and cleaning of the engine.
Summing It Up
An engine that passes the inspection and tests could be considered worthy of modification. A minor rebuild during modification, such as new gaskets and seals, should be considered, as well as replacement of certain components that are typically a hassle to replace when the engine is fully assembled, such as the thermostat, hoses, belts, and water pump.
Rebuilding
If, after inspection, a rebuild is in order, it is best to pull the engine and further inspect crankshaft and connecting rod clearances to determine if machine work is needed. Consult an engine rebuild guide for your particular engine to determine inspection specs.
If the engine checks out, a rebuild can be far less expensive than a new engine and can be performed by a do-it-yourselfer to save some expense. Certain things such as engine cleaning should be performed by a professional to ensure all contaminants are removed from even the hard-to-reach places in an environmentally sound and efficient manner.
In addition, consider a professional valve job while the engine is out and apart, again another overlooked job so easily done at this time. If rebuilding an engine is not in your bag of tricks, a good engine shop should be able to perform the rebuild, including checking proper clearances and machining if needed.
Bent pushrods from a past broken timing chain combined with age, dirt, and grime made this engine run poorly, and after years of neglect a rebuild was needed. Bored .020 over, cleaned, and new bearings made this engine like new again. During assembly, a new performance cam, intake, and a fresh coat of factory color paint made this engine ready to go back into place.
Replacement or Swap
If the existing engine is beyond repair, a decision to replace the factory engine with a same type or swap to another type needs to be made. Typically, replacing the engine with the same type is the least expensive option because the expense of a swap and the costly items such as adapters, radiators, mounts, exhaust, etc., that go with it will not be incurred.
If the choice is replacing the engine with a similar type, consider factors such as engine size; perhaps an AMC 304 would be best replaced with a 360 that bolts right in.
Finally, deciding to install a used replacement engine in good shape is considerably less expensive than a crate engine.
Performance Parts and Modification
Increasing the performance of your Jeep’s engine can be achieved with a variety of additions and modifications. Because the goal is to build a Jeep that performs well on- and off-road, the modifications are not to build a 1,000-hp race engine but rather a good-torque and good-power engine. The beauty of off-roading a Jeep is that horsepower is not always as important as it may be in other automotive circles.
A rebuilt AMC 401 in this Scrambler provides enough horsepower to easily propel this Jeep in the passing lane. The nature of the 401 and thoughtful performance additions keep the engine producing plenty of high horsepower for some fun while maintaining the low-torque “tame” for those slow, controlled trail maneuvers.
Much of the day on the trail is spent moving at slow speeds and excess horsepower usually results in poor drivability, overheating, and broken parts. This fact allows a Jeep owner to invest a reasonable amount of the Jeep funds into performance parts that actually make a difference without going overboard. The saved funds can be put into things that matter more on the trail, such as traction systems, gearing, suspension systems, and everything else.
Here, I generalize on performance parts and focus on common modifications that enhance performance.
Factory Engine Modifications
Because most Jeep owners find themselves with a factory engine, much can be done to both the AMC V-8 and I-6 to enhance their performance. It is fortunate that both of these engines (for the most part) can serve the Jeep well with no modifications, but really, why would you want this? Because the engines are so usable out of the box, this suggests that anything that is done only serves to make the engine better.
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.
