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Guest Message by DevFuse

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4WD ring & pinion in 2WD?

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10 replies to this topic

#1 Guest_Brettm57_*

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Posted 12 August 2003 - 06:34 AM

Will 4 speed standard 4wd ring and pinion gears fit in a 2wd 3 speed automatic tranny? I want to gear a 2wd auto tranny as low as I can. From what research I've done, the 4wd standards have the lowest geared ring and pinion. Also, I don't have a very good picture of it, but the service manuals I have show what they call transfer gears that send the power from the back of the transmission to the pinion shaft. Can any swaps be made to lower the gearing here? In case you haven't seen my other posts, I'm planning on building a '85 2wd auto hatch into a 1/4 mile bracket racer. This car will see no highway use at all, so I can gear really low here. Ideally, I would like to end up with the engine turning about 5000 rpm in high gear when the car crosses the finish line.

Thanks in advance for your help.

#2 Guest_OH Noah_*

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Posted 12 August 2003 - 08:23 AM

Your car most likely has a 3.454:1 differential right now, with a 0.974 reduction gear.

In looking over my books, it appears you're seeking the 1.026 reduction gear from the '83-'86 4wd AT.

I've never done tranny surgery before, but it certainly looks like you could swap the 4wd reduction gears into a 2wd diff. The part#s are as follows:

Reduction Drive gear = 31450AA020 (38 teeth?)
Reduction Driven gear = 31448AA050 (39 teeth?)

Moreover, you could probably use the ring and pinion from a 3.7:1 car.

The 3spd autos in '87+ cars with the EA82 engine had a 3.7:1 diff.
The '80-'82 autos also used the 3.7:1 diff.

It's possible that some models (XT maybe) may have even come with a 3.9:1 differential that you could use.

You will need to use the ring and pinion from a 2wd 3spd auto though, as the pinion gears are different in other trannies. (The 4wd auto has a longer shaft on the pinion gear.)

So, in summary, if I'm right about what you've got then you're starting with a total front reduction of 3.454 x 0.974 = 3.364 and you could end up with 3.7 x 1.026 = 3.796 or maybe even 3.9 x 1.026 = 4.001!!

#3 Guest_UltimateRX_*

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Posted 12 August 2003 - 12:38 PM

The answer is no, the ring and pinion gear in the tranny are totally different even between a 2WD standard tranny and a 4WD tranny (4WD is longer and won't fit in a 2WD box), the auto pinion gear is way different. Since ring and pinion gears are match sets, they can not be mixed without major rework and shimming.

#4 Guest_Brettm57_*

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Posted 12 August 2003 - 12:38 PM

So you think the reduction gears will interchange between 2wd and 4wd? I take it they are both splined to the shafts? That's definitely a step in the right direction.
I thought of another possibility: Will a 3.900 ring and pinion from a standard 4wd tranny interchange with the 3.700 ring and pinion in a 4wd auto tranny? I'm thinking maybe install a 4wd tranny in my 2wd car, but leave the rear driveshaft off. There should be creative way to cap the end of the case so oil won't run out. I wouldn't think there would be much of a weight difference between a 2wd and a 4wd tranny. That would give me the 4.001 reduction you're suggesting.

Thanks for your help, man. I'm getting pumped on this project now!

#5 Guest_OH Noah_*

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Posted 12 August 2003 - 01:36 PM

<img src="http://www.fortunecity.com/silverstone/subaru/1/USMB/Differential1.gif" style="border:0;"/>

You can see above that there are different pinion gears for AT and MT and 2wd and 4wd. Part 1 is the 2wd AT pinion.

<img src="http://www.fortunecity.com/silverstone/subaru/1/USMB/AT_ReductionGear1.gif" style="border:0;"/>

I don't think that using a 4wd tranny would help you at all. It WOULD be more weight and you'd still need to swap internal parts to get what you want.

The way I see it, you want to get a tranny (or at least the differential section of it) with 3.7 or 3.9 gears in it. Then you can put the reduction gears (parts 3 and 9 above) from a '83-'86 4wd into it. It looks like this would require re-shimming the pinion gear, but I don't see any way around that.

#6 Guest_Brettm57_*

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Posted 12 August 2003 - 08:32 PM

Thanks for the pictures, Noah. That helps me out a lot. I'm definitely going to research this some more.

#7 Guest_OH Noah_*

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Posted 12 August 2003 - 09:59 PM

Don't mention it. I'm enjoying learning something new here. I always wanted to know more about the auto trannies.

That said, I just noticed that my FSM says you have to press the reduction drive gear in and out. Sounds like it'd be more of a project for a tranny shop than for a shadetree mechanic. Still, I think it should technically be possible.

#8 Guest_Brettm57_*

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Posted 13 August 2003 - 04:47 AM

That doesn't really bother me, I've got access to some big tools (press, etc.). I also have access to a pretty complete machine shop, so I have the potential of building some neat stuff.

I'm thinking about just keeping my eyes open for some junk tranies so I can take them apart and see what goes where.

Thanks again for the info.

#9 Guest_Brettm57_*

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Posted 13 August 2003 - 09:24 AM

Noah, can you tell me what transmissions #2,3, and 4 pinion shafts go in? I'm guessing #2 is 2wd std, #3 4wd std, and #4 is 4wd auto?

Also, I was looking in my '85 hatch owners manual, and it says the 4wd Brat auto has a 1.026 reduction gear and 3.454 R&P, and the rear end has a .933 reduction gear and a 3.700 R&P, which works out to about 3.5 total reduction. Are the transmissions you're taking about actually have a 3.700 R&P? Could you scan the page you're looking up the gear ratios on and post it here?

Thanks again for your help, I really appreciate it.

#10 Guest_OH Noah_*

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Posted 13 August 2003 - 11:09 PM

#2 is 2wd 4&5spd
#3 is 4wd 4spd
#4 is 4wd auto

My gear ratio info comes from my nearly complete set of '80s owner's manuals. I was only referring to front diff ratios since you were talking about a 2wd car. ALL 4wd automatics used a 3.7 rear diff. The rear reduction gear was used to match that 3.7 rear to whatever was used up front. Yes, I meant that the '87+ EA82 autos had 3.7 ratio front R&Ps, as did the '80-'82 EA81 autos. (From '83 on, EA81 and EA82 3spd autos are essentially the same tranny. The earlier autos had a different torque converter, but were otherwise basically the same.)

I'll scan some more stuff for you later. This sounds like a pretty cool project, and I'm happy to be helping.

#11 Guest_Brettm57_*

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Posted 14 August 2003 - 04:47 AM

You more or less answered my next question, are the EA81 and EA82 3 speed autos interchangable? Do the bell housings match up? If that's the case, it would pay for me to find a "87+ EA82 2wd auto, then find a 4wd auto to get the reduction gears I need. That would make about 3.8 total reduction. That's where I would like to end up. Running the numbers through my Excel program, that puts the car at about 95 mph at redline in high gear, pretty close to what I'm looking for. At that gearing, I don't think it will take a whole lot of engine work to get into the 16/17 second range on the quarter mile.

Thank you Noah.

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