RS tranny on USDM rear

[rah1482]

Thanks to Patrick (CO VR4)and Ryan (RTS)for their help. Grilled my mechanic on your comments and here's his side ; Reason its ok to use the RS tranny on the USDM diff is that my rear diff has the same gear ratio as a JDM matic. That being the case, he matched the RS tranny with a JDM RS transfer case, thus making it compatible. He assured me further by noting that; my car has run 600 miles and nothing has happened to either my tranny or differential. If they were mismatched or going against each other, it won't even reach 100 miles. Makes sense ?

 

[bazeng]

rs's run the 3.9 ratio
usdm run the 3.5 ratio

 

[CO VR4]

The final drive ratios are the kicker. Either they match or they don't. There is a long thread on this board you can find if you search for it that will identify the tranmissions (and their final drive) by the tranny ID numbers. If you have a JDM market tranny with the different final drive, your front wheels will be driven by the tranny at, say, 3.9 revolutions per _______, while the rear driveshaft will spin at the same speed, but your rear wheels will turn at the final drive of the rear end, which is 3.5xx in a U.S. domestic market rear end. Thus, your front and rear wheels are driven at different speeds. You'll either be dragging the front, or pushing the rear. It will be only a matter of time until one or the other fails, and fails big. The only solutions if you have this problem are to change the final drive in the transmission, or find a RS rear end, so both match each other.

Whether you're in this situation or not is important to find out. You can check the tranny IDs, or you can compare the # of revolutions of the input shaft and the output shaft of the tranny, counting them carefully to see whether they match the RS final drive or the USDM final drive.

 

[belize1334]

My understanding is that final drive ratios come into effect before either the front diff, center diff, transfer case, or rear diff. That being said, what is important is that the transfer case (which uses a ratio of r to lower the rotational velocity of the drive shaft) is compatable with the rear diff (which uses a ratio of 1/r to get the speed back up for the wheels). So the final drive of the transmission is irrelovant. What matters is the ratio of the transfer case w.r.t. the rear diff.

Someone chime in here if my order of operations is wrong.

Incidentally, an easy way to check after the fact would be to put the car up on stands and place it in first gear at idle. Then just count the number of revolutions that the rear wheels make in a minute and compare that to the number of revolutions that the front wheels make. If they're the same, then you're gravy. The transmission might have scaled things up or down a bit but as long as the same thing is happening at all four wheels you won't burn out your diffs, which is of course the concern here.

 

^Nope.

The tcase ratio is the same for 3.545 rear diff ratio, and MOST of the 3.909 diff ratios. The Evo3 has a different one even though it has a 3.909 rearend, as it has a different front diff ratio. The tcase input shafts are different between the two diff ratios types, though.

You CAN use a RS gearbox with a US rearend IF it is an early RS version, eg 1987, 1988, 1989, and MAYBE early 1990, as they used the 3.545 rearend. You cannot use a late model or "Evolution" VR4/VR4RS(late 1990 to early 1992) gbox with a US rearend, UNLESS you change the front diff crownwheel and pinion to the US ones. A something to watch here is that both the US pinion, and the "Evo" pinion have 15 teeth, but are cut at different angles.

Basically, if its out of a 3bolt JDM VR4, you're apples...if it's out of a 4bolt VR4, you're not.

This is probably why, even though you have a RS box with a US rearend, you have had no problems as you have an early RS box.

 

[CO VR4]

That's what I said.

Go to the transmission ID list, and it will have tranny ID numbers for those that have the correct ratio and those which are incompatable without major modification.

 

Rs trannys will NOT work with our rear end gears. i drove on mine for almost 1.5 years (thousands of miles) because the previous owner didnt tell me nor know that he put an RS tranny in it. the guy who rebuilt the tranny recently said a good condition center diff can hold out for quite a while, then lock. mine didnt even blow up, but it temporarily locked. i found out the hard way driving on the highway, and ended up with the rear tires locked and going sideways through eisenhower tunnel at 80 mph. the RS transfer case is IDENTICAL to the usdm transfer cases. only 2g's and evo 3's are different, and their t-cases wouldnt make it work anyways. if your box is a true RS box with the RS final drive, you are in for trouble using the USDM rear end final drive. end of story. i have had people pm me about locking the center diff even with some early RS boxes, (88-89) as they had the different ratios as well. check the numbering on your gearbox.

 

[Wilfongtsi]

Sorry to drag this back up but I have a question, I lucked into a switchable trans and t-case(I didn't even know they were until the other day) anyway my question is when were these available on the jdm galants? I found out my 4WS, 4 bolt rear is 3.909, and the trans and t-case should match right? Its all getting put in fri. so please help, I tried searching for the tranny id link with no luck, and the tranny lists I have seen dont list the switchable ones. The code on my tranny is CIUBPH FC1235, Thanks in advance.

 

[Scientist]

Also ques

I have a JDM Vr4...not sure exactly what year but it has a 3bolt rear.
Got it without transfer case, drive shaft and rear axles.
Recently got those back. I bought a 4 Bolt rear diff to swap on...what part numbers should i look for to make sure i'm gonna be ok?

 

No, only some early and all Late RS tranny's have the 3.909 final drive. The switchable tranny's are the regular final drive from what i understand. Rarely did any RS have 4WS. is your rearend out of a USDM car? what car is it out of?

 

^Oddly enough, I bumped into someone yesterday who has a 4ws RS. His is an 1990 E39A RS with 4ws, but no ABS, no sound deadening, RS trim seats, windup windows, no rear wiper.

According to Ralliart, production option rules for the E38a RSes were hard and fast for homologation rules, but the as the E39a RSes weren't being offically rallied, you could option/de-option them.

But all that said, who really knows what the Japanese owners have changed after purchasing the car originally?

 

you are correct sir, rarely were any RS's built with 4WS, if they were, they went under the E39A designation, instead of the standard RS E38A. That might be the case only in late 89 models however. in 90-91, they were all E39A cars, but most didnt have AWS even then.

 

^Umm, well my is 1991 E38a RS, as verified with a Japanese CAPS. There are about 5 in New Zealand that I know of, and MaxPain also has a 1991 E38a RS in Finland(?).

 

[Scientist]

Quote:
No, only some early and all Late RS tranny's have the 3.909 final drive. The switchable tranny's are the regular final drive from what i understand. Rarely did any RS have 4WS. is your rearend out of a USDM car? what car is it out of?


I have a 4WS JDM 3bolt rear on now...but I bought a 4bolt from a 2G from a member on this forum. Haven't bolted it up yet

 

A 2g is the same USDM ratio. 3.454. you will eventually lock/blow your center diff with a 2g rear diff.

 

[Scientist]

isn't that if i have an RS tranny or the newer ones?

oh well is there a way of knowing what rear diff my car currently uses?

 

Quote:
^Umm, well my is 1991 E38a RS, as verified with a Japanese CAPS. There are about 5 in New Zealand that I know of, and MaxPain also has a 1991 E38a RS in Finland(?).



i must have it incorrect, i couldnt remember if it was all the non AWS cars designated as an e38a or how i wrote it, lol. its been a while since i had to know all this stuff since i sold mine. i actually even typed it out that way, then went back and edited it becauser i thought it was the way i said, ha.


as for the scientist, I dont think this thread even apply's to you. if your car isnt an RS, then you have the normal final drive. a 2g rear-end still wont work though.

 

Quote:
No, only some early and all Late RS tranny's have the 3.909 final drive. The switchable tranny's are the regular final drive from what i understand. Rarely did any RS have 4WS. is your rearend out of a USDM car? what car is it out of?



for clarification and archive purposes, this response was directed at Wilfongtsi.


scintist's situation has nothing to do with my quoted post above

 

[Wilfongtsi]

So since I have the switchable trans and t-case, they should be out of an early one? Which ratio should they have? There have been a couple posts about the old ones having the same gears as ours, but I've also seen some posts sayin the old ones didn't and then did and then? I have no idea what the rear end is out of, but it is a 4 bolt 4ws 4 lug, with 3.909 gears. I've already put the trans and t-case in but I haven't switched out my stock gvr4 rear yet, but my car is not running yet, so if I need to I can put the other rear subframe under it. Worst case scenario could I get all 4 wheels off the ground and see if front and rears are rotating at the same speed? I know this probably sounds stupid to some people, but I just don't want to break anything. Anyway any and all help is greatly appreciated.

 

I just did a search and i cannot find anything that would indicate that the switchable tranny's ever came in anything other than the normal final drive ratio. but, i cannot find anything about that particular tranny at all. I would try jacking the car up on four jackstands and putting it in gear as you said, thats how i confirmed that mine was indeed an RS tranny. you have to make sure your brakes are not dragging at all, otherwise it will make the results off. put a piece of tape on each tire and see if they turn the same rate.