CJ5 Engine Swap Choices

Article by Mark Trotta

Early CJ Jeeps (1954 to 1969) have a short wheelbase and weigh about 2,200 pounds. The best choice for an engine swap is not a V8. Aside from requiring too many modifications, it'll have more power than the rest of the truck can handle. Combined with the fact that there isn't a lot of room under the hood, a V6 or even a modern four-cylinder are clearly better choices.

engine swap choices for CJ5 Jeep

Drivetrain Concerns

The stock Jeep drivetrain is rated at 1/4 ton. A V8 would make more power than the rest of the truck can handle. Replacing the rest of the drivetrain (transmission, axles, etc) doesn't make sense, because the frame can't handle the power. This is particularly true of the early flat-fendered models. You'd end up with a top-heavy vehicle that just spins tires.

Off-Roading Concerns

If you're planning on taking your old Jeep off-roading, be it mud or dirt or gravel, remember that speed isn't a factor. The added weight of a V8 on the front end will make for slower maneuvering, plus you're more likely to sink into mud (have a winch ready).

CJ5 engine swap choices

V6 Engine Swap

Any V6 will give you all the power that a short wheelbase Jeep can handle. Consider the size of the engine compartment. A V6 configuration (versus a straight six engine) makes for a relatively compact motor, which fits easily into a tight engine compartment. The two most common V6 swaps for an early Jeep are the 225 Buick and 4.3 Chevy.

Buick V-6 Engine

Introduced in 1966 in Jeep CJ and C101 models, the Buick-designed "Dauntless V-6" produces 155 horsepower at 4000 rpm. That's nearly double the horsepower of the original four-cylinder engine. Since this motor was offered optionally in CJ models from 1966-1971, it's a fairly easy install.

Displacement of the Buick motor is 225 cubic-inches. The block and cylinder heads are cast iron, and the motor is externally balanced. The valve lifters are mechanical, so they'll need adjustment from time to time.

The Buick engine's firing order, 1-6-5-4-3-2, is known as the "odd-fire" pattern. The heavier, thicker flywheel must be used in Jeep vehicles, which increases torque and helps dampen vibrations of the odd-fire design.

CJ5 engine swap

The original three-speed manual transmission used behind this engine is not known for it's strength. However, the standard Buick bell-housing will bolt up to a SM420, making it an easy transmission swap. (The SM420 was used in Chevy and GM trucks from 1947 thru 1967.)

Chevy V-6 Engine

Another early Jeep engine choice is the 4.3 litre Chevy V6. Introduced in 1985, this motor is basically a small-block Chevy V8 with the back two cylinders cut off, so parts availabilty is no problem. It's compact size fits well in vintage Jeeps.

A popular engine/transmission combo is a Chevy 4.3 with a GM TH350 automatic.

AMC 2.5 Four Cylinder

This was the motor that was in our old 1989 Wrangler. Truthfully, I always thought it was under-powered on the highway, but for off-roading it was fine. It accepts GM small V6 and four-cylinder bolt pattern (found on many GM transverse-mounted engines) bell housings. A popular engine/trans combo is AMC 2.5L motor and Borg-Warner T-5 transmission.

A four-cylinder engine will give you the best mileage you can get from a CJ Jeep. If you do a lot of highway driving and want better acceleration, the 4.0 six would be the better choice.

Longer Frame

Soon after AMC bought out Jeep in 1970, a new frame with six cross-members and lengthened by 3" to accommodate American Motor's six-cylinder engine. Fenders and hood were also lengthened.

4.0L Straight Six

If the AMC 2.5L had two more cylinders, it would be the 4.0 motor. These motors were installed in Jeeps from 1987–1995, making them plentiful and cheap.

Four Cylinder Iron Duke Engine

Produced by the Pontiac Motor Division from 1977 to 1993, the 151 cubic-inch "Iron Duke" was the predecessor of the AMC 2.5 engine. In the eighties, GM began selling the engine to AMC for use in Spirit, Concord and Eagle models, and also in base-model CJ's. The engines in AMC vehicles continued to use the Chevrolet V8 bell housing pattern.

Several significant changes were made to the 2.5L in 1987, including an improved cylinder head and serpentine belt with an automatic spring-loaded tensioner. Through the years, power output went from 85 horsepower up to 110 horsepower.

Installing The Motor Is Only The Beginning

Installing different engine mounts and bolting up the motor is only the first step in the engine swap process. After that starts a multiple series of "mini projects", including hooking up the transmission, cooling and exhaust systems, and wiring it all up. At this point, you may consider upgrading the brakes and suspension.

CJ5 engine swap choices

Read: Restore An Old Jeep