charchri4

I'll join the chorus of guys looking for Tbird rear end. In a perfect world I’d like an 8.8, 3.55 locker but I know the odds of finding that is pretty slim. So I’m open to whatever you might have.

Thanks!!

MRM331

8.8's can be found in many more models than just the Thunderbird. Quite a Lincolns have them.

7.5's are only found on T-birds.

-Jason

tbone heller

My 7.5 actually came out of a Cougar.

MRM331

Sorry, you're correct, they can come out of a Couger as well. A few of mine have actually. I tend to think of them as the same car when I'm searching junk yards.

-Jason

charchri4

Thanks guys. From what I am seeing it is quite a bit cheaper to build an 8.8 then a 7.5 due to the availability of factory lockers in the 7.5s. My other concern is the drive train I am using has already eaten 2 GM 7.5 rear ends so I'm kind of liking the 8.8 idea. Thoughts?

MRM331

You're going to pay a bit more to use the V8 Roadsters or Boss Frog hangers for the 8.8 than you would for Monster Miata's 7.5 kit. Monster Miata can adapt their rear kit to work with a 8.8 but I'm not sure of the added cost. If you cannot find a locking 7.5 I'm pretty sure the locking unit can be added to an open one.

Here's a few options:
Differential Carriers - Ford 7.5 in. Axle Model - SummitRacing.com

Ford used to sell a carrier for $250 but it looks like they no longer offer it for the 7.5. The chapest is a "PowerTrax" unit for $411. The factory limited slip in the 7.5 I picked up is not the best but only cost me $35.

You're right though, it looks like the cost of adding limited slip to a 7.5 is higher than an 8.8. The Ford racing unit for the 8.8 is still avaible of $260 or so.

As far as breakage goes, I don't believe anyone has actually broken a Ford 7.5 in a V8 Miata to date. I could be wrong though.

-Jason

MRM331

Nice start of a build blog BTW Jim.

-Jason

Vic Harder

I went with a 7.5 and added a LSD carrier from summitt. I broke my mount once... and one of the inboard FORD tripod/universal joints, which is surprising considering the outboard universal is still the tiny mazda OEM part. The 7.5 diff is plenty strong.

charchri4

Thanks Jason. Locking 8.8s are pretty easy to find because all the super coupes and XR7s had them. I have a line on 2 of them in the $125 range shipped. Add about $225 for gears, bearings and seals and I would be in business. Assuming the 8.8 factory locker is any good I would be in good shape for about $350. I'd love to have a Mark 8 aluminum case to save the 25 lbs but none of them ever had lockers so it would be more pieces to bring together.

Good point on Martins 8.8 set up cost difference. I did not ask him if it is more. He mentioned he uses the same axles and hubs with either just the hanger is a bit different. He also encouraged me to go 7.5 because it fits so much better and he feels it will hold back the LS1 just fine.

I don't know, the old adage there is no replacement for displacement always seems to hold true and the memory of my Camaro being dragged back from the track with a blown rear end behind a - gawk - Ford is very fresh in my mind...

charchri4

Thanks Vic. Are you running Martins hanger then that broke? Also any idea what our HP is at the crank? I'm about 425 and I wonder if that is more then the 7.5 was designed for.

MRM331

Thanks Jim, as a die-hard Ford guy I love this picture^^^^^

If you find out what the added cost to Martin's kit to upgrade to 8.8 is please post it up.
-Jason

charchri4

Ha I knew you would enjoy that Jason! I put it up on my facebook page for all my old Mustang buddies to give me both barrels over. It was great fun!

For what it's worth I hold no bias at all other than a general distain for anything that smells like rice. This Miata is the first foreign car I have ever owned and had it not been for Mr Shelby blazing the trail for us with US muscle in the best chassis avaliable I doubt would not have it now.