[RallyBob]

The non-turbo Isuzu rear end (which people use for the disc brakes) has the same size ring and pinion/differential as the Opel. Therefore it is really not any stronger than the Opel differential (not sure about the side gear/pinion gear hardness compared to an Opel, but this is the weak link in the Opel diff. due to the brittle nature of these gears...they explode under duress).

The alternative is to install a limited slip diff. in either the Opel OR the Isuzu rear axle, but they are expensive and hard to come by, and you still have the relatively weak torque tube design to contend with. I'd try to find a relatively small and common rear axle that you can get parts for. Sure, a 9" Ford could be used, but those are heavy in comparison and physically much larger to try to fit to an Opel. But a Toyota 7.5" or 8" truck rear axle, or a Ford 8" rear axle, or a smaller GM 10-bolt would work (I prefer the front-loader rear axles, easier to work on).

Bob

 

[RallyBob]

The Isuzu housing is about 4" wider than the GT housing. You'd need substantial flares to fit it to the car, or you'd have to use front-wheel drive offset wheels.

As far as cross-referencing parts from other Opels.....no, they're not easily obtainable nor cheap. A limited slip (ZF) would set you back about $1200 or so, a GM bearing and shim set another $400, plus the labor to install it. And then if you decide to change gear ratios, another $300-$900 depending on the ratio. And then you STILL have the weak torque tube, which requires at LEAST an overhaul of all the parts if it's gonna take the abuse of an engine with more torque. New 'donut', new side mounts, new top/bottom bumpers, possible new T/T bearing. More $$$.

A swap is more labor intensive, but far cheaper in the end. Travis went this route for his, GT, narrowing a Toyota truck rear axle, but he also was able to pick up a limited slip and 4.88 gears for a LOT less money than anything Opel.....and it's a ton stronger. I mean, they put Small Block Chevys into Toyota 4 X 4's, and even with the added weight of a truck plus V8 (3300 lbs +), they hold up to most normal use.

Bob

 

[RallyBob]

What is the availability of limited slips, OEM or otherwise? Nice to have a stronger rear axle, but if you're putting out enough power to break stock Opel rear ends, you need a limited slip to put the power down to the ground for sure.

Bob

 

[RallyBob]

I'm not knocking any of the readily available US-made differentials, but there's something to be said for a front-loader like the Toyota. If you set up an extra 'chuck' assembly, a gear swap can be made in 15 minutes. Try that with a rear-loader!

Bob