I just came across this thread and went outside to check out my 88 Automatic GXL parts car. I could only do a visual check right now as I'm getting ready to leave town for a bit.
Here ya go.

 

Ok, we've now concluded that GXL Automatics do have LSD's. But, we still haven't concluded whether these LSD's are 3.9 or 4.1.

As reference, this observation has been made:

I have never run across an auto tranny that had the gray gear, which is 4.1 They all have orange gears, which is 3.9.

So, if we could get you to do this one more check, finally the mystery can be solved.

 

Well, how many does this make now that have seen the LSD tag on an automatic? I've seen one myself, sirx7 has a picture, how many does this make now all together?

 

My 86' GXL, which was automatic, has a limited-slip diff. The speedo gear is also orange. I guess that makes the LSD 3.9?

 

I pulled a S4 LSD from a Automatic GXL and checked by turning the half shaft mount on full turn to see how many turns the drive shaft mount would turn.

Formula, if 3.9 then the driveshaft would turn just under 4 turns, if 4.1, then just over 4 turns. It turned about 4 1/4 turns.

So, guess the color of the speedo gear isn't a indicative as thought. At least the answer to this mystery is solved.

 

Check out this thread for what S4 cars had LSD and had what tranny and had....

https://www.rx7club.com/forum/showth...hreadid=136126

 

If you check the parts manual you will find that the gear set(crown wheel and pinion) are the same part no for normal and LTD slip differentials.The part no for manual and automatic cars are different(as you would expect per Icemark's earlier comments).So yes an automatic S4 GXL came from the factory with a 3.9 LSD.

 

I, also, still say there was a 3.9 LSD. I once got a 87 GXL automatic, and the only thing wrong with it was a bad rearend. THis thing was destroyed, missing some teeth, and Im not sure why, because it was babied otherwise.

Anyhow I replaced this one with a LSD out of a 5sp GXl I had for parts. I forgot to change the speedo gear at first. I went for a drive and the speedo read quite a bit off(10mph) and the rpms/speed also changed. I changed gears(to the gray one) and it worked fine from then on and was accurate, but the rpms stil lwere different.

 

You know Kevin I was just gonna ask that, of how much of a difference between the 3.9 and the 4.1 is in MPH, (see I have been considering dropping back the 3.9 onto my 'vert since with my tire profile the T2 4.1 has the speedo all off).

 

Well, as I remember it was close to 6-7mph off, as measured between milemarkers at 60mph with a stopwatch, not the most accurate way to do it.

Also by way of my shabby *** old math, if standars is 4.1 and youre changing to 3.9, that's a difference of .2 in relation to 4.1, and that .2 is about equal to 5% of the original value of the 4.1. The ratio doesnt affect speed per se, but rpm in relation to speed, therefore if you went 3000rpm at 60mph before with a 4.1, if you swap to a 3.9 you will now go 5% less rpm at 60mph with the 3.9...in this illustration 2850.

That's an oversimplified way to look at it, since differnet models had different 5th gear ratios in teh tranny itself, and though I dont know the actual numbers as far as rpm vs. speed, you get the idea.

FWIW< I like a 3.9 better...Im all about doing some road travelling, and I like to put her on 100mph and see the tach sit closer to 3krpm than 4, like in a turbo II.

 

Mark,

If your goal is to get the speedometer reading correctly, you could just to to a speedo shop. I had that problem when I put a GSL-SE rear axle in my '79 (I miss that car).

They made an adaptor that screwed in between the tranny and the speedo cable that brought my speed readings back in sync. As I recall, it really didn't cost that much.

 

I either read somewhere (or dreamed) that 91 automatics DID have a 4.1, due to the "one model" approach of 91 NA's. Anyone have a 91 w. AT that they can look under or verify the rpm @ 60 mph? This may have been incorrect info however, which I probably read on a forum, or in one of those overview car books.

I agree that this is a nice mod in a 5 speed model, but I sure would like to try a 4.1 or 4.3 in my AT (it needs it!). Every little bit helps in an AT, even though fuel mileage would probably be saddd.

 

S5 Verts were the ONLY automatics with 4.1 gearing. BUT, they also only came with an open Diff, NOT LSD.

If you want drop in LSD for a NA, ya gotta go S4 GXL or S5 GTUs. But, you'll need to stay with your Automatic Driveaxle.

 

Actually 89 and 90 model verts with AT came with a 3.9, or at least according to all info I have (magazine road tests, consumer reports, etc.). I could not find anything about 91 models however, which were in question.

 

The '88 Vert was only offered in 5 speed,and all came with a 3.9 rear diff.

'89-91 offered the automatic option, but were the only automatics that came standard with a 4.1 Diff from the factory. It was to compensate for the extra weight of the Vert. All S5 Verts came with Open Diff 4.1 gearing.

If you have a S5 Vert with anything else, then someone swapped a different diff prior to you taking possession of it. Not that that is a bad thing.

 

sorry if this is a real newbie question but this has been pretty confusing to me with all the debates going on. If i wanted a very fast car off the line, which LSD would i want? Off which car would i get this? well let me guess

fast top speed=3.9
fast take off=4.3

am i right or is it the opposite?

 

you got it right

 

cool thanks

 

Not true. Mazda has a p/n listing for 86-88 NA LSD with a 3.909 R&P. GXL's with AT would have used this LSD drive unit.

Hot_Dog
90 GXL
02 RSX-S

 

Hehehe; lets shake it up some more! I have a S5 Auto Vert with a 3.909 open diff. Where the idea of all S5 verts having the 4.1 idea came about, I do not know. I just got a GTUs 4.30 rear, and when I swapped them I counted the turns on the stock one. And it did not make it 4 turns around. I also have the white gear for the 4.30.

 

Oh yeah! Mines a 91 and is now a manual.

 

The stock speedo gear in my tranny was black? WTF? The GTUs gear is white. But my 91, with a factory installed automatic, and stock differential, was an open 3.909. Maybe like the rest of the RX-7 legasy, Mazda mixed it up a, little just to **** with us.

 

My question is- Would a 91 Auto GTU have an open diff?
I recently bought a 91 GTU parts car, hoping for an LSD for my 87 sport.
Well before I installed this "what i hope is a LSD", I drained and refilled the gear housing of the Auto GTU with Lucas 80-90w gear oil. Hope they oil will be good for drifting...anywho-
After reading many "do I have LSD or open diff?" threads, and the test of "turn one wheel and see what the other wheel does".
This is what I've found.
Well, with the wheels on, only one wheel spins when turned by hand. But with the half-shafts or axles unbolted from the diff, they spin opposite of each other.
So is this an open diff or maybe the off chance of a 3.9???
Ill go spin the wheels to see what ratio it has. just over 4 is the 4.1 right?

 

You have a 3.9 open diff.

 

I just pulled an lsd from an 87 automatic, so, yet another confirmed auto with lsd, not sure if its 3.9 or 4.1 though

Just a quick ? this should bolt up to a 88 manual vert correct?