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M46 Swap / Differential question

Brad780B

New member
Joined
Jun 13, 2015
Location
Larksville, PA
Hello folks! First time poster here. i recently purchased a 1990 780 with an M46 transmission swapped in. The rear differential was never swapped out so it still has the original one. The problem I've encountered is that 1st and 2nd gears have reeeeeeealy short legs on them. I've been told that this is because of the automatic differential. I've gotten conflicting advice on what I need to change the differential to. My questions are this:

1) Do I need to go up in number (say to a 4.10 gearset) or do I need to go down in number (say to a 3.31 gearset). Keep in mind there are numerous performance mods to the car as well (19T turbo, RSI Stage 2 cam, Custom 3" exhaust, chipped, etc).

2) I know I need a differential from an IRS car. Can I only use one from a 740/780/940 IRS car or will a 960/S90 diff fit as well?

Thank you very much in advance for your advice. It's very appreciated.
 
1. You need lower number,3.31 is the longest OEM diff.The higher the number the shorter ratio.

2.740/940 never had IRS.You need a diff from MK1 IRS car like 760,780 and pre 94 960 but not the estate.
 
91-94 960 sedan is irs. 91-94 wagons are solid axle.

I do believe that the 780 axles are 26 count vs 27?

anyways 3.31 is the differential you want from a 92-94 960 sedan. if you want a 3.73 which i'm sure you already have then go with a turbo 780 or 940se sedan.
 
1045 indicates a mk1 limited slip irs diff. 1035 is the same, but with an open differential. Dont worry about wehter it came from an estate or not, because the estate cars didn't have irs before 1995. And by then the diffs are different and marked 1055 or 1065.

If you don't like the ratios in first or second gear, why not just change into third?
 
Yes, it came from a sedan and will work in your 780. That is exactly what you want if you are looking for taller gears. The 92-94 wagons were not IRS equipped. They were live axle. That carrier will also have a G80 locker in it.
 
Thank you very much for the advice guys. It's really appreciated. Looks like I'm going to put an offer in on that differential tonight. You all have been really helpful.
 
The other way you can go is keep the rear gear you have now and swap in a T5 transmission. You have a bit more power than stock, so it would be good insurance. And the T5 shifts a lot nicer IMO.
 
Thank you very much for your response, Slevin. It's really appreciated.

Is there a way to tell if a 960 diff came (say, like this one: http://pages.ebay.com/link/?nav=item.view&alt=web&id=311057874807&globalID=EBAY-US ) came from a sedan or an estate?

Thank you

Your 780 should have IRS. I think they misread your post, since Slevin referred to 740/940.

760/780 1988 - on should be IRS with nivomat self leveling, since it was the high end model. Look underneath. Easy to tell if you have solid axle vs. independent.

962706B4-1B87-4E46-878C-CF9ED34571F4_zpsbgz6v3nk.jpg
 
Last edited:
Lookforjoe: it definitely has IRS, that I do know. But the Nivomats have been removed and Khaplenke coil overs have been installed.

Greg Wong: Ive considered the T5, I don't need to change to a taller rear end with that? Maybe I'll just say screw it and forget the 3.31 gear and go right for the T5 transmission. My lowball offer on the differential wasn't accepted last night anyway.

Again, thanks for all your advice.
 
Lookforjoe: it definitely has IRS, that I do know. But the Nivomats have been removed and Khaplenke coil overs have been installed.

Greg Wong: Ive considered the T5, I don't need to change to a taller rear end with that? Maybe I'll just say screw it and forget the 3.31 gear and go right for the T5 transmission. My lowball offer on the differential wasn't accepted last night anyway.

Again, thanks for all your advice.


I would go directly to the T5. They shift much better and can handle 2X the power the M46 can. With a T5, the 3.73 gearing is perfect. BTW, you should keep a watch for a 1991 780 Coupe or 1991 940SE being parted out. Both those vehicles came with the G80 locker and the rear sway bar. It is quite an improvement over what your car came equipped with.
 
That diff has been in my watch list on ebay for ages.I am after a 3.31 diff too but I think that is a bit pricey(however i dont know how much they go for in the US)not to mention the shipping to Europe. :-(
 
I have an IPD rear sway bar. The locker would be nice though. Most of you guys probably know this car. Its Jesse's (JesseBeardtone) old car.

It's my understanding that the GM T5 is a better fit than the Ford T5? Is there any difference between the T5 out of the V6 firebird/camaro and the V8? What about the one from the S10?

Thanks again!
 
response

haha I thought that car sounded familiar.

Yea I think it might be best to go with the trans swap considering the power upgrades that car has
 
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