Any performance advice for 2.5 4-cylinder?


Supporting Member
Jul 2, 2022
North Carolina
Just curious if anyone has any performance tips for the 2.5? I've read a little on. the subject and there really isn't much promising information out there. Not really interested in a turbo. There's a guy on you tube that installed a M90 supercharger on a 2.5. Ultimately there has to be an upgrade to the ECM/PCM.
I'm looking for more of a mechanical upgrade like head work or injector/intake mods.
Thanks ya'll.
Honestly, axle gearing is most likely going to be the best bang for the buck. Gear it lower and it'll accelerate faster. If you're just driving local and on trails, this will make you the most ROI for what you're looking for. Going to (possibly) lose some top end, but from what I've read about the 2.5, they don't really like much over 60mph anyway.

You can go cheap and find lower geared axle assemblies in the classifieds. People part these rides out frequently. Plenty upgrade to the FSJ axles and sell off their earlier upgrade attempts with the stock axles. Fakebook Marketflakes almost always will have listings popping up.

The second option is to go all out building yours and add traction aides or even do the 8.8 swap if you want.

Going to the multi-hole injectors is on my long term list (4.0), but not for any performance gains. Just a better atomization of fuel for smoother idle.
The whole "increases performance/mileage gains" thing with injectors has pretty much been debunked from what I've seen.
Kinda what I figured. Gearing seems to be the best way forward. I supposed that there are little things one can do to improve the little 2.5. Not much in the line of engine performance or aftermarket parts. I read that even the cold air intake is rather unless since the factory air cleaner is built for max airflow.

Thought about swapping in a 4.0, have one on the garage floor, but there is too much modification needed to mount the motor. Moving motor mounts and whatnot. And besides, grand babies will eventually be driving this jeep so keeping it stock and slower.
The 4.0 isn't exactly a modern powerhouse either. But that may be my street friendly 3.07s and the slushbox auto :LOL:
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I've read that 4.56 or 4.88 is a decent choice for gearing the YJ. Currently running 33's and it gets along, just not in overdrive (5th). Filled it with mid grade fuel the other day and it got a little peppy! Woo Hoo!
Might go down the ring and pinion gear rabbit hole this spring.
I'd advise using a gear ratio calculator like this one:

Plug in your original tire size and ratio on the one side, than you can play with your new ratio and new tire size on the other side. You at least want to match your revs with the new setup. Keep in mind, larger tires and age affect the power a bit more than originally, and maybe compensate by going a bit lower on your ratio. ie: You do the math and a 4:10 is about the same as original in the rev dept, then go to 4.56s.
Well, that was interesting. Thanks. So, I can do 4.56 gears and expect 2762 RPM at 70... That's just a little spooky though. Not the RPM, but just thinking about going 70 mph in my YJ! =0
I like my 4.88 gears. I like them with the 31x10.50s, I like them with the 33x12.50s. Still a turd , but it will hold freeway speeds in 5th and climb most mountain passes in 4th. I almost installed 4.56 gears, ultimately glad I went with 4.88s.
My little 4 banger Jeep will do 90mph on 31s or 33s (eventually), and will run 80mph all day long.
That sure seems fast enough for a jeep with a 94" wheelbase.
I swapped a '94 4.0 into a '95 that came with a 2.5....Ran into problems because of a wonky rear end that came with the '95 so I used the 3.07 gears out of the donor and wound up with a YJ that runs like a watch but still is under powered for my taste. So I'm getting the kit together to small block it with a 5.3. and stay with the rebuilt AX-15 that's in it now. Tons of research states that the AX-15 is well suited for up to 350 horses and 300 ft lbs torque.....
Might want to put a heavy clutch in it....
So, to be open kimono, I have two 93 YJ's. The black one I'm trying to keep the stock drive train so my grand babies can learn how to drive a stick and be spunky enough to hang at highway speeds. I have a red YJ (not pictured) that is the project. I have a 4.0 and a SBC 350 with a TH350 setting in the garage. But I think I've pretty much decided to go the LS 5.3 route with the red YJ.
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So, to be open kimono, I have two 93 YJ's. The black one I'm trying to keep the stock drive train so my grand babies can learn how to drive a stick and be spunky enough to hang at highway speeds. I have a red YJ (not pictured) that is the project. I have a 4.0 and a SBC 350 with a TH350 setting in the garage. But I think I've pretty much decided to go the LS 5.3 route with the red YJ.
Personally, I like having a 4.0 in a Wrangler, but having ran LS vehicles for almost 15 years, you won't be disappointed with that 5.3 in anything. There's a reason my Wagoneer now has a gen 3 truck motor in it. I'd wager you get better fuel economy than the 4.0 as well. I'd recommend gearing the axles something like 3.73 or lower and utilizing an OD trans for full potential of performance and economy. That said, the 4.8 in the Wag does fine and cruises at interstate speeds with the 727 and 2.72s.
I have a 2.5l with a 8.8 rear with 4:10 gears. It is the slowest truck I have ever driven. Feels like a Toyota 4x4 pickup. My son has a 1990 YJ with a 5.0 ford v8 with a c6 and a np205 transfer case. It runs like a raped ape. I suggest not worrying about power. It is not happening.
I have a 2.5l with a 8.8 rear with 4:10 gears. It is the slowest truck I have ever driven. Feels like a Toyota 4x4 pickup. My son has a 1990 YJ with a 5.0 ford v8 with a c6 and a np205 transfer case. It runs like a raped ape. I suggest not worrying about power. It is not happening.
I had to laught at your post. My 75 FJ40 with engine mods got smoked by my buddies naturally aspirated VW rabbit diesel. :cool:
redux 1
causes or upgrades.??? is it normal or not now? not told, I will make it more like a physics class answer. ( i can name 50+ causes so I won't)
  1. Engine is bad, compression at 100psi and is a very sick and old tired engine, as many are this old. (bad as in worn out but still runs)
  2. EFI not happy, inputs to it dead wrong ( the PCM is a computer for sure and wrong inputs to any computer gets you lies outputted.
  3. Fuel pressure dead wrong Wot. 41psi is correct. (sealevel) (hand meter gauge it)
  4. ACTUATORS BAD. these are PCM computer outputs. and if injectors are clogged there will be no full power, 1/2 clogged. or fuel filter clogged?
  5. drive train excess friction from flywheel to tires hitting the ground, including bad brakes dragging. or huge wide tires.
  6. the hill is just too steep for 80 HP or forgot to down shift it?, with tall tires even 4th gear acts like OVER DRIVE slug mode. 33 tires are tall !
  7. the PCm never sees ever fuel pressure is wrong and power is weak unless you get DTC for lean burn. code 21/22 maybe. 02 dead.
the 1996 up cars have codes for fuel trim 100% dead or stuck max lean. not ours.
newer cars have fuel pressure tests, failing, but not ours. we use a hand gauge to find it. easy.(OBD1 ways and means)
the car does not have misfire detection as newer do, OBD2 all have it by law USA< 1998 Canada next.
the PCm has no CAT tests so if the CAT is 1/2 melted inside the power can be 1/2 power so unbolt the cat and test. (remove it and test) or?????
there are 3+ cat tests one is vacuum test @manifold,, that shows vacuum needle acting backwards at WOT.(or gunned)
the 2nd test is using pressure gauge, 1-5psi put in the place of the pulled O2 sensor, and back pressure is say 1 to 2psi at WOT.
if higher the cat is melted, and blocking the exh. path huge.
many shops uses this data 2.8 psi max 2500 RPM, back pressure, finding this out is not in the shop manual. but all shops know what to do.
there is thermal test that shows front to rear CAT temps increase greatly (hot engine) the output is 100f more degrees than front of cat. if both the same on this CAT the CAT is dead, ( many of them in side now are gone blow out long ago)
test 4?
run with no cat, (or buy a new one your call) if bumping the cat gently makes it rattle inside it is bad. omg I posted 4 tests

not told 50mph limited in what gear and what RPM and if on flat ground or not not told. (just 50 told)
the car really is too heavy for any 80HP, engine. (corrected HP this is)

it will be more power on a cold foggy day,
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Jeep is in North Carolina east coast, not smog checks (yet). Over 30 years old and qualifies for historic license plate, no more state inspection.
Pretty perky for a 4 cyl even with 33's. All I really want is to be able to sustain speed on the highway. Don't really do much dirt, maybe some sand on the beach.
93-2.5L, magic.
QOD: #1 is the car test for smog or not? if yes, if you mess with the engine, it may fail all smog tests. for sure if the CEL lamp glows all the time.
some states still do the IM240 rolling sniffer test. does your state?
Lucky you, NO.

but only you know for sure, some states do visuals on old cars. (under hood) like CALIF does. (mad)

2855 lbs stock curb weight, 85 more for 4L

well the car is heavy, mines like a tank and needs a powerful engine to make it go fast. (
not told by you under what conditions power lacks at 3200 RPM is max HP. so use gear that lets it go 3200 rpm.
in the dirt are you , having problems only.
deep snow? or just going up hills?
in 2wd or is the problem in 4wd.?
or just in the street is it a problem , see how versatile it is (it's forte")
or from 0 to 25mph town it is slow. (sure is)
or from 25 to 50mph. too slow to accelerate
no clues by you when it is slow doing what? (sure all things, sure) but not told.

2.5L is limited by 4 things,
  1. (not counting a switch to Nitro Methane fuel),,, can i watch?
  2. (not counting make car lighter first) this car is not a Freeway flier, sorry it is not.
  3. Displacement (engine swaps are costly and lots of work if runs EFI) (harness/pcm swaps )
  4. N/A or Induction boosted or not, (turbo etc) and or higher flowing head. N/A naturally aspirated. I beat the price scares you and blowing out any old engine with boost , cracked pistons etc) or mill head get higher CR, and more power and the PING of death this PCM can never handle.
the car is a tank so remove the heavy hard top (if has it) and go rag to make it lighter and faster?
the metal on the car is thick (that I like) and has heavy metal frame. No light unibody here.
no way to cut off metal and still have jeep to be lighter/faster.

or sell it buy the one with a bigger engine, this is the best way . saving a ton of grief, labor and trouble)
if the car has huge tires.
get rid if 30" or tall tires. (they cause power loss ( way too heavy) and cause gearing to be wrong too tall tires ( C= Pi x D)

or the silly brakes drag as mine did on the rear, with a park cables X3 fully rust seized and dragging brakes off.

I have seen u-joints so bad they chattered, in motion, and wasted huge power as they spun. (and the huge noise they make)
put a 4.0L stroker in it?
for more torque. all the 50 ways here, this guy is wild with mods. must be a day job?

what if the engine is not running right now?
Air cleaner packed in dirt.
02 sensor bad making it not go to closed loop and runs way too rich now and even fouls spark plugs?
bad map sensor, or vacuum to it routed did wrong or hoses cracked.
bad fuel
clogged (partially) fuel filter.
fuel pressure not 41PSI at WOt OR kEY ON 3 TIMES. MEANS FPR IS BAD AND PRESSURE WAY TOO LOW TO GET FULL HP. (or weak fuel pump)
the fuel pump shunt test is 75psi, is it?
the FPR on the rail loves to fail this old an all cars this old. <<< reality.
weak spark.
injectors partially clogged.
I will stop here can do more, I just do the tests and let the chip fall...
ok 1 more...
distrib timed wrong so injector sync is dead wrong.


Here is the basic history of the 2.5L and the AMC sixes.

The 2.5L (the old OHV valve engine used through 2002-ish) is basically a 4 cylinder relative of the 4.0L. They share costs by using similar machining operations, and many common parts (such as valvetrain components and internal engine parts that have nothing to do with length). The bore is the same as the 4.0L but the 2.5L has a slightly shorter stroke when compared with it's bigger (younger) brother. I'm going to venture a guess that the shorter stroke was probably done to reduce NVH. Largish 4 cylinder engines (without balance shafts) are not known for their smoothness and I would imagine the longer stroke exacerbated the issue at the higher RPM's this engine needs to run at to make power and keep up at highway speeds.

As a matter of fact, the 2.5L technically pre-dates the 4.0L. It was originally based on the 4.2L (but got a new large bore block and a new cylinder head with many flow improvements) and launched in the early to mid 1980's if memory serves. It was the 4.0L that picked up on the 2.5L's 3.88" bore (an increase over the 4.2L's 3.75" bore) and was given it's own stroke (3.41") that was a compromise between the 2.5L's short stroke (3.19") & the longish stroke (3.90") of the 4.2L, probably to improve high speed smoothness and control emissions (long-stroke engines are notoriously difficult to clean up from an emissions perspective). Ironically, the 4.0L's stroke is fairly close to the original Rambler 232 cid I-6 (3.50" stroke) that the entire family of AMC sixes and fours owe their design to.

Both the 2.5L and 4.0L engines share some of the same "warts" such as the ever popular rear-main seal leak, but by the same token they have the same strengths. They are both robust, simple to repair, cheap to operate over the long haul (not just talking fuel econ here) and capable of providing long service easily in excess of 200K if maintained properly. The 2.5L did start off a little rough though - early engines from the '80's suffered from really excessive piston slap (small harmless amounts of piston slap have always been an issue with this engine family), so bad in fact that the "slap" was quite loud and could ultimately lead to piston/rod failure. I knew some personal friends with a first year XJ 2.5L that had such excessive piston slap that a piston collapsed a skirt, ****ed in the cylinder and snapped the rod in two. Apparently this wasn't a terribly uncommon occurence back in the beginning and many Dealers made quite a bit of money replacing short blocks in early 2.5L powered XJ's and other AMC products.

To be honest with you, were I to own a 4 cyl Jeep, I'd rather have the old 2.5L like yours than the newer 2.4L, even though the 2.4L has a slight power advantage. The 2.4L has some issues with head gasket sealing and is just a needlessly complex design for a simple vehicle like a Wrangler - I just can't see them lasting as long overall as the old 2.5L. I hear a lot of folks on here with 2.5L's that have several hundred thousand miles on them, but I don't recall any similar claims from 2.4L owners, but perhaps they're too new yet to make a judgement.

I also like my 4.0L enough (from a design perspective) that I have absolutely no interest in a Hemi swap or some other V-8 swap that some folks are pining for. I'd sooner put a 4.2L crank in my 4.0L and build up a stroker than go through the hassles of a V-8 install. With the possible exception of a small-block Chevy, I can't think of an available V-8 that has the potential to provide as much trouble-free service as the 4.0L can.
Here's some more 2.5L history from:
AMC-Jeep 2.5 liter four-cylinder engine

The Jeep 150 cubic inch four-cylinder engine was introduced by AMC in 1982, for the 1983 model year. The engine used a carburetor until given throttle-body fuel injection in 1986 (except in the Wrangler, which switched in 1987); in 1991 it switched to multiple-port fuel injection, and in 1993 moved to sequential multiple-port fuel injection. The major change over the years has been the different fuel delivery methods, though minor changes and refinements were made as well.

The I4 uses overhead valves with hydraulic lifters and a cast iron head and block. It is said to be a 258 ci AMC straight-six with two cylinders lopped off, bored to 3.88", and de-stroked to 3.19". The head's chamber and port design were later used with the perennial favorite Jeep engine, the 4.0L Straight-6.

While clearly superior in power to the Plymouth-Dodge 2.5 liter engine, which produced at most 100 hp and 135 lb-ft of torque, the AMC engine co-existed with its corporate sibling; it did not even make it into the Dodge Dakota until 1996, when it replaced the 2.5 liter four-cylinder (a stroked 2.2 liter engine) which was no longer cost-effective to manufacture, since the cars that used it had been discontinued. The 2.5 would eventually be replaced by the corporate 2.4 liter engine (producing only a little more power). The Dakota dropped its four-cylinder option entirely.

W. Paul Tippett, president of AMC, was quoted by the New York Times as saying that it had made sense to buy four-cylinder engines from GM at first, because demand was low, but with the public using more of the engines, it made more sense to develop and build their own. The popularity of GM's new four-cylinder cars may well have made supplies questionable as well.

While some believe there were no cars made with this engine, a very small number of early Eagle Premiers - modified Renault Medallions - were built with the 2.5 in 1988 and possibly 1989. However, most found their way into the CJ7/Wrangler, AMC Eagle, Cherokee, Comanche, and Dakota.
In 1997, the Dakota's 2.5 liter adopted a "quarter-wave" tuning chamber in the intake duct and a Helmholtz resonator mounted atop the throttle body to reduce induction noise and provide a pleasant sound. (Click for details.)

The 2.5, as used by AMC, used a General Motors bellhousing pattern, instead of the AMC bolt pattern, because it was a replacement for the Pontiac "Iron Duke" four-cylinder purchased by AMC while they adapted their straight-six into a four-cylinder.

In 1997, the engine was noted as having flat followers and hydraulic lifters. The redline came at 5,550 rpm. Fuel was standard 87-octane unleaded. Oil capacity was 4.0 quarts, coolant 9.0 quarts; emission controls included three-way catalyst and dual heated oxygen sensors. Gas mileage in the Wrangler was 19/21 manual, 17/19 automatic.

Bore x Stroke 3.88" x 3.19"
Displacement 150.4 ci 2.5L
Main Bearings 5
Valve Configuration Overhead valve (8 valves), flat followers, hydraulic lifters (1997)
Construction Cast iron block and head
Redline 5,600 rpm
Gas needed Regular (87 octane)

Jeep 2.5 engine 2002-1997 / 1995-1991 / 1990-1987 / 1986-1982
Compression 9.2:1 / 9.1:1 / 9.2:1 / 9.2:1
HP 120-125 @ 5,200 / 123@5250 / 117 @ 5000 / 105 @ 5000
Torque 140-150 @ 3,250 / 139@3250 / 135 @ 3500 /132 @ 2800 RPM
Fuel: Sequential Injection / Multi Point Inj. / Throttle Body Inj. / 1 Barrel Carburetor
See less

2000 2.5 TJ, 5spd, 33s, 8274
I'm looking for more of a mechanical upgrade like head work or injector/intake mods.

ever do Iron head milling by hand (called porting)???? not easy work nor CHEAP (IRON =hard) and risks damage to the head, or breaching the water jackets there. or who knows what.? there are custom rockers for the engine that are called high lift but must not cause valve to smack top of pistons.
add more or bigger valves?, you tell me how? , there are no custom heads for this engine to that level of extreme. (that I have seen )
you do not know that? when you go WOT (wide open throttle) the PCM now goes to open loop ( wot throttle max or fast right foot are both PCM full ENRICH mode events) even pushing throttle fast does this too. on all EFI cars actually.(gasoline)

and is no longer 14.7:1 AFR but goes to like 13:1 and 13:1 is max power , 100% free and EPA legal. free of costs, amazing no?
Maybe CAT removed will make more power (bolt on CAT option? and bypass pipe) (NSA is reading my post now and telling EPA what I said)
13;1 is called PCM ENRICH MODE (aka open loop) below.
the higher flow injectors will not make more power, lacking more air.
The air intake path is not restricted here. exhaust is.
I see no racing heads sold for any 2.5L jeep engine,
pure physics after all its 13.1: 1 now WOT, in fact perfect if FPR is not dead is it. I told you low power (1 of 50 cause) FPR bad does that.
(more displacement or boost)
or custom heads not sold but sure any shop will port it at $150 per hour and no warranty if it cracks the head day2.
if the O2 is bad and it goes way too lean power will drop but never at WOT the O2 is OFF LINE AT WOT.
THIS answers are for warm 180f engine not cold, this is hot mode proformace, 5 gas physics. the STOICH is 02 closed loop cruising.
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one cure (not in my list( Obummer said) pump up dem tires. (he nailed it)
more speed.
as the car goes slow to fast say 0 to 40mph it is not much drag.
and when you go faster it must then compress the air. now it really gets hard. motorcycle drivers all know this, for sure freeway fliers do.
wiki quote, i added the 37)
Above 60km/h,(37mph) wind resistance grows with approximately the square of speed, becoming the dominant factor at high speed
so the brick compresses the air unlike any car on earth mostly, ( or as other bricks do)

this jeep is 2 times the normal drag coefficient of most cars. flat grill , and flat wind shield, more like flat box kite effect. or steel square box.
the only question is , how to make the engine pump more air, it is after all just and air pump. (and compressor)
you need to add more air, cheaply, ( a bigger engine or forces induction) or extreme costly engine changes.
so the key word is cheap (less expensive) labor costs , down time costs. etc.
more speed is costly. no matter the car. (vehicle , not in outer space)
the only freebee is pump up dem tires. (kids bike pump borrowed)?
or just down shift. (if must for hills) I must down shift mine for mole hills. or rolling over dimes