Getting more horsepower out of 2.5L -
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post #1 of 25 Old 04-29-2013, 03:18 PM Thread Starter
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1993 YJ Wrangler 
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Getting more horsepower out of 2.5L

Alright. I know there are countless threads on this topic. But, I thought I'd start a fresh thread thats a little more organized. I was wondering what the best (cost efficient) ways to gain a little more horsepower in the 2.5L 4cyl engines. Specifically I have a 1993 yj. Only things I have done to it engine-wise is a new radiator and fuel pump to get it back in running order. The best bets I've heard are

2.5L-4.0L Throttle body swap
electric fan
open flow intake
increased flow exhaust
increased flow headers

Please tell me what you think. remeber talking specifically 2.5L motors, no engine swap suggestions because I wanna keep the four angry squirrels for as long as possible. Just would like a little more ponies.

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post #2 of 25 Old 04-29-2013, 03:20 PM
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there isn't much your best bet for biggest gain is gears

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post #3 of 25 Old 04-29-2013, 03:31 PM
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if you have good cylinder pressure and your engine is tuned up...that's about as good as the 2.5L gets, so don't waste money on headers/intakes and such things.

I'm afraid horsepower gained versus the $$ spent on parts isn't very good on the 4cyl engine.

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post #4 of 25 Old 04-29-2013, 03:31 PM
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Regear x2

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post #5 of 25 Old 04-29-2013, 03:59 PM
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In order of cost:

Engine swap

Even with the proper gear ratio the 2.5 isn't quick, but it is reliable and off road has more than enough torque for trail riding or rock crawling. Its a pretty good little motor, just don't plan on running 80 through the mountains easily.

Everything else is a waste of money IMHO if you're doing it solely for HP.

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post #6 of 25 Old 04-29-2013, 04:24 PM
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I can go everywhere the rest go in the club which is with all the new Jeeps I am usually the only one on leaf springs when I show up. Always the oldest Jeep there. I am a little slow on the hills but travel is not an issue. Off road the 2.5 is great and haven't had any troubles but yes on the road it is weak but were both the same old and slow. You can get some HP gains out of some of the mods and if you can tinker you can do it. Costs a few bucks but can be done. Check that is all they do over there might find some useful threads.

89YJ 2.5L TBI, AX-5, NP231 SYE, Ford 8.8, Yukon Grizzly, Tom Woods Shaft, 33" BFG T/As, 4" lift, Warn VR10000s.

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post #7 of 25 Old 04-29-2013, 05:43 PM
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This horse is dead friends. The 2.5l was designed for longevity and reliability. It's the little engine than can. It will never be a 6 Cyl, but it's pretty damn good for what it is.

The most commonly referenced article for 2.5L engine mods for HP is this one:

It's fairly well established that, with the 2.5l, the more extreme you go on the off road scale (ie. larger than 30" tires) the less you will get out of the engine for on road use. If you have time and money, or just like to tinker, the mods in that article seem like reasonable paths to take.

Short of looking at the set up on the whole, factoring in tire size, gearing, weight (ie. hard top, Spare tire swing outs, etc.) you probably won't get the results you are looking for.


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post #8 of 25 Old 04-29-2013, 05:46 PM
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I've done the throttle body upgrade, matched bored the intake, installed an cold air intake, and four port fuel injectors. I can say that I feel a little seat of the pants improvement. But if I would have not done the mods on a budget, ie craigslist, ebay, Jeep forum sales, I wouldn't be able to say it was worth it. I'm into the mods maybe 145.00 bucks.

But I'm happy with my 4 cylinder and have really not wanted more power on the trails. And it's not a daily driver so no hi-way time or speed needed.

good luck,

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post #9 of 25 Old 04-29-2013, 05:48 PM
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Best way to get more power out of the 2.5L is to invert it to 5.2L.

All about that unibody life.
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post #10 of 25 Old 04-29-2013, 08:07 PM
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Any HP gains you get will all be in the upper RPM range. Totally useless for an off road vehicle.

Gearing will get you good off road crawling. If you want to run with the big dogs down the highway, you will have to go 4.0 or a V-8 conversion, sorry.

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post #11 of 25 Old 04-30-2013, 06:34 AM
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Totally useless, come on. That is way harsh. The 2.5L is a capable engine and most places you are trailing you don't want to power through anyways. The 2.5L is a good engine and I would listen to the suggestions here and try
All the suggestions except Old4X that is. I dont think he has had his coffee yet this morning! Good mornin' Old4X!
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post #12 of 25 Old 04-30-2013, 08:40 AM
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I would say leave it alone. The 2.5 is as reliable and easy to work on as they come. I pull a 17' fiberglass glasstron boat all over the place with it. Has no problem getting it in and out of the water as well. Sure I'm not flying down the road but it goes ok and I don't pull on the highway. The boat is a little to big for high speed with such a small wheelbase. Even though I'm going slow I still have a smile from ear to ear, but I think that's because it's so damn fun to drive at any speed.


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post #13 of 25 Old 04-30-2013, 10:04 AM
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Like others are saying, gear reduction is your friend. You just have to have a balance in your setup, gears vs tire size.

I just got back from 5 days in the 2.5L YJ. Tennessee Mountains Jeep Jamboree. 2 days on the road, 3 days on the trail.

On the road, my 33.3"x10" tires in 4th gear on the highway, with 4.88 gears in the diffs was happy running 60-65mph all day at 3k rpm. Up the mountains and down the mountains. This is the first setup I have had on my YJ that worked so well on the road.

On the trail the 33.3" height is great for clearance, you are only giving up a little ground clearance compared to guys on 35's. But the 4.88 gears and 4-lo 1st gear is enough to spin the 33's like a tractor on obstacles. You don't run out of torque.

I also added the Muncie transmission, SM465 with granny low 6.8:1. All I can say is wow, having the ability to grab that low gear on technical things like rock gardens is just perfect. A 4:1 transfer case kit would give you similar capability. Gives you the ability to apply maximum torque and power, but at a crawling speed. The engine screams, but the vehicle barely moves Gives you a lot of control, especially on descents.

1995 YJ "Mabel", 2.5L Muncie SM465 np231j 4.88's HP30 Lockright JK 44 Detroit 33s CJ clip

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post #14 of 25 Old 04-30-2013, 10:20 AM
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Agreed, the 2.5L mas more than enough power for what it is. It doesn't need to be pushed to get to highway speed, you only need to push it just as hard as any other 4 cylinder its size. Mine did 65 no issue whatsoever (that is, until all of the lifters started to go south and it lost most of its power- that was one of the PO's fault, not mine), I could hold a reasonable 60mph going up a long 4.5% grade on the highway if I kept my foot in it a bit in 4th. It likes to spool to 3500rpm getting onto the highway, runs around 2800rpm or so just fine in OD.

I never had issue on the trails. My buddies always seemed to get stuck (more balls than brains), and I never had issue pulling them out. Has plenty of torque, hell it would pull uphill easily at idle in 4 low.

All about that unibody life.
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post #15 of 25 Old 04-30-2013, 11:13 AM
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He biggest gain

I have tried and done all the mods except boring, and then I did this I went with the Mile marker Hub conversion, and it is a real improvement the little engine will cruise very well and the HP gains are very noticeable. I run 33s also please do the Speedo gear change, this will help you better Quantify any results. of course trail issues were never a problem so once locked the Jeep performs very well.

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