ok well after replacing the rear wheel bearings in my 86 n/a, the bearing sound did not go away, so it looks like i need to replace the rear differential...now should i replacce it with a turbo differential or a n/a..because i know the turbo differential are geared a little lower than the n/a's....top speed isn't really a thing for me but i am looking to improve my 1/4 mile and 1/8 mile times along with being able to do a lot of autocrossing...would the closer gearing help me? are these two differentials interchangeable? and the turbo has limited slip to right?

 

No...

All are 4.10:1 with the exception of...
3.90:1 on all verts (or was that auto verts?).
4.30:1 on 1989-1991 "GTUs" models.
All J-specs (all turbos) came with 4.30:1 rear ends.



-Ted

 

so then it wouldn't matter if i pulled it out of a 89 n/a or a 87 turbo ll?

 

Yes it would. The T2 models use different halfshafts which you also would need to change if you had an NA.

BTW: all automatics were also 3.9 and did not have LSD(even the series 4 GXL models that had an auto)

And the 89-90 GTUs was the only series 5 model besides the T2 to get a LSD.

 

no , the turbo rear end won't bolt to the N/A driveshaft...

 

so what happens if an auto N/A gets a J-spec replacement engine (turbo). What diff would it have? 3.9:1 or 4.30:1? LSD? Could there be either or would a specific diff be needed?

 

In North America if you kept the auto NA diff it would be 3.9 non-LSD (for as long as it lasted) no matter what engine.

Done right a Jspec conversion would swap the entire drive train including the tranny/driveshaft/diff/half shafts.

However like Ted mentioned, if you had a Jspec diff it would be for a T2 and 4.3 gearing.

 

Is there an AUTO j-spec diff that runs 4.3? Or does this mean that because its auto, it is not done right and has to be 3.9?

 

You might be able to get away with swapping the T2 flange on the differential with the NA one, as I believe the rear-ends are the same size.

 

Just get an NA from a GXL... Make it simple, go to a local junk yard and buy one. Its not worth all the trouble, the car wont even have enough power to really notice a difference in the gearing on the rear end...

 

Well, if you wanted to ever go beyond the limits of a N/A rear-end you could do the turbo, but of course it is easier to use the N/A. However, you can save the time of getting the j-spec rear and get a 4.1 from the US instead.

 

People underestimate the NA rear end... Its alot stronger then you all really think... It held up PERFECT to my 13 sec Turbo 6 port...

 

Cause it's TORQUE that kills drivetrain parts not horsepower.&nbsp the rotary doesn't make that much torque in the first place.

There's a bunch of guys who run V8's that CLAIM they are running NA drivetrains with very little problems.&nbsp Somehow I find it very hard to believe the NA drivetrain can handle a 10-second pass from a V8 engine!



-Ted

 

How did you distinguish the diff was bad?

It might be worth it to check your half shafts to see if they're bad. It a lot easier to change those than a whole diff. Unbolt one half shaft, jack the car up and start it, put it in gear and listen for the humming noise. If the noise is gone, then you need a new half shaft, if the noise is still present, then shut the car down and unbolt the other half shaft. Now try it. If it still hums then it's one of two things, diff or driveshaft. If you can get your hands on another driveshaft(make sure it's from an NA) , try that and see if it still hums. If it does, then you've narrowed it down to the last component, the diff.

 

Well, if you run the stock sized tires, the tires will probably spin before the diff breaks.

I was reading an interesting site yesterday, a TON of articles (over 370k of text!) on modifying Cummins turbodiesels. Just imagine driving something with 800+ hp and 2700+ lb-ft of torque at the wheels! Anyway one topic they discussed was "Will my drivetrain stand up to double the power and torque of a stock engine?" and the answer was 'YES if you're not stupid! If you give it full throttle in the lower half of the transmission (first several gears) then yes you will break things, however if you only use the power in the higher gears then you will be fine!"

That's an interesting point. An engine making 150lb-ft of torque in First gear is sending, what, 450 lb-ft to the differential. That's about the same as a 300lb-ft engine in 3rd gear. So, in 3rd gear and up, the diff could handle the load of an engine with twice as much torque. With stock sized tires you probably won't have any traction in 1st and 2nd gears anyways (maybe not 3rd either, looking at the torque figures!) Now if you get better tires so you can use full power in the lower gears, then yeah you'll probably break lots of stuff, no question.


BTW - Back to the truck thing, here's a visual image that shocked me: One of their customers can break his drive wheels loose from a ROLL at 85mph while towing an empty trailer, on flat dry ground.

 

One of my friends did some work on a Cummins Dodge. It was a low 12 second truck with the AC on

 

Not Dodge... Kenworth and Peterbilt and all them...