Regearing my MJ

[summerinmaine]

I have an 87 4.0L with a BA10/5. Dana 44 in the rear, 30 in the front.

 

So I assume I have 3.07 gearing, right? Fifth gear is virtually useless, even on the flats. I've gotten it into 5th on a downhill, but have no real reason to do over 70-75mph in the rig (I go to Baja or the desert, then everything is dirt track).

 

What's the best (i.e. reasonable cost) way to re-gear to boost the performance a bit? What are my options for gear-sets? I've heard of 3.55, 3.73, 4.10 and 4.54.

[FxRacing282]

3.55's will do you good if you want a lot more go to 4.10 but your top speed will go down the drain

[Pete M]

Plenty of vehicles on the road today came with 4.10/4.30/4.56 gears. Your top speed won't suffer as much as logic seems to dictate. My Dakota has 3.92s and even with the 28" tall tires it has no trouble doing 90.

 

What size tires do you have? If they are 30-31s, I'd have no problems suggesting 4.10s.

[JohnQ]

How hard is it to install new gears?

[Eagle]

The tire size you run will determine what your optimal gearing is, although if you can't use 5th gear at 55 to 65 MPH in level terrain, you must have a very sick engine, because the '87 4.0L has a torque peak that's down around 2000 or 2200 RPM and a 65 MPH cruise in 5th gear is right in the fat part of the torque curve.

 

Nonetheless ... 31" tires with 3.73 gears is exactly the same overall final drive ratio as stock tires with 3.55 gears. I originally bought the 3.73s for use with 30" tires, but ended up running 31s instead. It's quite driveable, but 4.10s would have been better. If you run anywhere near stock tires (i.e. 225/75 or 235/75) 3.73s will be a good compromise between performance and fuel mileage.

[DirtyComanche]

The tire size you run will determine what your optimal gearing is, although if you can't use 5th gear at 55 to 65 MPH in level terrain, you must have a very sick engine, because the '87 4.0L has a torque peak that's down around 2000 or 2200 RPM and a 65 MPH cruise in 5th gear is right in the fat part of the torque curve.

 

 

I have to agree. I never understood the 'I can't use 5th gear' guys. When I DD'd my MJ, I used it all the time. That's where the engine LIKES to run.

[Pete M]

How hard is it to install new gears?

 

It takes a few special tools and a whole lot of patience. You essentially have to completely and properly assemble the entire diff in order to check the gear pattern and guess how you should move the shims around to get the pattern to be good. Then disassemble the whole thing to move the shims, the reassemble and check again. Then repeat until the gear pattern looks good. It's part science and part magic and it's one of the few things I would gladly pay a pro to do for me. If you do it wrong, you can waste your new gears or even the whole axle.

[HOrnbrod]

The tire size you run will determine what your optimal gearing is, although if you can't use 5th gear at 55 to 65 MPH in level terrain, you must have a very sick engine, because the '87 4.0L has a torque peak that's down around 2000 or 2200 RPM and a 65 MPH cruise in 5th gear is right in the fat part of the torque curve.

 

Nonetheless ... 31" tires with 3.73 gears is exactly the same overall final drive ratio as stock tires with 3.55 gears. I originally bought the 3.73s for use with 30" tires, but ended up running 31s instead. It's quite driveable, but 4.10s would have been better. If you run anywhere near stock tires (i.e. 225/75 or 235/75) 3.73s will be a good compromise between performance and fuel mileage.

 

Don't know how long ago you bought the 3.73s Eagle, but I looked into buying them when I changed gears - no one had them. Been back ordered for a year or so. So I went with the 4.11s, and glad I did. Worked out well, and gives a bit of room for tire expansion.

[Pete M]

I have to agree. I never understood the 'I can't use 5th gear' guys. When I DD'd my MJ, I used it all the time. That's where the engine LIKES to run.

 

With 150k on the ticker, my 4.0L '88 was fast and peppy from 0 up to 80mph with the 28" tall tires I bought it with. Then I got 30s and cursed the day. It sucked at stop lights and above 65 on the freeway. Swapped to 3.55s and it was like a whole new truck again. :D

Combining 3.07 gears with a manual transmission was a mistake.

[DirtyComanche]

Regarding gear setups:

 

I wouldn't call it horrible. I did a toyota one last weekend. Okay, it was a big PITA and consumed a lot of time. Had the diff been bare it would have been way easier since most of the time was wasted trying to get the old pinion out without a press...

 

 

Anyways, the only trick to a gear setup is buying extra bearings or pulling off the old ones to make setup bearings. Then having the time to play with it. You can get a good guess for the first pinion depth, then quickly determine if it is right or wrong.

 

 

You do need special tools though. A couple bearing pullers, a dial indicator for gear lash and a base for it, a couple brass drifts and bearing drivers, and a torque wrench or 2.

[summerinmaine]

The tire size you run will determine what your optimal gearing is, although if you can't use 5th gear at 55 to 65 MPH in level terrain, you must have a very sick engine, because the '87 4.0L has a torque peak that's down around 2000 or 2200 RPM and a 65 MPH cruise in 5th gear is right in the fat part of the torque curve.

 

Nonetheless ... 31" tires with 3.73 gears is exactly the same overall final drive ratio as stock tires with 3.55 gears. I originally bought the 3.73s for use with 30" tires, but ended up running 31s instead. It's quite driveable, but 4.10s would have been better. If you run anywhere near stock tires (i.e. 225/75 or 235/75) 3.73s will be a good compromise between performance and fuel mileage.

 

My tires are stock (235/75/15) though I might decide to increase a bit at a later date (I have a 4" lift). The engine isn't sick, exactly, just tired. It has ~300K on it AFAIK (but 155# compression across the board). I can hold 65 in 5th on level ground, but any gradient at all starts to show (and since I don't live in fly-over country, gradients are everywhere :brows: ).

 

So an engine rebuild is in the near future (and I'll probably do a stroker), but I'm trying to approach things systematically, knowing that I'll want gears, tires and engine rebuild in the not-too-distant future. Just wondered if re-gearing could pep me up some until the engine craps out on me.

[rockhardzj]

Okay, it was a big PITA and consumed a lot of time. Had the diff been bare it would have been way easier since most of the time was wasted trying to get the old pinion out without a press...

Tip:

If you don't use a press, put the nut about3/4 back on. Get out BFH, and give her a few good smacks in the face, and see how she likes it. The gal will be looser than Paris Hilton by the 3rd hit. :D

[HOrnbrod]

Okay, it was a big PITA and consumed a lot of time. Had the diff been bare it would have been way easier since most of the time was wasted trying to get the old pinion out without a press...

Tip:

If you don't use a press, put the nut about3/4 back on. Get out BFH, and give her a few good smacks in the face, and see how she likes it. The gal will be looser than Paris Hilton by the 3rd hit. :D

 

Paris Hilton LOL. Damn sure I could not do her any good. ;)

 

More better to stick a 2x4 between the nut and the BFH to soften her up first.

[DirtyComanche]

Don't think I didn't try that. I've done gears before. Somebody left this third out in the rain and she just plain didn't want to do anything for me.

 

 

I used an 8lb sledge and wound up mushrooming the old pinion so it was about 50% wider than it used to be. That was doing over-head blows.

[HOrnbrod]

Don't think I didn't try that. I've done gears before. Somebody left this third out in the rain and she just plain didn't want to do anything for me.

I used an 8lb sledge and wound up mushrooming the old pinion so it was about 50% wider than it used to be. That was doing over-head blows.

 

Yeah, had the same problem on my third attempt (after raod testing) to remove the pinion when I did my recent regear. Had to use overhead blows too. Not a 50% mushroom, but maybe 25%. BF impact wrench w. frequent cool-downs did allow me to get past the expanded threads and do the correct torque finally on the nut though.