Honestly, the 8.8 is just as strong as a 9", actually a bit stronger when comparing a typical 9" and a typical 8.8".  The 9" built its reputation on a handful of N case rears used in ultra high performance applications.  In trucks I've seen their weak links many times.  The pinion bearings are too close together necessitating that pilot bearing.  Under load its super common for the case to break right at the pilot bearing bore.  Then the case is junk and likely so is the ring and pinion.  The cross pin in the factory diffs is also fairly weak.  I've seen those shear a couple times.  Then the axle spline count, which I believe is typically only 28, same as the 8.8. 

For ease of a disc brake swap, strength and parts availability the 8.8 is a very sound choice.  I've dealt with them a lot as well, and I've seen them fail under hard use, but if you build the rear from the onset that won't happen.  My '89 GT project I kept the 8.8, added a Yukon limited slip with 31 spline axles, 3.73 gears, solid pinion spacer, studs for the caps, and a TA girdle cover.  I have no fear it will withstand everything I throw at it including sticky tires and 450HP (possibly up to 600 on nitrous). 

Honestly, in a street car, with street tires, there's not much point in spending a bunch of money on a super strong rear.  You'll spin the tires long before you shock load the rear enough to break it.  If it has an 8" in it now I'd probably just leave it, regear it if needed, and add a limited slip if it doesn't have one. 

Only reason I see for a nine inch is the quicker ratio change. If you are going to not change ratio very often, I would do the 8.8 which can save you on unsprung weight on the rear of an already light weight car. All that mass bouncing around back there is bad for ride and handling.

I ground out a pinion bearing with the toolpost grinder so it hand pushes on and off to use for set up. I have four different gearsets pre-setup for the housing in the race car, so I can change one in about an hour. The old MG I used to race was like the nine inch and I could do it in less than half an hour with the pigs all set up ahead.

Last edited by DC (7/18/2019 6:31 PM)

MS wrote:

Use Ford brakes. The aftermarket stuff just doesn’t stack up quality-wise. Also consider future replacement parts availability. The MS Cobra or GT rear discs all use all Ford replacement parts.

That is really solid advice.  I have a rear disc setup on my Blazer that uses a bunch of cobbled together and modified parts.  All stock parts, but its Cadillac calipers, front rotors of a different year truck with the center bores machined larger, hoses off a Monte Carlo, etc.  If something goes I'll have to go back to my file on the truck and figure out what the hell I have to get it back up and working again.  That's stock parts that should be available in the future, but its not a straight forward setup, because it didn't all come off the same vehicle.  With aftermarket parts you'll have to pray that the manufacturer is still in business, and still produces the setup you have. 

As I modify my '67 and build my '89 I am using as many of the same parts as I can, all Ford OEM, so that I have not only stock parts that will always be available, but also interchangeability between the vehicles.  Both will eventually run '94-'04 Cobra brakes front and rear, both run a T5 trans, etc.  If one is down and I have a failure on the other I can just swap what I need and get the driveable one back on the road quickly.  It reduces the amount of spares and service parts types I have to keep on hand as well.