Car Culture & Entertainment

The Top 10 American Performance Engines of the Last 30 Years (#10) AMC/Jeep 4.0L Straight-6

Editor’s note: The 1980s was a transition period for engine technology in America. The iconic carburetor gave way to fuel injection. Cubic inches were out, and liter designations marked a new era for engines — one when power and fuel economy were no longer mutually exclusive. This modern engine age has featured some of the best innovation, technology, and performance yet.

So what have been the best powerplants of these last 30 years? We asked you to help us answer that very question via Facebook and Instagram. Factoring in your votes and comments, we’ll unveil the Top 10 over the next couple of weeks.

Jeep 4.0L straight six


#10 – Jeep 4.0L Straight-Six

You’ll be hard pressed to find a more passionate tribe of engine loyalists than those who worship at the automotive alter of the beloved 4.0L I6 (often referred to as a straight-six).

This little engine that could is best known for reliably powering a variety of Jeep utility vehicles from 1986 through its phase out in 2006. But the famed inline-six lived an entire life before Chrysler stamped its name on it. The modern straight-six engine was born in 1964 after American Motors Co. built it to power a variety of AMC passenger cars and Jeep vehicles, as well as a number of International Harvester light-duty trucks (International Harvester bought the engines from AMC for Scouts, Travelalls, and other pickups.)

The “modern” 4.0L was introduced in 1986 for the 1987 model year and has been “bulletproof.” It’s widely recognized as one of the best engines ever built, often performing dutifully for upward of 300,000 miles. The 4.0L I6 kicks out 190 horsepower from a 242 cubic-inch cast-iron block and cylinder head, hydraulic lifters (with non-adjustable rockers), and seven main bearings.

In 1991 and beyond Jeep applications, the engine was fuel injected which, according to Jeep Tech, helped it perform “much better in off-camber, bouncy, and steep situations. It also calibrates better for changes in altitude. Fuel economy is also improved.”

Engine Specs

Bore x Stroke – 3.88” x 3.41”

Displacement – 242 (4.0L)

Compression Ratio – 8.8:1

Torque – 220 ft.-lbs. at 4,000 rpm

Top Modifications

If you own a 4.0L, there are plenty aftermarket parts to keep it running as strong — or stronger — than ever. According to Summit Racing, here are some of the more popular items for the engine:

Exhaust Systems (21)

Radiators (20)

Air Intake Kits (20)

Air Filter Elements (18)

Oil Filters (17)

Headers (11)

Alternators and Generators (10)

Computer Programmers (7)

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  1. slant six was different, slant six was used in cars like dodge dart cause they slanted for better hood clearance

  2. Zachery Lewton says:

    4.0 jeep six was fuel injected from introduction the 258 six had a carburetor

  3. that’s not why the slant six is slanted it was so mother mopar can get a better rod length mopar has the best rod lengths in the business and they came in b-dody’s as well.

    • John hanson says:

      I’ve had 2 225 slant 6 engines tough as mopar also put the leaning tower of power in Ebodies.

    • that is exactly why it is slanted to one side for the lower hood lines on the dart. see

  4. The first few years were not as tough. Seen many with a hole in the block or a rod knock with less than 160k on them. The later reinforced blocks were better.

  5. My 98 Cherokee with the 4.0 currently has 413k trouble free miles on her and to third VERY day I stomp on the peddle and just destroy these new soccer mom “Jeeps” I’m not sure they’ll ever make another engine as reliable. Bought it twenty years ago almost right outta high school, took 4 years to pay off and while my garage is full of different models and brands today, she’s still my favorite… rust and all.

  6. Anyone can buy a new Jeep, a real man keeps the old ones running.

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