2.5L performance?

[ron521]

Originally Posted by warpedredneck

i just got my first jeep, 87 comanche 2.5. having said that ive been a mechanic and fabricator since, well as far as i can remember.
isnt the 2.5 a general motors motor?
i was under the impression that the 2.5 is what they called the iron duke motor. it this is the case there is a lot of stuff available for this engine! cams, pistons, roller rockers even performance heads.

can some one tell me if i'm right or wrong?
thanks
jeff

There is a Wikipedia article about the AMC 2.5 which states that it was a new design which replaced for the GM "Iron Duke" engine, which Jeep had used for a few years. Here's a link to the article:

AMC Straight-4 engine - Wikipedia, the free encyclopedia

Essentially, the AMC 2.5 amounts to the AMC inline six with two center cylinders removed, bored out to the largest size the tooling centers would accomodate, with a shorter stroke allowing better high rpm capabilities. Actual horsepower compares favorably with 2.4 engines from Toyota and Nissan.

[Junkpile]

Like Ron said, the Jeep 2.5 is a different engine than the old GM Iron Duke. I think the Iron Duke eventually evolved into the Quad 4 engine that came in some GM fwd cars, like the Old Cutlass Calais from the 80s-90s. It's a damn good four popper.

The key to making the 2.5l pop is to set the rest of the Jeep up around it. You aren't going to make huge power with it normally aspirated, but the Jeep it's sitting in can still be pretty quick. I raced mine forever, riding on 36" TSLs and with the STOCK axle gearing, and it did well. I have somewhat gone through my old combo on here before if you want to know, but some secrets need to stay secret, and there are people I used to race against who read these forums now... The main key to it all is to keep the weight down. If you have to have 1 ton axles and HD truck gearboxes then you might as well get used to riding in the slow lane. The tradeoff there is that a lightweight Jeep will break less parts, so it works out. Mine weighed about 2900 pounds, it would actually roll a few smoke trails off those 36s if I launched it at the normal 4K RPM on pavement, and it would cruise right past 100mph on the street. Now granted, it was no hardcore trail Jeep, but I could still do pretty well out in the woods if I dropped it in low-range.

[guardian7]

Let me just start off by saying that every Jeep is different. I don't know why that is but they are.... well, except for the fact that they will just empty every pocket of money that is. After performing all the mentioned mods on my 2.5L in my opinion the 4.88's was the biggest gain but costly, my second best gain was a 4.0L throttle body swap which was cheap and easy. All the small mods do add up to some real gain but nobody ever does them all at once to get a good before and after comparison. The Flex-a-lite E-fan is a nice mod but dont use the straight blade because they are very loud, use the S-blade models!

[woodie92]

Originally Posted by Bounty__Hunter

I wouldn't necessarily spend the time and money to regear at 31" tires, but definately would at 33". I run stock 4.10 axle gears with 33" tires and, with many performance mods, it does okay. Of course regearing will get you the most noticable performance improvement.

my 92 2.5l is running 33's and stock gears, i did rhe 4.0 throttlebody swap.... helped greatly on take off lag. with danna 30/35 what do u think is best gear? gonne look into that after i do tb spacer and k&n

[jfrabat]

My understanding is that the best gearing for a 2.5L and 33's is 5.13, but the highest you can go with the D35/D30 axles is 4.88 (which should be about the same as a stock setup).

[woodie92]

so are there easy axle swaps? pocket book friendly options!

[guardian7]

Axle swaps can be expensive for a TJ, many axles are not setup with the proper mounting brackets. If you have a YJ it may be easier to find a dana 44 or ford 8.8 which can drop in without much work or cost because there already setup for leaf springs. These axles can be found in junk yards fairly cheap and and often have the same lug bolt patterns.

[Bounty__Hunter]

4.88 would work well for you with 33" tires, and the engine mods help a lot.

I run 5.38 gears and 35"/36" tires (on/offroad) and it's geared just right for the 2.5L and 5spd. Of course I've added a perf. cam and a bunch of other engine mods.

BTW I've got 62mm throttle bodies (7mm larger than the 4.0L and no taper) for sale if anybody is interested. Only $85 + a refundable core charge. Great improvement in power and throttle response. You'd be surprised how much the throttle plate has to move in a stock throttle body before air gets past it, due to the taper beneath the throttle plate.

[jfrabat]

Originally Posted by hummer-recovery-inc

If jeep could've gotten more ponies from it, they would of.

You mean in the same way that Jeep decided to sell the 20 gal tank as an option when all tanks in YJ's were really 20 gal? You make the 2.5L too good, and the 4.0L sales go down...

Originally Posted by hummer-recovery-inc

Adding three ponies is not worth your time nor is it noticeable, assuming you actually get them.And you may even lose some low-end.

Adding 3 ponies is about a 2% power increase for the 4-banger.. Do 5 mods that ad 3 ponies a pice, and you got a 10% power increase from stock. And THAT is noticeable.

In my case, I did the e-fan conversion, put a bored out 62mm TB, a 62mm TB spacer, improved the ignition system with an Accell coil, LiveWires, and Performance Distributor rotor and cap, gapped spark plugs to .45, installed 19# Bosch Ford Mustang yellow-top injectors, put in rocker rollers (1:1.6), use only synthetic oil, K&N filter in stock box, Gibson Cat-BAck system, and removed the 2 restrictions for air of the stock box, and not only do I feel the power difference (I run 31" tires, and when going up some grades that used to require me to downshift to 4th, I can now cruise over them in 5th), but my mpg improved by almost 1 mpg. And I have not lost any low-end torque.

Originally Posted by hummer-recovery-inc

Your 2.5l wiil serve you well, leave it alone and regear your axe's.

I do grant you that the best performance gain will come from regearing, but that is also the most expensive mod.

All the mods listed have run a little under 1K; just regearing will be around that much (between labor and parts). Combined, though, the little 4 banger will really hold its own with some of the tired 4.0L out there (granted you, not the ones that are in shape).

Felipe

[mrmopar02]

Hey, do what I do when I need more power when I have a slow vehicle, just strap a turbo or nitrous to it.

[dave123]

If you have an electric fan, I would put an override switch in the dash so that you can turn it off when going through water. Seems like it would be a problem otherwise..