Advice on gaining more horsepower on the 4.0

[shortee1fourtee]

Hello all, and thanks in advance for any suggestions or help on the subject. I’m sure I speak for more than just myself when I say at this point in time, Money is TIGHT! lol As much as I would love to build a brand new engine overhauled and filled with all the goodies available, I just can’t at the moment. But I am looking to squeeze in a few more horses in the ol’ straight 6 if possible, and who else better to  ask than the Comanche club family ! Was curious if there are any interchangeable parts I might could find at places like pull’a’part, maybe cylinder head for better performance, intakes, etc etc. 

[jpnjake]

91-99 4.0 cylinder heads bolt directly on and have considerably better flow, the 91-2006 intake manifolds are also better, 91-99 being regular high output intakes and the 99-06 being horseshoe with a bigger plentum 

99/2000 depending on the application was the start of the small exhaust port heads so for power I’d avoid those 

 

all this being said to put on a Ho intake you should have the head to go with it or your ports will not line up, and you will also need to adapt your tps and delete egr. 
 

Other things that’ll help is a Regearing of your axles or Bored out throttle body 

[shortee1fourtee]

Is there a swap for the rear end? Or is it something that’s I’m gonna need to tear down and rebuild myself? Either is fine, just curious.

[cruiser54]

CPS advance mod.

Have a new headpipe installed without the crush.

Index the distributor.

Freer flowing muffler/exhaust.

Bored throttle body. 

[jpnjake]

9 hours ago, shortee1fourtee said:

Is there a swap for the rear end? Or is it something that’s I’m gonna need to tear down and rebuild myself? Either is fine, just curious.

Depending on the application different Cherokee's and Comanches got different gear ratios 

here's a chart my dad made for it about 2 decades ago: 

 

Also a common swap for a rear axle is an explorer 8.8 and those come with 4.10’s pretty often 

[Pete M]

if money is tight, leave it alone and focus what funds you have on preventative maintenance.   Its a lot less fun, but a shiny new intake and bored TB won't get you home each day in your 35 year old truck.   

 

also, more horses will inevitably lead to more money spent on gas.   

 

but also, also, gears will give the biggest seat-o'-the-pants bang for your buck.   

[89 MJ]

Gears, a high altitude CPS (advances timing 5 degrees), and a bored TB. 

[shortee1fourtee]

Very true words Pete, but even with a few more horses, it still beats the old 81 Dodge d150 v8 I have also….man that things loves some gasoline. 😂  Also thanks cruiser for the tips. I’ve already been in the process of going through your list of tips. It’s a lot of help and much appreciated. Jpnjake, what year model explorers came with the rear end you mentioned? 

[shortee1fourtee]

Also I have a small issue at this very moment in time. Ordered a new intake/exhaust gasket for the 89 stock 4.0 due to mine was leaking and just old. But the dummy’s sent me one that was for a 99 to 06. Website says it’s not compatible with my truck but placing the old one side by side with the one they sent, I really can’t see much difference. I can return it and get my money back. But due to the holidays it will most likely be next Monday before I get the other gasket. Can this one be used? Like. I said the bolt pattern matched perfectly, and I really need to get it back together asap. 

[jpnjake]

2 hours ago, shortee1fourtee said:

Very true words Pete, but even with a few more horses, it still beats the old 81 Dodge d150 v8 I have also….man that things loves some gasoline. 😂  Also thanks cruiser for the tips. I’ve already been in the process of going through your list of tips. It’s a lot of help and much appreciated. Jpnjake, what year model explorers came with the rear end you mentioned? 

I think it’s something like 96-01 explorers have them, just make sure to google what they look like to confirm you have it, and it should have an axle tag on it to tell you the ratio on it, although sometimes they can be missing. 

[jpnjake]

This will also help with identification https://www.blueovaltrucks.com/tech-articles/identify-your-ford-truck-axle-from-the-door-sticker/

 

[shortee1fourtee]

2 hours ago, shortee1fourtee said:

Also I have a small issue at this very moment in time. Ordered a new intake/exhaust gasket for the 89 stock 4.0 due to mine was leaking and just old. But the dummy’s sent me one that was for a 99 to 06. Website says it’s not compatible with my truck but placing the old one side by side with the one they sent, I really can’t see much difference. I can return it and get my money back. But due to the holidays it will most likely be next Monday before I get the other gasket. Can this one be used? Like. I said the bolt pattern matched perfectly, and I really need to get it back together asap. 

anyone can confirm whether or not I can use this?

[cruiser54]

I don't think you can use it. You'll have vacuum leaks. 

[Pete M]

another option for rear axle upgrade is the 03-07 Liberty which has disks and like 99% of them will have 3.73 gears. 

[shortee1fourtee]

Well thats no good cruiser54. I was really hoping to catch a break for once, well I guess I will return it and order another. Thanks for the info . And hey Pete, was wondering if you knew what came with the 05 Jeep Grand Cherokee with the hemi in it? Just curious, because my friend has one he just pulled the engine from for his truck. So I could get anything else off of it for near nothing. 

[Pete M]

wrong bolt pattern. 

[Eagle]

On 4/6/2023 at 11:52 PM, shortee1fourtee said:

Hello all, and thanks in advance for any suggestions or help on the subject. I’m sure I speak for more than just myself when I say at this point in time, Money is TIGHT! lol As much as I would love to build a brand new engine overhauled and filled with all the goodies available, I just can’t at the moment. But I am looking to squeeze in a few more horses in the ol’ straight 6 if possible, and who else better to  ask than the Comanche club family ! Was curious if there are any interchangeable parts I might could find at places like pull’a’part, maybe cylinder head for better performance, intakes, etc etc. 

 

I'm tempted to ask why you want to throw money away in search of more power if money is tight, but that's your call.

 

However, I WILL ask what you mean by "power." Look at a torque/horsepower graph for any internal combustion gasoline engine, and you'll see that the torque curve usually has a peak somewhere around 2500 to 3500 RPM, after which it flattens or starts to fall off. The horsepower curve, on the other hand, keeps climbing right up to redline. So the simple answer is to run the engine at higher RPM (which means either gears, or don't use overdrive) if you want more power.

 

But most of us don't want to drive around at 5,000 RPM on the freeway. It's noisy, and it wears out the engine pretty quickly. When most people ask about power, what they really mean is that they want better acceleration and low- to mid-range performance.

 

What are you hoping to accomplish?

[shortee1fourtee]

Good point Eagle. I would like better acceleration and low to mid-range performance for sure. I guess I’m mainly looking at my options, different routes to take, and seeing what interchangeable parts are out there that will improve performance at a low cost. Money is tight, but gathering all the info and advice from everyone here at CC will help me come up with a game plan and point me in the right direction. I’m a knowledge junky, so to speak,🤓. 

[Pete M]

as I always say: research is free!