UsmclovesXJ

I go for a nice flow master 40 with 2.5 in pipe

UsmclovesXJ

Maybe this will help... Off naxja.org
Basic Modification tips for the XJ 4.0 I6
Gleaned from a post on the 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
(note pulled that post offanother thread had a lot of infor in it exhuast wise)

soxfan1534

i have a 93 cherokee with a flomaster 40 series muffler. i want to put headers and i was wondering whats the best brand to get that i can pick up cheap (around $100) that gives it a good loud sound

UsmclovesXJ

all of the headers ive seen online have been 225 or more..i think you can get a nice banks header or borla used for in your price range id sudgest putting up an ad in the watned section.. localy ive had trouble finding a cheap header to upgrade mine..i sudgest banks or borla thats my prefrence but maybe not others.

Nperryman

My muffler is practically falling apart. Will just a muffler upgrade give better performance/MPG? Or do I need a new tailpipe too? Hat do you recommend? I don't want anything too loud, maybe just a little deeper than stock.

UsmclovesXJ

if you dont want anything to loud id get the flowmaster 40 delta flow. and a 2.5in tailpipe..or just a tad bit louder but not to anoyying to me is the flowmaster 44 nice deeep tone at idel and a deep hum at highway speeds.

jeep bandit

What is louder. The 40 or 44?

UsmclovesXJ

the super 44

onlyinajeep726

I've got a Cherry Bomb on my XJ.

UsmclovesXJ

very short glasspack

onlyinajeep726

Yes, 8" body to be exact. It doesn't do much at all in the way of muffling, which is what I wanted. It still sounds good and hums like a V8. I wake up the neighborhood when I leave for work in the morning and put stupid sounding ricers to shame.

I also have a modified intake... I think I've gained around 20~25 HP.

UsmclovesXJ

ohh my nice intake

onlyinajeep726

I made a custom splash guard to go along with it to keep water/mud from hitting the filter. Soon, I plan to enclose the intake entirely and running an extension out of the cowl vents like someone did on here.

UsmclovesXJ

im working on mine and going to run into the cowl.

onlyinajeep726

When you do, make lots of pictures... lol.