View Full Version : Basic Modification tips for the XJ 4.0 I6

Ed A. Stevens
December 21st, 2005, 17:53
Basic Modification tips for the XJ 4.0 I6
Gleaned from a post on the NAXJA Modifications Tech Forum by Ed A Stevens.
Edited by Jeff Veach

Think about what is already good about the 4.0L engine, and then try to improve what needs help.

The engine already has a tube header. Even the early log style tube header flows way better than an iron manifold. The problem is they crack at the #6 cylinder (poor design) so a header replacement is a good choice (but not from a performance standpoint -- only ~3 hp maximum). The HO header is better, than the RENIX header, but not as good as a long tube aftermarket header.

As with any pump, the engine efficiency improves about 1% for every 1 psi drop in the exhaust stream. The stock exhaust from the cat-back restricts about 6 psi (up to 6-10 hp) compared to a 2.5 inch pipe system and a good muffler. The talk about loosing low rpm power is not valid as long as the head pipe from the collector to the cat is still 2.25 (it determines the resonate rpm and flow velocity). A 2.5 inch head pipe only moves the collected resonant rpm from ~900 to ~1400 rpm, so it really does not change much. The best is to get the benefit of the cat back without loosing the resonant flow scavenging at low rpm (your choice).

The intake, head ports and valves flow well. The HO high ports really do not offer an advantage until 4500+ rpm (well beyond most users). Large valves and porting only help if you are reaching past this rpm. It's a wast of money unless you want to move the rpm power range higher. Chamber polishing and piston top polishing helps prevent detonation. Talking to your machinist about 0.045 quench clearance, and decking the block to match with the right head gasket, will help the compression a little without promoting detonation. The 96 & later 4.0L-HO graphite skirt pistons run quieter (and fit all 4.0L's).

The intake of the stock system sucks (from a performance standpoint). It was designed for emissions and noise control and is restrictive. The same style rule of thumb applies, that a 1% increase in power is possible for every 1 inch of water gage reduction in restriction. A K&N style flat filter helps ~2%. An open element cone style air filter will do way more for power than the header (about the same as the cat back exhaust). Save the stock box and system for emissions testing.

The factory RENIX throttle body flows ~325 cfm (the HO ~350 cfm). Both will flow well past 5000 rpm, at the mild volumetric efficiency of a near stock build up. Unless you reach higher in rpm or go big cubes with a stroker the return is not that great for the expense of a large TB (~3%).

The ignition on the 4.0L is very good. The system can be improved with a high altitude CPS to allow advanced timing. The RENIX system has the advantage of the knock sensor so going too far and promoting ping is not an issue. Good plug wires and cap/rotor help, and the stock E-coil is better than most factory systems up to 96. An MSD-5 or 6 will help a tired motor, and areas with poor fuel (rual Mexico), but is not needed for most users.

If you want to know about cams, The Mopar stage 1,2,& 3 cams are very mild. I run an Isky cam listed on Greg's spreadsheet on the strokers list (near identical to a stage 3 Mopar cam). It is locomotive like in building low rpm torque. Greg also runs this same cam in his stroker MJ, and agrees it is a torque monster, but not too powerful past 4500 rpm. The power is still better than stock past 4500 rpm, and it will rum to 6000 or more, but it drops off quickly compared to the punch you feel from idle to 4000. The HO cam (IMO) offers nothing over the regular 4.0L cam in a RENIX engine. Crane, Competition Cams, Isky & others all make 258 cams that will work well with the 4.0L MPI.

The cam problems start when you make choices to improve the rpm past 5500, where the low rpm vacuum suffers. This high rpm range is where the injection system really needs to be matched with large injectors, high flow throttle body work, and port/valve work (and throwing on a 38/38 Weber or 4V carb is much easier to tune). The MPI also cannot resolve poor intake vacuum readings at low rpm (big lumpy idle cams). I don't think this is where your going, but it is worth understanding.

You may also enhance the injection some with a switchable 1000 ohm resister in the temp sensor wiring (to throw it into rich warm up mode) or add an adjustable pot MAP bias bridge (to bias the A/F ratio for more power in both open and closed loop modes -- works good with the other mods, but is not much good on it's own). These MPI modifications are simple but not really needed unless you want every little bit of power (and have a G-Tech or other performance measuring tool to tune the gains).

Happy Trails!

Ed A Stevens