This isn't rocket science. You need Spark, Fuel and Air for combustion. With my 2.5L I did all of
the above and noticed a hugh difference. But that's the key, you have to do them in conjunction
with each other. Doing one will only net small changes and you might notice a small gain but
you still have choke points in the complete system.
Open up your air intake by getting rid of your stock air box. There are two choke
points or restrictors in your stock air box. By doing so you will greatly increase the air
flow and temp of the air being mixed. This is where a cold air intake comes into play and
allow cooler, more dense air into your system.
Open up your throttle body and port your intake to match. If you just throw
your 4.0L (larger)TB on your intake, there is a restriction of the Intake being smaller. If you
look down through it, you will see the extra surface area that has to be ground down to
match the TB. A TB spacer doesn't do much to the equation but I have found that it does have
a cooling effect with the helix design and therefor does increase the HP upgrade a bit.
Increase your fuel delivery to match your larger air flow This is where you upgrade
your stock 17lb fuel injectors to a better 4 pintle Bosch Type III EV1 19lb injector. This
allows more fuel to be mixed and atomized with the cooler, more dense and higher air flow
volume. Stock injectors have only one pintle or jet hole. Bosch type III EV1 19lb injectors
have 4 pintles or jet holes for a better spray pattern and mist. Depending on how you drive,
this equates to a more efficient burn for better gas mileage or a hotter burn for better
power and throttle response at the top end.
Hotter spark to burn the mixture. By upgrading your coil-wires and plugs, you allow your
system to burn off the better spray for more power. I used a super coil (Screamen
Deamon) coil, Live wires and Champion Plugs gaped to .080 for a much hotter burn.
Open up your exhaust for your new system to breath. Getting rid of the stock muffler
and cat and replacing them with high flow units allows your system to aspirate
(breath) much better and allows your system to move the exhausted gases out at a
I also went to an 14 " electric fan. By eliminating the belt powered fan, it reduces drag
on your little 2.5L 120Hp motor. Here are a few things to think about. Approximately
for every year old your motor is, you loose 1 hp due to age wear and tear. Stock your
new 2.5L motor is only 120Hp. My YJ is 19 years old. Theoretically, i'm around 101 HP.
small gains of 4-5 HP is noticeable on this size motor. Someone that has the 4.0L at 265Hp
probably won't notice a 5 Hp gain. I have not tested my motor but I think I'm in the 143 Hp
range for power. Not bad for a stock tired motor.
You may or may not notice a difference when you only do one of these upgrades.
Unless you do them in conjunction with each other, there will always be a choke point
in your system and it will not work up to it's capacity. In my experience, all of these
upgrades done together really do make a much peppier 2.5L motor. Will it ever be a
4.0L NO. But it is much better then a stock 2.5L will ever be.
Here is an article from 4banger.com which specializes in 2.5L motors.
I cut and pasted the article so you wouldn't have to join the forum to
see the link.
Yesterday a 4.0L TB from a '98 Cherokee arrived by UPS -- to be installed on a 2000
2.5L TJ. With the help of a neighbor who teaches auto mechanics at the local high
school (absolutely cool woman who decided to make it a day project for some of her
students), I dyno-ed everything in stages. Checked the air flow, too.
Results, with stock (2.5L TB & no spacer)as baseline:
(1) Poweraid spacer only -- +2 HP, 8% less airflow (Evidently, the helix bore, lips
of which extends beyond intake & TB opening, actually cuts down on airflow. But
the spacer does "cool" and extend air volume, hence horsepower increase.)
(2) 4.0L TB only -- + 6 HP, 16% more airflow (no surprise here)
(3) 4.0L TB & grind out intake opening to match lower TB opening -- + 13 HP, 29% more
airflow (Wow! Grinding out the intake opening did make a difference!)
(4) 4.0L TB & Poweraid helix spacer & grinded-out intake opening -- +16 HP, 2% less
airflow from #3 above(overlapping helix bore is still cutting off airflow, but spacer
is still cooling & increasing air volume)
(5) 4.0L TB & grind out helix bore in Poweraid spacer to match lower TB opening
& grinded-out intake opening (the Big Kahuna) -- + 24 HP, 39% more airflow (Wow
again; the optimal configuration, obviously)
It should be noted that grinding out the helix in the Poweraid spacer still leaves
a partial helix in the walls -- the main difference being that the helix now has rounded
edges rather than sharp ones. My auto mechanics-teacher neighbor suggests that
these rounded edges are actually more aerodynamic than the original sharp ones,
and will still "spin air" -- so perhaps the "pulse-organizing" effect I mentioned in an
earlier post will be retained, if in fact there is such an effect.
So, to reiterate, I ground out on the helix bore of the Poweraid spacer, and ground
out the intake opening, both to match the lower opening of the 4.0L TB I swapped
in. Dyno-ed & tested airflow, results: +24 HP, 39% more airflow.
Seat-of-the-pants impression: idles smoothly, more torque on the low end, smoother
acceleration through the midrange, and better performance above 2700 RPM. No
hesitation, no engine codes.
One MAJOR suggestion: if you grind out your intake opening, be VERY CAREFUL
about aluminum filings in the intake manifold. We stuffed everything with tack cloth --
sticky cloths which can be purchased at a hardware store -- rather than shop towels.
The tack cloth caught about 95% of the filings, but there was still a small mess
in the intake. We vacuumed a lot of the leftover out with a shopvac, but we still
had to use tack cloth attached to a long, thin screwdriver to "mop out" the runners.
On the TJ there are also four open plugs/hoses attaching to the intake below the
TB -- you'll have to clean those, too. It took longer to clean out the intake than
it did to bore out the opening, but it can and needs to be done.
Finally, should say that the TJ has a two-stage K&N filter system, an MSD ignition
system & coil, high-gapped Champion truck plugs, a Flowmaster delta-40 catback,
and a case-full of Mobil 1 synthetic oil. All or part of which may have a synergistic
effect on my results, since mods usually affect each other.
More Info: (1) The intake is soft aluminum, so we used a medium tungsten rasp,
followed by a fine fluted grinder bit, followed by a fine polisher. I'll admit it's not
even -- hard to get consistency around the runner walls which are nearly flush
to the intake opening. The spacer is a harder grade of metal, and trickier, even
when clamped in a vise. Doesn't look pretty anymore, at least on the inside.
(2) Tested at 1800 RPM and 2800 RPM with an average between the two -- but
the results were fairly similar.
(3) We figured the following: At 2800 RPM, torque was up by about 16% with #5
option above. At 1800 RPM, torque was up by about 14%.
Again, folks, consider the mods and the synergistic effects -- your numbers may
be different depending on what you have your rig, mileage, mechanical conditions,
altitude, etc. I give this just as a general indication.
Ambient air temperature: 74 degrees Humidity: about 35%
First, let me say hello to the group. I am a tech for Jeep and Chrysler, electrical
and drivability are my areas of expertise. Now I'm going to add my .02
1. I have done this mod on my own 92 wrangler with a 2.5L engine with incredible
results. The 2.5L has a K&N stock replacement air filter w/ a stock air box... the
restrictor in the air box cover removed, a tri-y design header (manufacturer unknown,
was given to me), stock exhaust minus the cat, an Accel super coil (direct OEM
replacement) w/ Belden spiral core wires and factory plugs gapped at .035". I have
4.10 gears w/ 35x12.50 General MT's and I can say this motor screams. The motor
in stock form would not pull the jeep w/ 33" tires. I did all the mods except the
throttle body and put the 35's on. At 65 mph into a 15mph headwind, I would have
the throttle pushed to the floor just to maintain speed in 4th gear, with the throttle
body swap, it was like putting a 4.0L under the hood. Under the same conditions,
I could maintain speed w/ 1/4 to 1/3 throttle opening, and my DRB III confirmed
it. It felt like I added 15 to 20 hp., bottom end torque improved and overall drivability
improved. Fuel milage remained constant, or slightly better depending on how heavy
my foot was.
2. Now on to the AIS motor. If you look at the AIS motor and housings on th 2.5
and 4.0L engines, you will notice tha the idle air passage seat where the AIS needle
seats is considerbly smaller on the 2.5L than the 4.0L, so is the needle on the AIS
motor itself. You must transfer the AIS motor and housing from the 2.5L throttle
body to the 4.0L throttle body, if you don't and you use the 4.0L housing and motor,
you may not get your idle speed down to where it should be. Your AIS motor will be
fully closed or close to it (0 to 2 steps). On TJ's this can be a problem with the OBD
II, since it can trgger a code in the PCM (target idle not reached), so be sure to
transfer it to the 4.0L throttle body. Be careful with the gasket it is not available
separately. If done right and you have access to a DRB II or DRB III or a code scanner
your idle steps should be around 14 to 18 steps in neutral, air off, if not you can fine
tune to get it in that range by adjusting the throttle stop screw, if you turn it more
than a 1/2 turn in either direction, you better check your installation.
3. Yes you do have to trim the intake manifold opening, mine had a 1/8 lip around
the base and it does have a profound effect on air flow, enough to cancel the gains
of doing the conversion. I used a die grinder with a wide fluted aluminum bit, on the
vehicle wuth rags stuffed in the intake runners. A shop vac to remove the filings
was used and make sure all filings and rags are removed. I trimmed the hole to the
size of the gasket (4.0 and 2.5L gaskets are the same).
4. The PCM's adaptive memory after 100 miles of driving was practically unchanged
after the swap meaning the PCM saw no ill effects from the swap. Keep in mind
that the MPI fuel injection is a speed/ density based system and does not see an
increase in airflow like other systems that use a mass air flow sensor, instead it
sees changes in MAP value and intake temperature (density in the intake manifold)
crankshaft speed and throttle position, coolant temperature in all modes plus the O2
sensor at idle and part throttle and then using complex alogrythims calculates the
injector pulse to provide the correct fuel/air raio. For all practical purposes you do
not need to worry about it, the computer seems to compensate for the swap with
5. For those of you who still feel the need for more fuel, you can try injectors from
a 4.0L engine. These run approx. 10% richer, single cylinder displacement is about
10% larger on a 4.0L, so these will work good if you need more fuel delivery without
screwing around with the pressure regulator on the fuel rail (91-95 only, TJ's regulator
is in the fuel pump). The PCM will only compesate for the added fuel in closed loop
operation only (idle and part throttle)!
I did this swap months before I even came across this site (my own idea) in search
of more power, knowing what it would take to install a 4.0L in my Jeep, even with
the resources available, and I can tell you I am very satisfied with the results. I hope
the information I posted is useful and can be used to your advantage.