The legendary Jeep 4.0-liter inline-six has been transformed into a naturally aspirated 5.0-liter powerhouse by Keith Newcomer of Newcomer Racing, challenging the engine’s long-standing reputation as a low-end workhorse rather than a high-revving horsepower machine.
From 4.0 Liters to 5.0 Liters
For decades, the Jeep 4.0-liter inline-six was known as an unkillable cast-iron engine with strong low-end torque. Factory models were celebrated for their ability to pull hard over rocky trails, but they were not known for producing raw, high-revving horsepower or dominating the drag strip.
Newcomer Racing took a different approach by boring and stroking the heavy straight-six block to 5.0 liters. The result is a naturally aspirated engine designed to demonstrate the potential of raw displacement, even alongside modern turbocharged four-cylinder engines with impressive factory output.
Custom Valvetrain and EFI Setup
The engine received extensive internal modifications, including a steel crank, custom JE pistons and a Ford El Rock head. The combination produces a massive 13:1 compression ratio.
Newcomer Racing also addressed the classic straight-six flat-tappet failure issue by engineering a full hydraulic roller valvetrain. The setup is secured beneath a custom billet valve cover equipped with a dual breather system.
Air and fuel delivery required additional fabrication and testing. The team compared a sheet metal carbureted intake with a CNC-ported EFI manifold fitted with a 75mm throttle body.
For the wiring and engine management, the build uses a stripped-down 1996 Cherokee factory wiring harness. The early OBD2 computer was tuned using HP Tuners.
434.1 Horsepower From the Naturally Aspirated 5.0
Dyno tuning eventually required a return to 1.8-ratio rocker arms and the installation of 420-pound double-nested valve springs to eliminate top-end valve float.
The finished naturally aspirated engine produces 434.1 horsepower at 5900 rpm and 424.2 lb-ft of torque at 4700 rpm.
The engine is destined for a dedicated mud-drag Cherokee, where massive wheel speed is required to clear thick sludge from the tire treads.
Why the Cast-Iron Engine Still Matters
The naturally aspirated pushrod engine’s output stands against the standard-output twin-turbo Hurricane inline-six, highlighting what can be achieved through a heavily modified, displacement-focused setup.
The 5.0-liter AMC block also stands apart from the modern focus on touchscreen sizes, hybrid architectures and brand reliability scores. Built around heavy-duty iron and a purpose-driven mechanical design, the engine is headed for a Cherokee built specifically for demanding mud-drag competition.
