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Guest Message by DevFuse

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Turbo & 3.90 5spd D/R Q's

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10 replies to this topic

#1 Free Range

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Posted 02 October 2003 - 02:25 PM

Here is a few random questions for all ya'll turbo fanatics

1) Anybody, besides myself, running 3.90 5spd D/R in a turbo?
Moved from an RX tranny to improve torque, results have been amazing, except I have to walk into 2nd gear because syncros can't handle 10psi boost:headbang:

2) Along the same lines, has anyone put 3.90 gears into a full time tranny?
FWD is cool, good milage and massive torque steering, but running 4WD on dry pavement with a locked center diff on the ol' 3.90 D/R, grenades more axles than it's worth...

3) lifted turbo's, what ratio of tire size&width to axle breakage have people experienced? I.E. at what point does too much traction cause excessive axle damage.

#2 MilesFox


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Posted 02 October 2003 - 03:17 PM

i put a 3spd in an ea81 turbo, to replace the automatic. having the low gear will let you pull some quick rpms, i accidentally aqualled(not exactly, but broke loose) the tires doing a clutch drop in 2nd hi, i thought i was in 1st low!

anyway, the automatic wouldnt let the tires break free in mud, let alone shift right.....

the 3.9 gear is a lot more fun than a 3.7--i got a 5spd single range in my gl-10, and it didnt want to pull in the sloppies.

i had a 5 inch lift on the ea81, didnt affect the axles, the ea81 turbo automatic uses the same axle as the regular trannies.
when you lift a soob, the axles will be in the same alignment relative to the suspension, because its all a body lift.

if your car is already turbo, being it had(has) an rx tranny, or a single range 5spd, it will already have the s27(or 29? turbos have more)plines for the turbo axles.
if you are going to use a regular 5spd dual range, you will need to find the non turbo axles, or swap DOJ with the 25 spline

i know of board members who have swapped 3.90 gears into 3.70 gear LSD, and the rx 5spd is not much different than a normal 5spd, and i would suppose that the gears would swap into a an rx tranny without fitment issues!

for the tire question: I have a 5spd dual range with a n/a ea82 motor in my 83 wagon, had a set of 16 inch chevy rims drilled out with 205/55 tires, i broke the inner doj turning from a stop while getting on it, but that is because the boot had been gone for some time, the axle coming from a parts 88 wagon

#3 Dr. RX

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Posted 02 October 2003 - 05:35 PM

Yes, I'm running a 3.90 D/R on demand 4WD, I have a 3.90 VLSD rear diff out of a Legacy Turbo. This is in my 86 GL-10 Turbo 4WD Sedan.

I just picked up a steel work bench today and how have a place to mount my vises (one at each end) (man, that sounds a little kinky). Anyway, now I can do the 3.90 LSD conversions a lot easier.

#4 shanonsf


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Posted 02 October 2003 - 06:26 PM

ok fellas,
since we are the topic. I got an EA81T, 5spd D/R w/ ctr locker, and a 3.7 all waiting on the bench for motivation & answers.

is the 3.9 quicker/better than the 3.7? why?
is the 5 spd D/R w/ ctr lock more trouble (reliable?) than the effort?
I ask because I live in San Francisco, and gas is brutal and I got 4wd D/R if I need extra traction................everything is paved anyway. Would the current 4wd system be enough for rallyX since i'ts on dirt/grass anyway??? plus easy to find when blown..

trying to figure out if I should complete my RX-ification or

BTW- the KYB, sway bars, vented frt disc & rear disc conversion was worth it, so were the touring wagon seats:headbang:

rally on-
82 GLW Snow White

#5 Dr. RX

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Posted 02 October 2003 - 08:52 PM

If you are worried about gas milage, stick with the 3.70. Eventhough that 3.90 makes your car quicker off the line, it also runs at higher RPMs at freeway speeds. The center lock is not something you are going to use on a daily basis, only when tranction dictates it's uses, and since it doesn't snow much in central Cal on the coast, you probably won't use it that much. I believe the RX tranny also has a different set of gears then the normal D/R, giving it a close ratio gear box set. This is designed to take advantage of the power band. But for day to day driving, not that important. If you currently have a D/R, why not just have someone (or do it yourself) convert a 3.90 to a LSD?

#6 shanonsf


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Posted 02 October 2003 - 09:39 PM

I think i need to take a ride in an RX

hey RallyRuss:wave: ...anyone

ha ha ha


#7 baccaruda



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Posted 02 October 2003 - 11:58 PM

how hard would it be to put new synchros in my 88 RX tranny? and maybe convert it to 3.9 while I'm at it? :D

#8 shanonsf


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Posted 03 October 2003 - 11:15 AM


should I too consider converting the system to 3.9 instead of 3.7 before install? big difference?
what's the scope of the conversion?

quickness/torqueniess is very appealing for my rally X aspirations..........

what type of top end sacrifice will I be making.....?

bla bla bla

82 GLW (losta mods)

#9 Dr. RX

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Posted 03 October 2003 - 01:41 PM

My 86 Sedan gets about 21 to 23 mpg, I've heard of someone getting 30 mpg in a RX.

I can get second gear tire churp from my sedan on dry pavement, don't know about a RX.

#10 jmr052


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Posted 03 October 2003 - 02:03 PM

I was talking w/ CCR a few months ago about a diff and reduction gear swap. He said that the diff should be a direct bolt in replacement. He wasn't as sure about the reduction gear. We were talking about a regular D/R and the RX tranny, taking the RX to 3.9 gears. He also said that the swap isn't hard and that you don't need any special tools.

I have an extra of each and will do this when time and space allow. This was the impetus for the following thread I started a few weeks ago.

I wanted to know, with stock tires, what the difference will be between the gearing in the real world. I have done some of the math but the real world is always better.

'89 TTW, FT4wd D/R Diff lock, rear LSD, 5 little lugs w/ Outback rims.

#11 soobme


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Posted 03 October 2003 - 06:03 PM

I have an '85 turbo that I swaped the DR 5spd into with the 3.90 dif. and I love it!! I don't think I could come up with a better setup for what I use it for. I am runin' 27x8.50-14's and a welded rear dif. I just pull one of the rear shafts when I'm not wheelin' and that takes care of the bad street maners of the welded rear. ( I have little rubber caps that I put on the stubs to keep the splines clean) I think that the welded rear dif. is one of the BEST things I did for off road use. And if the roads get slik I can still use 4WD, it's just 1 wheel in the back pushin' and that still helps out a lot.
The rear shafts seem to be strong enough, but it's the stubs out of the rear dif that worrie me. I have yet to break one, but after seeing the "Subarubicon Gold" tape, I know that it's just a matter of time. But that's me, if YOU drive it eazy and don't "pull a Tony!" on big obsticals on the trail, I bet it would survive just fine.

WELDED DIFS FOR ALL!! :banana: :banana: :banana::drunk:

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