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The Novak Guide to the
AMC / Jeep I6 Engines

The 4.0L I6
The AMC I6 (Mopar 4.0L Version Shown)

The AMC I6 is probably the most recognizable and longest-running Jeep powerplant in the brand's history. It features smoothness, good torque and basic longevity inherent in I6 designs.

History
The AMC straight-six began life at AMC as the Typhoon, a 232 CID engine, in the 1964 Rambler American. It was installed into the FSJ Jeeps for the first time in 1971, one year after AMC's purchase of Jeep from Kaiser. It replaced the competing Buick 225 engine, (whose tooling and rights AMC would soon sell back to Buick). The installation of the straight six actually forced a serious design change in the the Jeep CJ5, as AMC designers had to lengthen the 1972 CJ's front clip by 3" to allow for its fitment into the engine bay. This engine was rated at 100 hp and 185 ft. lbs. of torque, not an apparent upgrade over the Buick V6, but since this was the year that the industry standard changed from gross power reporting to SAE net, the actual disparity was much smaller.

In 1975, the 232's displacement was increased to 258 CID, and they were built and sold side-by-side with the 232 being the base engine and the 258 being the upgrade. The 258's power numbers were 150 hp and 240 ft. lbs. of torque. In 1976, the 258 became the base option and the 232 was discontinued.

The 232 was not the first I6 installed into a Jeep. The Willys 226 "Super Hurricane" was installed in the Utility Trucks / Wagons and FC trucks through the 50's and 60's. Later, the 230 CID OHC Tornado was installed in the M715 trucks in 1966. That motor sort of didn't work out so well.

The I6 trudged through the 1980's with carburetion and increasingly complex (read: problematic and power-robbing) emissions controls. AMC, relying on their partnership with Renault of France, sent the engine to Renix (Renault-Bendix) for major re-engineering in the head and induction system, and minor changes within the engine's lower end. In 1987, the I6 was re released as a 4.0L (242 CID), port injection motor in the Jeep XJ / MJ Cherokee, Wagoneer / Commanche.

The 4.0L was produced alongside the carbureted 258, the latter being installed into the YJ Wrangler from 1987 to 1990.

It has been popular to install GM components onto the AMC I6 engine, including ignition, computers, TBI injection, etc.

Our response to these callers requests are to go ahead and use these GM parts, and to not forget the matching heads, block and so forth. Hey... we're an engine conversion company.

After Chrysler's purchase of the beleaguered AMC, and the dissolution of the AMC/Renault partnership, Mopar took about to upgrade the 4.0L to their own fuel injection and control system. In 1991 the 4.0L was released with Mopar MPI and a slightly improved head design, and dubbed the 4.0L HO PowerTech I6. The power rating was between 180-185 HP and 220 ft. lbs. of torque.

Horsepower was drawn back some in 2002 to meet tightening emissions standards.

Flywheels and Bellhousings
The I6 is internally neutrally balanced. It uses a 159 tooth flywheel / flexplate and a 10.5" to 10.9" clutch disc. Bellhousings (and automatic transmission flanges) interchange nicely between the AMC I6 and its 304, 360 & 401 counterparts. It should be noted that the earlier version of the 232 - up through 1971 - had a different bolt pattern. The block pattern was changed in 1972 to standardize this interface between the I6 and V8 engines.

4.0L Crank Position Sensor
A crank position sensor installed into a CJ (258) style bellhousing. Contact us for details.

Flywheels and flexplates do not interchange between the I6 and the 304, 360 & 401 engines due to balance issues.

1987-present 4.0L engines require crank position sensing (CPS or CKP), so any installation using an earlier 258 bellhousing requires a modification to accept this sensor. Novak assists many customers with these bellhousings. Note that 4.0L flywheels have a tooth pattern machined into them for the above sensor's operation.

AMC I6 (and V8) engines do not share block bolt patterns with any other engine, including all Willys, Kaiser and Chrysler engines.

Transmission Compatibility
The I6 is natively and otherwise compatible with several great transmissions. Some popular choices in Jeeps include:

Manuals, non-native upgrades

Adapting the Dana 300 to the SM-420 transmission Adapting the Dana 300 to the SM-465 transmission Adapting the Dana 300 to the T-18 transmission Adapting the Dana 300 to the NP-435 transmission
SM420 (*) SM465 (*) Ford T18 (**) Ford NP435 (**)

* Adapting the AMC I6 to GM SM420 or SM465 Transmissions
** Adapting the AMC I6 to Ford transmissions


 

Manuals, Jeep native

Jeep T14 Transmission T15 transmission T150 transmission The Jeep T176 Transmission The Jeep SR4 Transmission
T14 T15 T150 T176 SR4
The Jeep T4 or T5 Transmission BA10/5 The Jeep AX15 Transmission Jeep NV3550 Transmission NSG370
T4 / T5 BA10/5 AX15 NV3550 NSG370

 

Automatics, non-native upgrades

The TH-350 Transmission The TH-400 Transmission The 700R-4 Transmission
TH350 TH400 TH700R4 / Early 4L60-E

Adapting GM HydraMatics to the AMC I6 or the 4.0L I6

Conclusion
In 2006, DaimlerChrysler pulled the plug on the I6, ending its 36 year run in Jeeps. With the pinch on the automotive companies to tighten their fuel and emissions belts, DaimlerChrysler felt the need to put its efforts into a fundamentally different design; the 3.7L V6 for the 2007 model year.

Sources
- The Novak archives and installation projects
- Novak customers and their project input
- Allpar.com
- Jeeptech.com
- DaimlerChrysler
- D. DiChristopher

We welcome any contributions or clarifications to this article. Contact us here.