most Powerful Jeep Inline-6 Engine

When you think of Jeep engines, you probably don’t think of “power” but rather of the reliable workhorse inline-six designed by AMC back in 1952. Specifically, you probably think of the 4.0-liter version which was introduced in 1986 and lasted through the 2006 model year. However, not everyone likes to stay within the design box the OEMs originally established.

Enter Newcomer Racing, based out of Concord, North Carolina, who has an affinity for both Jeep inline-six-cylinder engines and copious amounts of horsepower. First, they did a low-buck stroker kit, which boosted power by over 50-percent. Then, they adapted a Holley Dominator EFI system to a 4.0L Jeep engine and doubled the stock power output on pump gas. However, that wasn’t enough.

This go-around, Keith Newcomer is looking to walk away with the most powerful Jeep engine in the world. The previous record is 692 horsepower and 777 lb-ft of torque by adding a turbocharger and methanol injection. While the 2JZ and RB26 engines might make it seem like making big power out of an inline-six is no big deal, the AMC 4.0-liter is a different animal entirely.

We’re either going to set a new record or blow this engine up. – Keith Newcomer, Newcomer Racing.

The Heart of the Matter

Starting with a “328” block out of a Jeep Grand Cherokee, chosen for its slightly thicker cylinder walls and additional webbing in the block. Newcomer then threw “the book” out the window and put a .120-inch overbore in each cylinder, resulting in a 4.00-inch bore. In order to strengthen the cylinder walls with such a stout overbore, the Newcomer filled the water passages with hard-block but left enough room to allow some water to flow.

After fitting a set of 1/2-inch ARP main studs and King main bearings, Newcomer dropped in a 3.895-inch crankshaft out of an older 4.2-liter Jeep engine to get a total displacement of 293 cubic inches – or 4.8 liters. The pre-1980 crankshaft is fully counterweighted, which is especially important in a long inline-six crankshaft.

The foundation for this build is a 4.0-liter Jeep "328" block out of a Grand Cherokee. While Newcomer went way past what is generally accepted as a safe overbore to get the 4.00-inch bores, the hard-block fill will help strengthen and support the cylinder walls. Additionally, half-inch ARP main studs and a modified OEM main girdle add strength and rigidity, as well.

The extra weight is a minor price to pay for the increased strength in this application. Additionally, a steel main girdle, which is found in the newer engines, is used with the ARP studs along with custom spacers to ensure the extra stroke will clear.

Unfortunately, when it comes to rods, there aren’t a lot of off-the-shelf options for Jeeps. However, Scat Crankshafts does make a set of forged I-beam connecting rods which will work. The “Pro Stock” rods are 6.125 inches long, and Newcomer then modified them with a bushed small end to allow for a floating wrist pin and added ARP 2000 rod bolts.

Here, you can see the difference between a fully counterweighted unit used in the build (far crank) and a more modern non-counterweighted crank. The fully counterweighted crank is significantly heavier, but also significantly stronger in Newcomer’s opinion.

A set of custom forged slugs from JE Pistons hang off the rod. With a 1.355-inch compression height, the pistons feature a 28cc dish to keep compression at 8.6:1. In order to free up some horsepower, the ring pack is .043/.043/2mm. Newcomer modified a stock stamped-steel oil pan to accommodate the extra stroke and reduce windage.

Newcomer turned to Bullet Racing Cams for the camshaft on this build. Surprisingly, he chose a solid flat-tapped camshaft profile for the build. That’s not for any specific performance reason, but rather because there isn’t a good solid roller lifter option out there for the Jeep 4.0 engine. It has 243 degrees of duration at .050 inch on both the intake and exhaust, along with .584-inch of lift at the valve. A billet-steel double-roller timing set keeps the cam and crankshaft in harmony.

Custom JE Pistons with a 28cc dish keep compression of the engine down into the 8.6:1 area in preparation for the boost being crammed into the cylinder. To handle the pressures, a set of SCAT 6.125-inch forged I-beams, modified to accept a floating .866-inch wrist pin, were used.

OEM Heads Rule

While there are aftermarket cylinder heads available for the Jeep 4.0, Newcomer wanted to keep an OEM head, since both the block and crankshaft were OEM pieces. However, that didn’t keep him from working the 1991 high-output factory cylinder head over to within an inch of its life.

Not only were the intake and exhaust ports opened up and smoothed out, but so were the combustion chambers. By laying back the chamber walls and opening up the area around the spark plug, the volume of the chamber is significantly increased to 66cc. The valve seats have been opened up to accept 2.055-inch intake and 1.650-inch exhaust valves with 8mm stems.

While aftermarket aluminum cylinder heads exist, Newcomer wanted to use an OEM head on this record attempt. He put a ton of work into the ports and chamber, opening up the chambers by 8cc to a total of 66cc. You can see the difference between a stock combustion chamber (right, near) and Newcomer's heavily modified chambers (right, far).

A set of COMP Cams 26918 beehive springs were used, which offer harmonics resistance, a 125-pound seat load, and 367 pounds of open pressure. Riding on the cam lobes are a set of Bullet solid flat tappet lifters with an integral oil gallery to keep everything lubricated, while a set of Scorpion 1.6:1 roller rocker arms translate cam motion into valve motion.

A Cometic .049-inch compressed thickness MLS head gasket seals the cylinder head and engine block with the help of ARP head studs. The factory mechanical water pump had to be gutted in order to fit back on the block, thanks to the block filler but makes for a great inlet for the remote electric water pump.

The valvetrain is important in this build. While larger 2.055-inch intake and 1.650-inch exhaust valves play a significant role in airflow, the LS-style beehive valvesprings are critical in eliminating some valvetrain harmonics seen in earlier tests.

Feeding the intake ports is a completely custom log-style intake manifold, called “the Cannon”. It’s outfitted with an eBay 92mm throttle body and six 80lb/hr fuel injectors. The stainless-steel turbo manifold is also a custom-fabbed unit by Newcomer, designed for the On 3 Performance 76mm turbocharger. With a billet compressor wheel and 76mm exhaust exducer diameter, the turbo is rated at over 1,100 horsepower.

The compressed charge will flow through an eBay 3.5-inch air-to-water intercooler before being ingested by the engine. Controlling the combination is the modified Holley Dominator EFI system, with a custom-fitted crank trigger, and one-off camshaft position sensor, and individual smart coils.

This On 3 Performance turbocharger features a billet 76mm compressor wheel and a 75mm exhaust turbine and a 0.96 A/R. Rated at 1,125 horsepower, this should be more than enough turbo for this engine.

Spinning the Pump

After some break-in pulls, the crew at Automotive Specialists — notably tuner Jeff Dorton —was ready to pour on 20psi of boost. While set up for methanol injection, the first round of testing was done with just compressed atmosphere and yielded 750.9 horsepower at 5,500 rpm (where the test was stopped) and 776.7 lb-ft of torque at 4,600 rpm. That pull blew the record away by almost 60 horsepower… and it wasn’t even a full pull.

After connecting the Snow Performance water/methanol injection system with a number six injector and setting the activation threshold at 12 pounds of boost with the injection ramping in to full-injection at 17psi, the team filled the tank with Boost Juice and fired up the dyno. This time they pulled a little bit of timing, went to 21psi and spun the engine to 5,700 rpm.

This time around, power jumped 110 horsepower to a peak of 861.3 horsepower at 5,700rpm, while peak torque jumped 105 lb-ft to 881.7 at 4,800 rpm, utterly destroying both the previous power and torque record. Newcomer feels this combination is far from maxed out and is capable of making more power.

As you can see, even with the low compression and no boost, the engine was a healthy performer. Then, the engine was able to break the previous record on just boost. Once they added in the water/methanol injection, the record went from broken to smashed, with final numbers of 861.3 horsepower at 5,700 rpm and 881.7 lb-ft of torque ay 4,600 rpm.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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