Giving a 21 year old motor some more pep.

[MrSimon]

My "new" 87 MJ doesn't have near the pep as my 96 XJ, and the XJ has 40K more miles. Maybe the difference is that the XJ has the HO, or maybe its because the MJ is almost ten years older ;)

 

So, I'm looking for a list of thins to do to my "new" MJ to help it run better and have a bit more pep.

 

All I know to do is a general tune-up (plugs, wires, cap, rotor, fluids, filters), but I'm sure there is more I can do.

[Pete M]

gears, gears, gears, gears, and... um... oh and gears. :D

[Eagle]

You really can't compare the two engines. Let me repeat: You REALLY can't compare the two engines.

 

The Renix 4.0L is a low-RPM, torque engine. The '87 was the first year for it, and it actually produced less torque and horsepower than the '88 - 90 but I still haven't found out what they tweaked in '88. However ...

 

... The '91 and up HO engine raised the torque peak from around 2200 RPM to well over 4000 RPM. It also changed the cam and the manifolds. The result is that the HO breathes freer and revs higher, so it feels like it has more "pep" -- because most Americans equate revving through the gears with "pep." The factory gave us what we wanted.

 

Except for me. I prefer torque. The natural shift point for a Renix 4.0L engine is about 2,000 to 2,200 RPM. The natural shift point for an HO is around 3,500 RPM. If you want your '87 to run like an HO, you'll have to cam it.

[james750]

You really can't compare the two engines. Let me repeat: You REALLY can't compare the two engines.

 

The Renix 4.0L is a low-RPM, torque engine. The '87 was the first year for it, and it actually produced less torque and horsepower than the '88 - 90 but I still haven't found out what they tweaked in '88. However ...

 

... The '91 and up HO engine raised the torque peak from around 2200 RPM to well over 4000 RPM. It also changed the cam and the manifolds. The result is that the HO breathes freer and revs higher, so it feels like it has more "pep" -- because most Americans equate revving through the gears with "pep." The factory gave us what we wanted.

 

Except for me. I prefer torque. The natural shift point for a Renix 4.0L engine is about 2,000 to 2,200 RPM. The natural shift point for an HO is around 3,500 RPM. If you want your '87 to run like an HO, you'll have to cam it.

 

This would explain a lot to me. ive been wondering why it has seemed not so "peppy" compared to the other xjs we have.

[comanche87]

if you want more power, get an aftermarket exhaust and get a throttle body spacer. since its a 3 bolt housing. here is the web site for it. its the only one i can find for the 3 bolt throttle bodys. http://rustysoffroad.com/mm5/merchant.m ... de=PER_AIR

 

i did that to mine and i feel more power from it. hope this helps. mike.

[comanche87]

oh yea. and a k&n air filter too. mike.

[89eliminator]

IMO i wouldn't waste the money on a throttle body spacer. i think the easiest "engine" power to gain is from a new hi-flow exhaust (header, cat, muffler), intake, and injectors.

[FxRacing282]

i would clean up ALL filters, do exhaust, tb spacer can't hurt it in theory, but a good cleaning and pouring water down the tb to clean up the valves isnt a bad idea, as long as you don't put in too much

[Pete M]

The first thing I would do is a good cleaning and keeping up with regular maintenance. If the cat, the fuel filter, or a sensor is not up to specs, that needs to be addressed before any aftermarket parts are even looked at.

 

Then I would do gears. :D Gears, gears, gears, gears, gears. Best bang for the buck you could EVER do. The right gears can make a 2.5L feel peppy and help a tired 4.0L smoke a v8 car.

[mvusse]

I can see the theory behind adding a throttle body spacer if you have throttle body injection. But what is it supposed to do on a 4.0? They use multi port injection.

[BLHTAZ]

and injectors.

To many people forget about these. Big source of power loss, mpg loss etc, etc... Click here for more info on injectors. I have personally put them in 3 vehicles now, my '88 MJ, my wifes '95 YJ and a friends '97 TJ.

 

Then I would do gears. :D Gears, gears, gears, gears, gears. Best bang for the buck you could EVER do

Definitley agree with Pete...you will notice more gain from this then anything. It is the most expensive option, but this is one of those "get what you pay for" things ;) .

 

I can see the theory behind adding a throttle body spacer if you have throttle body injection. But what is it supposed to do on a 4.0? They use multi port injection.

It only helps at higher RPM's as it is an air flow thing. It helps get a more directed & consistent air flow to the engine so it is not really noticeable all the time on these engines.

[Pete M]

 

Then I would do gears. :D Gears, gears, gears, gears, gears. Best bang for the buck you could EVER do

 

Definitely agree with Pete...you will notice more gain from this then anything. It is the most expensive option, but this is one of those "get what you pay for" things ;) .

 

 

Not necessarily the most expensive. I regeared my 90 for less than any aftermarket exhaust system. :D Heck, you can pick up a whole XJ for cheap and haul the remains to the scrap yard to offset the cost.

One caveat, please don't ever sink money into regearing a Dana 35. Grabbing a junkyard one is acceptable. :thumbsup:

[BLHTAZ]

True...I was thinking of actually paying to have the current axles done :cheers: .

[Eagle]

I can see the theory behind adding a throttle body spacer if you have throttle body injection. But what is it supposed to do on a 4.0? They use multi port injection.

It only helps at higher RPM's as it is an air flow thing. It helps get a more directed & consistent air flow to the engine so it is not really noticeable all the time on these engines.

This brings to mind one other difference between the Renix and HO versiuons of the 4.0L engine -- the HO has a larger throttle body bore. That means it flows more air at higher RPMs. I don't know how the original poster drives or what RPM band he spends his time in when looking for "pep," but the smaller bore is noticeable for those who like to live in the upper end of the power band. You can but bored-out throttle bodies, or you can grab a TB from a junk yard and hone it out yourself. It might make a difference, and it's an easy, bolt-on change. (Once you get it bored out, that is.)

[Randy in Maine]

I would first do a compression test properly (warmed up engine, wide open throttle, fully charged battery) to see what is happening in there right now. Then I would do the squirt of oil in there and repeat the test to see if that changes anything.

 

You may need to follow it up with a leak down test to see just where the compression is leaking out from.

[robfg67]

Pete, what specifically do you mean by gears? Are you recommending a rear axle upgrade to the Dana 44?

[Oizarod115]

Pete, what specifically do you mean by gears? Are you recommending a rear axle upgrade to the Dana 44?

 

nah, he means like, go deeper or go home

 

the stock 4.0-5speed combination used a 3.07 gear an upgrade to a 3.55 or 3.73 with the same size tire will give you more "umph" off the line and raise the rpm to a better range for power.

[Pete M]

Well, yes to the 44. Or an Explorer 8.8, or an 97 XJ 8.25. Spend your money on something that's worth investing in. Heck, you might even be able to buy the axle with the better gear ratio already in it.

Being a 4.0L/auto, your MJ likely has 3.55 gears in it. Essentially that mean that the driveshaft spins around 3.55 times for each rotation of the tires. Going with a lower (numerically higher) ratio of gears will give you moor grunt. What you get depends on what you want out of your truck. Since you're looking to get a lift and bigger tires, I'd go hunt for 4.10 geared axles. Explorer 8.8s can be found with 4.10s, or you can get the aforementioned 44 or 8.25 (in which 4.10s are rare but not unheard of) and pay to get it refurbed and regeared. With the front axle, you're pretty much limited to the Dana 30, but they are out there with 4.10s in them. Oh, and for reference, it can cost $300-400+ to regear one axle (that's parts and labor), that's why all the reference to junkyard scores. :D car-part.com is a good junkyard search engine.

A common trick is to get your axles, get them ready to go, and then swap them in when you do your lift. Less work that way. :thumbsup:

[Mattpioneer]

I was going to ask the same quisten about the gears yall were talkin about. what cam should i go with that is resonably inexpensive and still work killer. i have a stock 88 4.0 cam

[robfg67]

I have a stock '89 with the 4.0 & 5-speed. I have no plans to lift it, but would like to gain more power. What kind of gear change would you suggest?

 

Lastly what would the impact on MPG be?

[Eagle]

I have a stock '89 with the 4.0 & 5-speed. I have no plans to lift it, but would like to gain more power. What kind of gear change would you suggest?

 

Lastly what would the impact on MPG be?

My '88 XJ 4.0L 5-speed is stock with 3.07 gears. I get around 21 to 23 MPG highway.

 

My '88 MJ 4.0L 5-speed has been swapped to 3.73 gears and I am currently running stock tires on it. Highway mileage is typically 19 to 21 MPG, with the tailgate up and no tonneau cover.

 

The 3.07 gears actually leave the 5-speeds cruising well below the engine's torque peak at legal highway speeds, which is not ideal for efficiency/economy. 3.55s or even 3.73s will put you closer to the "fat" part of the torque curve at highway speeds, and that's really where you want to be running.

[robfg67]

Eagle, is a gear swap-out something a modest wrench can handle?

[Randy in Maine]

Does anyone here have a dyno sheet for a stock 4.0L engine? It might be interesing to look at.

[Eagle]

Eagle, is a gear swap-out something a modest wrench can handle?

I know some guys who do it themselves. I don't. The "gotcha" is that you have to have the pinion depth and the backlash set perfectly so the gear mesh pattern is correct, or you get noise and rapid wear. The "right" way to do it is to use a fancy gauge set to measure the pinion depth. I can't afford that tool. The shade tree method is to use white marking compound and "read" the gear engagement pattern, then adjust as necessary. That's what my friends do.

 

My mentor is a long-time stock car racer, shop foreman, and later service manager for the AMC/Jeep dealership where I used to buy all my cars (until new owners booted my friend because they wanted to bring in one of their cronies as service manager). He did all my gear work. Once he didn't have access to the shop's gear setting gauges he refused to touch gears because he felt is was too much of a chance to mess up and expensive set of gears.

 

So my position is, if Joe won't touch it ... I ain't gonna touch it. But the reality is, it ain't rocket science. If you're careful and you're willing to learn how to "read" the gears, I'm sure you can do it. Best approach would be to degrease, mark, and read the old gears before you take anything apart. That'll show you what they should look like. Then you just aim to duplicate that pattern.

[jtdesigns]

Has anyone swapped the HO head on the renix block?? How hard is that to do?