4.0L Engine Tech

This page will cover modifications to improve the power of the 4.0L I-6 engine. I do not personally have any experience with the 2.5L I-4, though I do know the Rimmer supercharger brings it to an equivalent peak level as the I-6, with extra low end torque.

There are several portions to this page, as we will cover, section by section, the improvements possible for the 4.0L.



Think of the motor as a giant air pump. The more air you pump, the more power you make. There is a more technical side, such as air velocity, compression, etc., though this is the basic premise. The volume of air generally affects the peak horsepower you can achieve, while a high air velocity guarantees good low end torque. If you enlarge the inlet/exhaust ports too much, however, you will end up with high horsepower at the expense of low-end torque, which is only desirable on a drag or race car. We do not have to worry too much about this, however, as no one makes 'race' ported heads for the Jeeps!
Also keep in mind that the engine works as a whole, and doing only intake mods without addressing the exhaust side will not realize the full potential gain. Only so much can enter the engine as can exit, and vice-versa. IT's not good spending a few hundred dollars on the intake and leaving the exhaust stock.
The several sections I will break the mods into are the intake element, consisting of everything before the engine head on the inlet side, the exhaust element, the internal components consisting of the engine head and block, the electronics portion, and various forced-induction systems, such as turbos and superchargers.

Turbo City does a very good job of addressing the intake modifications. They have made a multi-component package that takes care of all the necessary intake changes to decrease the airflow restriction provided by the stock units. They use a K&N cone-type filter for the least restriction, and make their own steel air tube to remove the turbulence-inducing stock 'accordion' intake tube. Airaid also makes a new air filter of this type, though theirs costs more than the TC tube and filter ($190), and piggybacks off the stock tube, to place the filter int he stock box location. The debate behind this, is that hot air hurst performance, being less dense then cooler air, and more likely to cause detonation, therefore an inlet in a cooler position should provide better power. The difference, in my opinion, on the TJ's though is the hood design. At any appreciable speed, the hood on the TJ scoops enough air to make this a moot point. If you look closely, there is over a 1/2" gap in the front, which should blow a steady stream of air through the engine compartment at any forward speed. One thing I may add to my TC setup though is a metal plate under the filter to deflect hot engine air migrating upwards, though I don;t know what, if any, effect this might have. The FTE company, in the United Kingdom, has actually dynoed the Turbo city parts, as well as their in-house exhaust, so I will use their figures in my discussions. Anyone local should give them a buzz for a Turbo City hook-up - I think they are the only authorized distributor on that end of the ocean.
There is also a cheaper alternative to buying a complete unit. Some have found that you can cut the stock inlet pipe just before the accordioned area, and tossing on a generic K&N cone of the appropriate diameter, for around $40.

FTE got a 6hp gain at the wheels off the K&N and air tube alone.

The next inlet modification is a bored-out throttle body. When I purchased my throttle body, I brought along a caliper to TC, and found that the new unit was something on the order of 6-7mm larger, I think (It's been awhile!!). If you look inside the stock unit, you will see a HUGE lip on the lower half. TC bores off this entire region, neat enough to look factory. I immediately noticed a quicker throttle response, and slightly better top-end. The I-6 is such a loud and vibrating motor that it's hard to feel an extra few top-end HP here and there, thanks partially to my polyurethane engine mounts!!

FTE found the throttle body, in addition to the other parts, gave a 7hp increase at the wheels.

UPDATE 3-7-01:

I have now tried the Poweraid throttle body spacer-  the helix bore yadda-yadda-yadda.   It is SH!T!  Don't bother- I got no gains, the helix thing is pure gimmickry, and it is smaller than the throttle body and intake bore!  Whoo-hooo!  The flies were still buzzing on this pile of crap when I returned it.

I am in the process of designing a cold-air type induction system based on a guttes stock airbox now.  We will see how it works when all is done.  I plan on replacing the element with an AMSOIL foam filter as well.

The next area to address is the Exhaust system. One thing to look for in a good exhaust is mandrel-bent tubing. This requires extra work, and will drive up the price of an exhaust, but the airflow gain is worth it. The tube will b an equal diameter the entire length with mandrel-bends. The stock pipe was of a machine-bent design- you could clearly see the diameter decrease to maybe 60% in each bend. NOT ideal for airflow.
I decided on the Borla cat-back system for three reasons- they have a million-mile warranty, they have mandrel-bent tubing, AND they use stainless steel in its construction. One less thing to rust on the underside of the Jeep. It is not the cheapest alternative, going for around $270, but as I learned with the system I had on my 5.0L mustang, you get what you pay for!
Turbo city offers their own version (non-stainless, but mandrel bent) for less, and some have fabricated their own systems using Flowmaster, and other mufflers. Just make sure to use mandrel-bent tubing, and not too large of a piping size (up to 2 1/2"), or you may lose some low-end torque from a lack of backpressure, or result in an excessively noisy vehicle. Borla uses 2 1/4" piping.

FTE found their in-house system gained 5hp alone, and around 7hp when other modifications were made, enhancing the 'package' performance concept.


The other common exhaust mod is a new header. Several companies offer new headers for the 4.0L Jeep, including Borla and Turbo City. I chose the Borla unit because of price (found it cheap!), and it is of stainless construction. The '98-99 Jeeps use P/N 17127.
I have not received it yet, so a review is pending, though Doug Campbell, one of the few other Jeep Engine enthusiasts around said it provided the most gain of any single item, including those I've mentioned, and a few others, possibly providing the most gain short of a supercharger. We shall see soon!

I decided to Jet-hot coat mine too. This is a heat-directing coating, which can dramatically lower underhood temperatures. While on the 5.0L list, a member claimed he could touch the header on his mustang 10 minutes after shutdown!! Another 'we'll see' item!

Many electronics modifications are now available for the 4.0L engine. Jet makes a chip for all years of the Jeep, and Turbo city makes add-on components as well. The new JTEC controller in the wranglers is quite an advanced unit, though more power can be had through some further compromise.
As far as I've heard, the Jet chip makes a slight power difference, at the expense of fuel mileage. I chose to try the TC parts, as their MAP sensor is adjustable to tune the fuel mixture in the engine, and they sell an adjustable timing modifier as well, for several extra degrees of advance. This is basically how a chip makes power anyways, with the benefit of tuneability. I have not tried the timing modifier yet, so look for that review in the next few weeks. The cheapos among us can make a TC timing module by reaming out the holes so it slides a few milimeters over, which is what they did, but I don't have that level of confidence in my dremelling yet, and I'll pay them $34 to do it for me. With the MAP adjuster, you're still at a third to half the cost of a chip, so you'll still make out pretty good. Ian, of Ian's Jeep page, told me the timing advance plus a slightly enriched fuel mixture seems to be just the ticket on the 4.0L. There is a link to his page in the reviews section.
The compromise I mention is the possibility of having to run higher octane gas. That is what you pay to un more timing for more power, though with the low 8.8:1 compression of the 4.0L, you might get away with 87 octane.. I'll have to ask Ian what he runs.
Hesco also makes an adjustable fuel pressure regulator for $189, for those who wish to adjust static fuel pressure.

Ignition mods are another common electronics modification. I've seen many different computerized products, such as Jacobs, MSD, Accel, etc., though I'm not sure how much extra benefit this might provide. I have heard of people having problems with the Jacobs unit, though I personally had a MSD in the mustang that ran great. On the Jeep, I decided to go with a coil swap only, which provides a greater charge to the plugs on firing. IMO, that's the benefit you'll get off a computerized system anyhow, so why pay more? The Accel coil smoothened out my idle a bit, and reduced an occasional jerkiness in the motor under load.


Another often forgotten, though possibly power-improving mod, is the addition of an electric fan. I couldn't tell you how much power it would make, or even how it feels directly, though others have claimed it made their motor rev up easier, and some even experienced a gain in fuel mileage, though possibly slight. The reduction of drag on the engine should be noticeable, however, as the large fan would not be using up crank power anymore.
The other advantage of an electric fan is the ability to shut it down for water crossings. With a constant-on power supply, it could even continue cooling the Jeep after you've left it, and shut down when the thermostat temperature has been reached.
As far as I know, Flex-A-Lite makes one for up to 1996 Jeep Wranglers, though their rep at the local Perfromance Motorsports 'show' told me it might fit, and to call their tech department. I do recall seeing this one for sale in Summit Racing's catalog for $299. You could probably build one up, or use a generic one to save money, though the real deal comes with a thermostat, *might* be a direct bolt-in, and pushes upwards of 2000 cfm, I believe.
My thinking is it may well be better than the stock clutch-fan setup, as while moving, the fan doesn't need to work to get air through the radiator, only during low speeds, and at idle, though at those speeds, the motor is not revving very high. An electric fan ALWAYS moves the same amount of air. The heavy duty alternator on the TJ should handle it fine as well.
I am currently corresponding with a few people who are in the process of adding an electric fan. If and when I add mine, or some conclusive result comes from theirs, I'll add more info, good or bad, on this mod.

UPDATE:

I have located a used fan setup exactly like this online.  I should be getting it some time next week, so we will definitively know what kind of improvement this gives.

UPDATE 3/7/2001

   I have unsuccessfully tried modding the fan :(    I tried the 16" AND the 18" Permacool fan.    I believe there is some odd thing with the fan shroud that makes it not work with the electrics.  There is an odd forward-blowing wind from the gap in the hood with the electric fan on.  And NO it was rotating the correct way!

My current theory is that the fan needs to be some distance towards the engine for the suction effect to work as the stock clutch fan does-  if you look at the pics, you can see that the fan is about 2" from the radiator-  perhaps flipping it and rever-se rotating the motor would put it in a position to better match the stock fan location?  Maybe time and boredom will have the answer.

The Permacool controller does work great!  And the fan pulls pretty strong. Just not enough, I guess.  Funny thing is the Grand Cherokee has a stock electric fan on the 4.0L!!  Maybe that one would work....

here's the install:

The Turbo City Timing module and MAP adjuster have been thoroughly tested as well.  As far as the MAP adjuster went, I didn't need it.  The TJ runs plenty fine without it-  with the MAP thingy set, it runs too rich.  The timing module does seem to get slightly better acceleration and gas mileage, and doesn't seem to require more than 87 Octane yet, so for about $35 bucks, what the heck.  You can always slide it back to the stock position anyways.

Internal modifications are usually more involved than most are willing to try. I have heard of ported 4.0L heads being used, as well as roller rockers. MOPAR sells a set, though they are very pricey, and you must use a spacer on the valve cover for clearance. I had a cheaper Crane set on the Mustang, though I can't say I saw an appreciable power gain. A side benefit is the reduction of oil temperatures thorough the reduction of friction, and more power through less friction as well. JP magazine did a test of the rockers though, and it only showed a 1.5hp gain. Pretty shabby, if you ask me, for a $600+ mod. If you could locate the original Crane part number and buy them direct, however, you might find them for less, though it would still be too much, IMO.


Hesco makes a 4.7L stroker kit for those who need to rebuild their motors anyways, or those with too much cash to waste. It should provide considerable increases in low end torque, and run like a factory motor.

For those of us insatiable, power-hungry drivers, this is the holy grail. Short of a complete engine swap, no other mod will come close to providing the increases afforded by forced induction.

Turbo City and Belltech Engineering currently provide two turbo kits for the 4.0L. Belltech's kit is considerably more expensive, but has more components, though I don't know which would run better. TC's kit is simple to bolt on, requiring one cut to be made in the exhaust pipe, and fitting on top of the manifold, and provides a 6psi boost. It sells for $2999. They use two extra injectors before the turbo to supplement fuel. My only issue stems from what I have read about water-injection systems on a Merkur XR4Ti. On the Merkur page I saw, the owner added such a system before the turbo, which resulted in the water severely pitting the compressor blades. I'm not sure fuel would have the same effect, but it's something to research first. Here is the Turbo City kit. Note the extra plate after the turbo where the injector is located.



SVI provides the only non-custom centrifugal supercharger on the market for $2995. They use a vortech supercharger at 7psi, and a Belltech adjustable fuel pressure regulator to spike injector pressure on boost for fuel enrichment, and a Crane ignition unit as well- all you need to finish up. Not bad for the price! Simple, but effective. Here is the SVI offering:


Hesco offers a Positive displacement supercharger as does Rimmer Engineering. They are similar designs, while rimmer uses an additional injector to supplement fuel, and Hesco uses a Kenne-Bell Boost-A-pump, which increases fuel pump voltage to spike fuel pressure to the existing injectors under boost. The Kenne-Bell unit is adjustable. Hesco's kit costs about $200 more, at $3650, though I think it is the better unit. Though I have only heard good reports about Rimmer's 2.5L kit, I have heard of problems with the fuel system in the 4.0L kit, causing erratic idle and other things. Hesco's kit puts out a claimed 275HP and 305ft/lb. torque at the crank- add that to about 25hp from the basic mods, and you've got quite a powerful combo. They say the SC on top of the stroker kit is incredible! Here is the Hesco SC from the Hesco ZJ jeep page. Check out the link for more Hesco info!

I myself am leaning towards the Hesco unit. The install is clean, they have at least one favorable result (ZJ page- the owner loves it!), and it messes with the least amount of stuff in the engine bay. SVI's kit is clean, but with a centrifugal blower, gain will be mostly on the top end.
Awhile back, Hesco sent me a flier with 1/4 mile and 0-60 results of two of their kits (back then I think they offered both a eaton-type and a Vortech-type unit).
The Vortech type got a 1/4 mile of 14.90 sec. and 0-60 of 7.2 sec, while the Eaton got a 15.0 in the 1/4, with an astounding 0-60 time of 6 sec.!!! You can see what that means- below 70mph, the Eaton is king. With a Jeep, you wil not need that high-speed boost, so the Eaton makes more sense. For those who want a highway boost, and more controllable rockcrawling with less power, the SVI unit or a turbo might be the way to go.
Remember too- their test car had those donut tires on. If you're running 35" tires, don't expect to be in the 14's!! I do expect, however, with all the other mods, plus a blower, to see low 15's with 33" or 35" tires, and appropriate gearing. If I get more, great! If not, well, that's what I expected! Look for a blower review in a few years.. unless I win the lotto!

UPDATE:

Belltech has lowered their price to a level comparable to the Hesco SC..  not bad for an intercooled turbo.  I have emailed them for some more information.  Hopefully we will be able to provide a more in-depth review soon.

Rimmer Engineering has gone OUT OF BUSINESS!!!!

They did rise from the ashes again, and are offering a new kit.  Problem is, it costs $5G's!!!  It's going to be a roots/eaton style blower with intercooler, so we'll see how it is.  Spending 1/4 the value of the Jeep for the blower is insane though.  The $3K SVI offering looks to be the most reasonable at this point- especially with all off-the-shelf parts, support in the future will not be a problem.

Hesco's offering:



And this is the Rimmer offering. Note how it replaces the entire intake manifold!  (Now DEAD!  this is the old kit)

This is Rimmer's Dyno Graph.

Here is a final summary of FTE's dyno findings in table form. I simply duplicated what's on their page. Check it out for a full review.
They found that the stock 'trumpet' on the box sucked into the hood at first, hence the disproportionately large gain from simply opening the hood. That is the true baseline figure.

Well, that about sums it up. You can tune the 4.0L for an extra 20hp or more quite easily, or go nuts for a 300+ hp monster. The options are out there, the choice is yours.

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