Automatics and crawl ratio? (I'm planning my 8.8 swap)

[W_A_Watson_II]

Picked up a Ford 8.8 rear axle out of a 1998 Ford Explorer. It's outfitted with 3.73 LS gears, (code D4, a 3200# disc brake axle). I started stripping it, and opened up the case, and found the usual black stinky gear oil. No water, and not looking too bad inside. The gears all look good. There is however 0.050 in ring gear backlash (spec is 0.008-0.015), and about .012 in pinion fore aft play. Feels like almost no pinion bearing preload, so with the end play, I'll have to re-torque the pinion flange bolt. I'll also have to re-shim the ring gear to get it's backlash within specification. The limited slip clutches seems to be good and tight.

Now to some decisions and parts planning. I'll by new rotors, pads, calipers, hoses, etc... ( new proportioning vale and improved master cylinder) as well as of course clean and paint everything. I'll also cut off the old spring perches, as I'll need to realign them in both width and rotation to get the proper pinion angle. With staying with a spring under suspension, and not having to use pinion alignment shims, I'll only loose about 1/8" in rear height with the larger diameter axle tubes on the 8.8.

The question is, why 3.73's, why not 4.10's? My thoughts are with the automatic, and a 4.6L engine producing ~240 HP & 315 ft.lbs of torque (33% more HP & 43% more Torque than stock) I should have plenty of power and torque multiplication. The stock YJ with 3.07 gears and tires (215/75R15) ran with a crawl ration of 20.46:1 and at 55MPH the engine was turning about 2,048RPM. With the 3.73's and the 33" tires, the crawl ration will be 24.86:1 and at 55MPH the engine would be spinning about 2,088RPM. So even with out all my extra power I'll have a little better crawl ratio and slightly higher engine RPM. With a torque converter multiplier of 2 (single stator TC), my crawl ration would be 49.72 to compare to a manual setup. I've seen people say the best ration from as low as 50, to as high as 85, with a few wanting 100 or higher. My M has a ratio of 76.3:1. 4.1 gears would only give me a craw ratio of 27.32:1, while s3.73's with a 4:1 transfer case would yield 36.55:1 (very close to the effective ratio of my M).

So my thought is to stay with the 3.73's and if I need a slower crawl ratio, upgrade to a 4:1 low range transfer case. I'll also be looking to install a new set of 2" lift springs to give myself a little more ground clearance, I already have a 1" body, and 1/2" in boomerang shackles as well as liberal wheel wheel openings. I may even stretch the rear axle back about 1" with the new axle spring perches.

Thoughts?

Oh, also got the on-board-air compressor hoses made and installed. The compressor works great, and with the hand throttle with the engine setting at ~2K rpm, it supplies plenty of air. Leaks down from about 115 psi to 90 in about two hours, so I have a small leak some place. I still need to install the arming switch and wire (will change to a power rather than grounding wire), as well as getting a inlet air filter.

[W_A_Watson_II]

Wow, after 20'ish views no opinions or thoughts on crawl ratio in automatics?

[wdln]

I think you've just put far, FAR more thought into it than the average bear. Certainly way more math.

I wheeled with an automatic pushing 35" tires, an I6-258 and 3.31 gears for a number of years. Performance was rather anemic. At some point after my V8 swap I upgraded to 4.56's. I'm certainly glad I didn't do 3.73 or 4.10's, and I'm very happy with the on and off-road performance (although it's used mostly off-road). And I feel like the higher gear ratio is a little easier on the transmission. However, I had never broken an axle shaft or U-joint until I upgraded the gear ratio. I've done it twice since. In the magazines I've seen this referred to as "moving the fuse". Multiplying the torque you can send to the wheels is going to make a weak point that much more likely to break.

The other change I made (before doing the axle ratio change) was going from a 999 automatic to a 727. The research I did at the time said that the 727's first gear was slightly higher than the 999's. Between that and the large, heavy tires I was running, I chose to err on the higher side for axle gear ratio to balance out the other changes.

So looking at your post again, it seems like you've talked yourself into being happy with both the low and the high speed ends of your Jeep's range if you upgraded to 3.73's. If you're OK with the performance, I think you would also be much less likely to break stuff elsewhere in the drivetrain than if you went with 4.10's or 4.56's in your axles. I bet the difference between 3.07's and 3.73's will be rather subtle, but I get the feeling that you're not going to be doing anything crazy with this YJ, right?

[W_A_Watson_II]

Dave,

Good points. I'm now leaning more tot he 4.10's, but that's based on some parts availability and cost. I agree, moving the fuse will get me, and I suspect I'll be breaking front axle u-joints after the upgrade.

What, me do something crazy? Well, perhaps not too crazy with he YJ until I get the tire under it like I have on the M. B-)

[Jon Martin]

Don't forget that YJ's with 4 bangers got 4.10 gears from the factory. Probably cheaper to swap in than regearing your existing front axle.

[W_A_Watson_II]

Good point, so if anyone knows where one is (cheap) let me know.

[matt4wheels]

You can find gen 2 explorer 8.8's with 4.10s stock. Usually found in explorer sports.

[W_A_Watson_II]

Picked up an 8.8 out of an explorer for $200. Bearing seem a little loose, to much back lash play, and more pinion in and out than I think would be safe. Will re-build it, but still need to decide on re-gearing the front, or find a front 4.10 to swap in and re-gear the 8.8.