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 accleration 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.
Cheers, GP http://www.forumco.com/jeffy/topic.a..._ID=6&CAT_ID=1
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 no problems.
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.