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Discussion Starter · #1 ·
Howdy,

I have a rotary-powered (12A) 71 GT (in progress).
I've been trying to figure out what rear end to use. Possibilities include:

This is what is kicking around the local wrecking yards.

Early 80's Mazda Rx7 GSL SE - Disk brakes, width could be accomodated by FWD high offset wheels or narrowing. Flange to Flange 57.125" Seems relatively easy to mount, as it's similar to the stock Opel.

IRS:

Mid to late 80's BMW 325 - disk brakes, only 3 mounting points for the whole thing! (2 for moustache bar, 1 for diff.) Problem is, the mounting points are 48" apart, and the Opel just ain't that wide. mounting points may be a problem. Flange to Flange 56"

78 280Z - drum brakes, very narrow mounting points, driveshaft is 3/4" offset towards right side of car. nice and narrow, but Macpherson strut, have to build some towers? Flange to Flange 54.5"

Flange to Flange distances refer to wheel mount flanges.

So... anybody had any success/horror stories with any of these units? :confused: I don't care about hacking up the rear floor pan, the car is so far from stock.

Thanks,

James
 

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1450 Seeker...
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A Popular non-Opel rear end swap that I have read a lot about is a Toyota pickup rearend. There are millions of them, plenty of aftermarket support, available disc-o's millions of ratios and limited slip possibilities too.

There is at least one thread on this site dedicated to the installation of Toyo P/U rear on this site. A search for toyota rear end should probably turn it up for you.
 

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Discussion Starter · #3 ·
Thanks!
I have read about that swap, I believe he narrowed the rear end of the Toyota. I wonder if narrowing is avoidable by using FWD wheels?

I have the Mazda rear end already, so if I go for a 'regular' rear end, it'll be the Mazda. I was just deliberating whether going to IRS would be any better (not only for handling, but also ease of installation).

I've read a lot of different opinions about IRS for the Opel, but I wouldn't mind reading a few more!

Cheers,

James
 

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'NO,......O P E L..G T!'
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jtb said:
Howdy,
Mid to late 80's BMW 325 - disk brakes, only 3 mounting points for the whole thing! (2 for moustache bar, 1 for diff.) Problem is, the mounting points are 48" apart, and the Opel just ain't that wide. mounting points may be a problem. Flange to Flange 56"
Hi James,

This is definately doable, just think you can also gain a LSD :D The chairman of the UK OC is currently having a GT built with this rear end configuration, I think it's a case of griding the points off an re-mounting......just think no more rear end lift on full throttle :D

If you need any more advise I may be able to get some info from the guy that's doing the conversion.
 

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Discussion Starter · #5 ·
Thanks, Mark!

I would be interested in some pics &/or dimensions for the BMW swap, if you can get ahold of them. It seems to me the ideal mounting points for the Opel are right where the trailing arms connect to the moustache bar for the Bimmer. But maybe there's a different point I'm missing

Cheers,

James
 

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Just curious on how you plan to overpower the stock GT rear end with a 12A rotary? Or are you just doing this for the disc brakes?
 

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Discussion Starter · #7 ·
Who said the Mazda 12A is stock :D ?
It has been street ported, is running a custom header, and a few other goodies which should (I've been told by the engine builder) put it around 180hp. Besides, the Opel rear end that I have is trashed.
And yes, rear disks are a nice bonus. The only reservations I have left about the BMW rear are available ratios and the cost of parts!
 

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Manta, GT, Isuzu, Toyota axle widths

I did a little research, and I came up with the following numbers:

GT rear axle width 52 5/8"

Manta A rear axle width 54 5/8"

Isuzu Impulse rear axle 56 3/4"

Manta B rear axle width 56 7/8"

Toyota pickup rear axle 55 1/4"

The Impulse width I have not measured myself, that dimension was posted here by GTJim some time ago. The rest I've checked personally from my own parts.
 

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Old Opeler
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DIFFerence

The Impulse diff is virtually an Opel Manta "B" diff which has very similar internals to the original GT diff ( same size Crownwheel & Pinion plus carrier) so there is not any 'strength' advantage. The IMpulse pluses are 3.9:1 ratio and disc brakes. Also the original GT drive shaft and torque tube will bolt up but the panhard rod is mounted on the oposite side and the spring perches are wider than the GT's. So some adaption is necessary. The horsepower limit would be similar to the original diff.
 

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GTJim,
You say that the impulse has the same size pinion and carrier. Can you actually rebuild a GT axle with a gear set from an impulse?
 

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Rumour Has it!

I have measured them both up and the parts are identical sizes - but I have not put Impulse stuff into a GT diff housing - decided to use the complete Isuzu diff instead. Also be cautious as the later "Turbo" Impulses use a completely diferent diff that does not have a torque tube. The "ordinary" Impulses have Opel diffs with torque tubes - just like the GT one.

All GM companies so the parts similarities are not surprising - I would think that the diffs were probably made in the same GM plant and shipped to each division for use. Certainly that was done with Opel motors and is still being done today.
 

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Travis said:
and 58 1/2" in '86+
Travis, this is only because you chose to use a 4 x 4 axle, and not a 2wd truck axle. I have five 2wd Toyota axles and they're all narrower than yours, and identical to each other. They're all (concidentally) '86 and '87 axles.

Bob
 

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RallyBob said:
Travis, this is only because you chose to use a 4 x 4 axle, and not a 2wd truck axle. I have five 2wd Toyota axles and they're all narrower than yours, and identical to each other. They're all (concidentally) '86 and '87 axles.
I went back and checked the info I collected when I did my conversion. '85 and earlier 4x4 is ~55"(what I used) and '86 and later 4x4 is ~58.5". There is no mention of the 2wd axles. I guess I had assumed that they had increased in width in '86 just like the 4x4's. I guess we're both right, depending on what you find in your local yard

-Travis
 

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GTJim,
My question was somewhat hypothetical. Since Opel differential gears are no longer available, it would be great to be able to find new gears to rebuild existing axles. If they would fit, would the impulse gears still be available?
 

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Mute point

Isuzu gears will fit - however Isuzu parts are even harder to find than Opel ones! The Impulse gears will come from 1981-89 cars so may just be still stocked by occassional GM dealers though I have no idea which GM Division dealt with Isuzus way over there in the US of A. Perhaps specialty diff gear distributors or rebuilders may be worth a try too.
 

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GTJIM said:
Isuzu gears will fit - however Isuzu parts are even harder to find than Opel ones! The Impulse gears will come from 1981-89 cars so may just be still stocked by occassional GM dealers though I have no idea which GM Division dealt with Isuzus way over there in the US of A. Perhaps specialty diff gear distributors or rebuilders may be worth a try too.
No aftermarket sources exist for these parts here in the US. I used to buy the Isuzu 3.90 gears and bearings direct from the Isuzu dealer here in the US. I can attest that they DO fit into the Opel, having installed numerous sets into Opel rear axles. I have two Opel differentials in my barn that were run for years (including racing) with the 3.90 axle ratio from an Isuzu. BTW Jim, here in the US the last year for the RWD non-turbo axle was 1985, after that we only got front-drive cars.

Bob
 
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