Drivetrain 8.8" vs 9" rear end swap?

BigPabby79 · Jun 21, 2019

We're building a '78 Cobra II into an autocross/road track car, which rear-end is the easier replacement for the factory 8"? From what I've read the factory 8" is 56" wide and you have to run quite a bit of wheel offset to avoid the leaf springs, what is the right width rear to shoot for to avoid clearance issues?

LILCBRA · Jun 21, 2019

I run a factory 8" with an 8" wheel with 4-1/2" backspace. With that and my tire sidewall, I had more clearance issues with the body than I did with suspension components. And, depending on the diameter of your wheel and tire, you may have to keep an eye on your front end moreso than the rear. I'm running a 165/70-15 (I think? I'd have to double check...) on the front and had to knock the body seam 90 degrees of how it was stock so it wouldn't eat into the tread when turning the wheel.

LILCBRA · Jun 21, 2019

Now, with all that being said, if I had to choose I'd go with the 8.8. They're more abundant anymore, lighter, just about as strong, and there's plenty of aftermarket support. They also seem super easy to shorten to your specifications. As far as a width goes, if you're not doing any heavy suspension modifications, I'd probably stick pretty close to the same stock dimension, maybe a touch wider to center your wheel choice. As I said, my 8" tire has 4-1/2" of backspace, so it only has 1/2" of offset, and that's probably about as much as you can go with everything being stock. Bumping it out an extra 1/2" on either side would net you a 0 offset 10" rim.

BigPabby79 · Jun 22, 2019

So is 56" the magic number?

LILCBRA · Jun 22, 2019

You've probably already seen these sites, but this one says it's 57".

http://www.mustangsandmore.com/ubb/FordRearDimensions.html

And this one says the same as well as giving a number of different axles - including some 8.8s - for comparison.

Rear End Widths

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www.carnut.com

It's kind of crappy out, so I'm not going to go outside to measure my car for confirmation today. Maybe if someone has an 8 inch out of a car they can measure it a lot easier and confirm. I used to have a spare, but it's long gone now.

BigPabby79 · Jun 22, 2019

I hadn't seen those, but that's perfect the 8.8 with the left side cut down should be a perfect fit. Thank you.

429MII · Jun 22, 2019

I have the 8" that came out of my car sitting in the shed, I will see if I can get a measurement of drum to drum.

For what its worth I would keep the 8" for anything small block just put good parts in it and good axles.

No specific need to go to an 8.8 or a 9" in my opinion, as the ring gear won't be an issue in that type of racing.
Axles and retention of axles, and cooling would be important, and a good yoke with a decent size u-joint.

Some will argue that once you spend all the money on good parts for an 8" you could have bought a 9" or an 8.8 (plentiful and cheap) and that is a possibility minus of course the mods to shorten a diff or special wheels to make it work.
For sure so price it all out first.

So, If money is no object I would go 9" for sure even just for pure availability and selection of parts for it, if not put some good parts in the 8" and go race..

Drivetrain 8.8" vs 9" rear end swap?

BigPabby79

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Rear End Widths

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