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.
AMC V-8
Like the Jeep I-6, the factory 304 V-8 is a good starting point in a CJ, even in its stock form. The messy emissions controls and equipment of the 1970s stifled the small V-8, but with a little help this engine can provide plenty of low-end torque and some extra horsepower to make the Jeep feel like a muscle car. Swapping an AMC 360 or 401 is a bolt-in job and can provide a larger boost to the Jeep than might be possible with the 304. The AMC V-8 has a fair amount of aftermarket performance parts available, and finding a proven combination is not a difficult task.
The Generation 3 or Tall Deck AMC V-8 was used from 1970 until 1991 and was available in 304-, 360-, 390-, and 401-ci configurations. These engines share most components such as cylinder heads, intake manifolds, camshafts, etc. A 304 or 360 are probably the most common and easiest to find. Both provide a good start to a Jeep engine build.
A few varieties of the high-output electronic ignition systems are available for I-6, including a less common but effective Ford TFI conversion. This system uses a more modern ignition system with a similar function and performance to the GM HEI system. The TFI conversion uses some of the stock components while most importantly replacing the Ford Duraspark module.
Even stock, the AMC 304 has plenty of torque for the Jeep. This restored 1978 Golden Eagle CJ-7 is still powered by the stock engine attached to the factory TH400 and Quadra-Trac. A set of fenderwell headers is about the only performance addition.
V-8 Stroker
Unlike the I-6, the AMC V-8 engine has few stroker options available, built mostly of homegrown combinations using mixed parts and special crankshaft grinds. The cost and compromise may make it not worth it for a Jeep used on the street and trail. Power and torque gains found in the I-6 stroker make the work make more sense. In contrast, the AMC V-8 running in its factory sizes serves the Jeep easily, even with light modifications. I suggest you skip this option; it’s just not worth the effort to stroke an AMC V-8 for a CJ.
V-8 Induction System
The stock AMC V-8 used a Motorcraft 2-barrel carburetor that was surprisingly well suited for both on- and off-road use. These carburetors were well known for reliability and maintenance-free use. A variety of intake manifolds were used on the CJ V-8; design differences were often to accommodate emissions systems such as exhaust gas recirculation (EGR).
A shiny new Edelbrock Performer intake manifold provides the perfect
