AMC I-6
The most common I-6 in a CJ is the 258-ci (4.2L), which is a good engine with excellent low-end torque. The biggest drawback to this engine is the Computerized Engine Control (CEC) system found on the 1980s-era 258. The performance of these systems was barely adequate when the Jeep was brand new; the last CJ rolled off the assembly line more than 30 years ago so it’s safe to assume performance will be considerably worse.
Clifford Performance has a long-standing excellent reputation for combining proven components to improve the Jeep’s I-6; this shop is a good starting point for ideas and solutions.
The 1980s CJ I-6 brought with it the dreaded CEC system. Seemingly miles of vacuum lines and incomprehensible carburetor system made this engine a poor performer even at its best. Swapping the CEC for a better system can free up much space.
Stroker Engines and Kits
Perhaps the ultimate in AMC I-6 performance comes from a stroked 4.2L. Stroked engines use larger bore pistons and a longer stroke crankshaft to increase the engine displacement up to 4.7L. Stroker I-6 engines can make up to 300 hp and 350 ft-lbs of torque. Hesco, Clegg, and Golen produce crate stroker engines and stroker kits that provide significant power and efficiency gains. The options available from these companies range from direct drop-in replacement crate stroker engines to turnkey multi-port electronic fuel injection (EFI) engines to full-on supercharged engines.
I-6 Induction System
Replacing the whole CEC is the best start for a 258, and best results come from either installing an aftermarket intake with a performance carburetor/throttle body fuel injection or installation of an electronic fuel-injection conversion from a 4.0L HO engine.
A few aftermarket performance parts companies, such as Offenhauser, make aluminum intake manifolds for the AMC I-6, and perhaps most notably, Clifford Performance with its extensive line of AMC I-6 performance products. These manifolds allow installation of performance carburetors or throttle body fuel-injection systems.
Typically, the manifolds are designed to use 4-barrel carburetors but can be adapted for other carburetor types. Unless the I-6 is capable of handling a large 4-barrel, that is, the engine was stroked and/or is running performance heads and a performance cam, a small 4-barrel carburetor should be the limit.
This CJ-7 runs a perfect example of a performance I-6 that is well paired with the Jeep’s transmission and drivetrain. A CEC-less, rebuilt 258 running a Clifford intake, Howell TBI, header, and TFI ignition lead the swapped T176 transmission and TeraLow-equipped Dana 300 transfer case.
The Howell TBI system is specially tuned for the I-6 in the CJ and provides computer-controlled efficiency to make the engine run at its best. TBI systems provide for quick starts and smooth trail running, especially in situations that flood most carburetors. The Howell kit includes all sensors, fuel pump, and components to easily swap out a troublesome carbureted system.
For a more stock engine, many Jeep owners have used the Weber 38/38 2-barrel carburetor with excellent performance results. The more adventurous Jeep owners convert the I-6 to a Howell EFI throttle body injection (TBI) system. This complete system brings the CJ a bit more modern. The TBI system performance is excellent off-road and improves starting in both hot and cold weather.
Mopar makes an EFI system for the 258 that is essentially a fully contained induction conversion for the Jeep I-6. This system replaces the entire induction system with a modern fuel-injection system nearly identical to the one found on the 4.0L HO engine that Wranglers, Cherokees, and Grand Cherokees ran until 2006. This system requires a fair amount of wiring and sensor installation but is installable by most DIY-type Jeep owners.
4.0L Head Conversion
The 4.0L engine used in Jeeps from 1987 until 2006 used an improved head design that is compatible with the 4.2L engine, with a minor modification to seal off the small, triangular cooling passages on the intake/exhaust side of the head with epoxy. A common practice is to lightly stuff the ports with packing peanuts (which dissolve), seal with epoxy, and smooth the epoxy when dry. With this swap, use of a 4.0L exhaust manifold or header is required; the stock intake may be retained if desired.
This swap, when combined with a performance carburetor, TBI, or MPI setup from a 4.0L, wakes up a tired 4.2L and increases engine efficiency and off-road performance. The longer stroke of the 4.2L combined with the head improvements can yield an I-6 with more than 200 hp.
I-6 Camshafts
A few high-performance camshafts can be found for the I-6 from companies such as Clifford, Comp Cams, and Crower. Cams that improve the low-end torque (in the 208 duration and .448 lift range) are the best choice for a Jeep that sees the trail and street. Clifford performance makes a complete cam kit for the Jeep 258 that includes all the components such as springs, pushrods, and lifters. This kit increases the I-6 torque curve to produce a flat torque curve off idle to 4,500 rpm, especially useful off-road at slow speeds.
I-6 Exhaust
Headers versus manifolds is a debate that will last as long as engines still have them. The I-6 was known to crack factory exhaust manifolds, but on the flip side headers are known to blow gaskets and leak exhaust. Proper headers increase flow, resulting in better horsepower and torque. Clifford, Hedman, PaceSetter, and Borla make quality headers for the I-6 in various constructions and configurations.
Stainless and ceramic coatings provide long life and good looks. Some I-6 headers use a 3-into-1 tube design to provide a dual exhaust; the traditional 6-into-1 remains most common. Performance differences between the two can be negligible and insignificant to the dual-exhaust routing headaches that occur.
A good muffler can also help with performance and make the Jeep sound a little better. A loud exhaust gets old very quickly on a trail Jeep; consider a muffler that sounds good but is quiet enough to not annoy. Flowmaster mufflers combine good flow with good sound and solid construction that can handle the punishment of off-road use.
A Clifford header provides smoother exhaust flow from the I-6 with some added tone to give the Jeep’s sound some personality. Headers can increase torque and power 30 percent, depending on configuration and engine specs. Dual- and single-outlet headers are available for the I-6.
I-6 Ignition
The Jeep I-6 used a points system until 1976 and the Motorcraft electronic ignition system until 1986. The Motorcraft system worked well but was prone to sudden failure without warning. Many Jeep owners made it a regular habit to carry a spare ignition module in case of a failure.
The high-energy ignition (HEI) system is considered the most reliable classic ignition system and only requires a single wire for power. A GM HEI-style distributor can be adapted to work in the I-6 with only a gear modification. These conversion distributors can be found ready to go and are easy to install. Davis Unified Ignition makes a top-of-the-line HEI-style distributor for the Jeep I-6 that provides the ultimate in ignition performance.
An alternative to a complete HEI swap is adapting a Ford TFI ignition to a Duraspark distributor.