Nowak1981

So I think I have an opportunity to get a 7.5" tracloc with 3.27 gears over at the tccoa forums. I see most people are running 8.8".. I'm trying to remember where I saw people using 7.5". This is mainly gonna be a cruiser and show car so I'm not needing anything super beefy. I'm still planning a a stock ls1 though. Ls2 if I'm really lucky.

Any suggestions, should I just stick with the 8.8?

MLKNR

7.5 is supposed to be easier to install and gives more room for the exhaust, but I'm sure I'm just talking out of my "Newbie, it's what I read knowledge" Only negative to the 7.5 is that it's getting hard to find parts for it. Like you I'm still undecided on which to go with. I see which costs less and go that way for the first go around.

.boB

Stick with the 8.8. It is bigger and heavier. But it's pretty much indestructible. No matter how you drive your car, or how long you own it, you'll never have to worry about the diff.

Always try to build things a little stronger than what you think you'll ever need. Words of wisdom from my father.

Nowak1981

Appreciate the advice, 8.8 it is.

tbone heller

I used to build differentials when I worked as a Ford service technician, & I have yet to hear of anybody breaking a 7.5 in a Miata. I saw almost no issues with them in T-birds, Cougars, or Rangers.....FAR LESS than with the 8.8. Whatever you decide on, have somebody that knows what they are doing go through it, replace all bearings, seals, & clutches. As with ANY differential, if it is not set-up correctly, it will fail.

charchri4

+1 on the room for exhaust. That's a big headache and the smaller the diff the better. The only reason I went with a 8.8 was for the gear ratio options and the price. With a stock cam and 15" wheels 3.27 works fine behind an LS and Martin has run 400+ hp through a 7.5 with no issues. I wouldn't be concerned with a stock LS1 and a 7.5 at all.

MRM331

The 7.5 is a much better choice but is getting hard to source. To make it worse Ford has for the most part disowned it a few years ago and no longer offers a traction loc or even a rebuild kit for it. You can get parts through the aftermarket but everything you would need for the 8.8 can be had from Ford Racing.

-Jason

Gabriel

I went with 8.8 because that's what i found at the junk yard... but I really like how Martin's 7.5 mount works... Stacey David on gearz used a 7.5... and it's lighter.. from everything i've read on it, you really do not need the extra strength of the 8.8...

Sunshine Guy

Here's a thought that might help someone searching for info on differential options.

The independent rear suspension, Ford 8.8 diff is an iron casting on the T-Birds, the most common donor. However, it was also employed in a full aluminum configuration on the early Lincoln Mk IV.

The Mark IV unit (unlike the aluminum 8.8 used in later Cobra Mustangs) has the same basic casting shape and mounting points as the T-Bird; it's just significantly lighter. My Lincoln, aluminum 8.8 complete with heavy-duty, high bias Torsen T2R differential only weighs about 12 lb. more than the stock Miata differential. If you are going to build/blueprint a complete assembly anyway, it might be worth starting with a lighter aluminum core.

They look pretty, too, glass bead blasted and coated with aluminum wheel clearcoat. I'll attempt to post a couple of pictures.

Gator Bait

Now that's a nice clean workspace. Curious on your plans for the fill port. Fluid cooling planned?

Sunshine Guy

Fill port...oh, darn, I knew there was something else I was supposed to put inside that thing! Does it really NEED oil in there???

I was indeed thinking about a set-up where the directional flow of the fluid coming off the ring gear at speed would push it out the back cover through a steel braided hose and into a little cooling radiator mounted somewhere in the subframe area where it could get some clean air flow (not off the exhaust). Then it could either drain by gravity back into the diff, or maybe use a small, electric pump activated when necessary by a thermostat in the diff.

I'll cross the bridge of figuring out those little details someday when I actually find time to start assembling big stuff again. By the time I actually get to work on this thing I may be too old to care about driving fast enough to need a diff cooler. I may need a differential warmer, LOL.

Thanks...I had a great little workshop (see pic) in Minnesota. Then I took a crazy, constant-travel job, moved to Florida and my garage has become the poster child for a hopelessly chaotic life, LOL. I won't be posting a picture of THAT mess anytime soon...

tbone heller

I saw a lot more concerns with the aluminum castings than with the cast iron ones. Mostly noise concerns generated from the casting flexing which would constantly alter the backlash of the ring gear. The extra weight of the cast iron unit balances some of the nose weight added by the V-8 & puts it low in the chassis. Just something for you to think about.

Sunshine Guy

Yes, I've heard concerns expressed about the aluminum 8.8" case flexing. I'm running a Ford Performance stud girdle to help stabilize the caps. On one hand, I've not heard of many Mark IV Lincolns having chronic differential problems, and that's a much heavier car. On the other hand, the diffs in those cars are typically receiving less torque, too. The aluminum 8.8 in the later Cobra Mustangs handle a boatload of power, but then they aren't identical castings, either. Hopefully I'll eventually get the chance to find out how it works.

The thought about offseting the weight gain in front is certainly valid. I've kicked around the idea of using dual (but compact) batteries forward in the trunk to put a bit more weight directly over the rear tires to help traction. However, as you suggested, the weight would indeed be better added down low.

charchri4

LOL OK Mr Sunshine what kind of engineer are you?

Gator Bait

Has anyone seen a report of blown rear ends on a converted Miata? There's bound to be some failure rate just from incorrect gear or bearing set up, but what I'm looking for actual failure due to not enough strength in the rear? I ask b/c I would think we are so severely traction limited that the safe guard is built in.

I hammer a locked 8.8 solid axle rear end rock crawling in my Jeep on 35" MT's. That thing just begs for more. Different animal completely casting wise, yes, but the shock loads the R&P absorbs are pretty darn high compared to what these cars will see. The U-joint is the safeguard on the Jeep, but when it goes I won't be doing a buck and a half either...

Sunshine Guy

I followed the action on another V8 Miata forum for several years, keeping up with numerous builds and their owner's experiences over time. I only heard of two guys whose diffs started getting loose and noisy. Both were 7.5" units that were pulled from wrecking yards and used "as-is," no rebuild; who knows what mileage or treatment they had endured. One was noisy when initially installed and it still held together for quite a while before the owner got around to rebuilding it. Neither experienced actual breakage.

I believe you are correct that the light weight and traction limitations are the saving grace in these cars. I have an 8.8 and a high torque motor, but if I were starting over I might choose a 7.5" just for packaging reasons (well, I DO like the choices in 8.8" limited slip units better). Martin tried to talk me into the 7.5" but then, why would I listen to a guy who's only experience is building over 100 of these cars instead of my shade tree mechanic intuition?

Sunshine Guy

Official driver of antiquated ideas, grand schemes, and broken dreams!

charchri4

OMG and a half I stand in complete awe and bow to your epic level of origination and cleanness. I can't remember a time I had more tools in my tool box than scattered over the bench and on the floor. I love it! Just wish I could live it...

Sunshine Guy

I wonder if this would be a worthwhile strength upgrade on an aluminum 8.8?
LPW ULTRA Cobra Mustang & Camaro Independent Rear Suspension Support Cover, IRS Differential Cover, IRS Rear End Girdle

Sunshine Guy

Bad memory on my part. It was the '93 - '98 Mark VIII, not the Mark IV.

02-VIVID

Whose differential mounting kit is that one you have? Is it from Martin?

MX-Brad

That is Martin's diff hanger and the torque plate is in the foreground.

V8droptop

Actually, on the high bias T2r, how many miles have you put on it? I've read about mustang guys ruining those torsens, any issues with yours? I have both the regular and high bias versions and am worried about installing the high bias unit and having it wear out or break. I guess the mustangs are just having them wear out after a lot of track use.

Sunshine Guy

My T2R has been completely trouble-free for several years! Of course, I haven't actually gotten around to installing it in the car yet... Sorry, can't give first-hand experience here, it is for the car I'm piddling at building rather than the car I'm currently driving and refining.

I have done a fair amount of reading about the Torsens in the various forums. The guys over at corner-carvers tend to be the most techie and reliable in reporting issues, IMO, and there have been a few people who have experienced issues with the Torsens. On the other hand, others have put thousands of track miles on a single unit without a failure.

My best advice would be to contact the folks at Reider Racing. They are a top-notch builder of racing diffs and they do a bunch of them. I remember reading something posted by one of the techs over there, or perhaps by someone who was quoting one of the techs, that, for many years, there were virtually no failures recorded for the Torsens. Then, suddenly, some people did have problems. Since the torque transfer is created by rotating parts developing friction against the case itself, it seems reasonable to assume that there was some change in metallurgy.

The people at Reider talked with the people at the Torsen factory who denied that there was any change of any kind. The Reider people inferred that if they used to never break and now they do, that isn't true. I believe they (Reider) started steering people away from Torsens for some particular applications. All this is from my fuzzy memory, so a call to Reider would get you an opinion from the horse's mouth. They built my unit, their workmanship was impeccable, and I would trust them to know the straight skinny more than anyone else I know. At the time I bought my T2-R, (before the mysterious problems emerged), Reider advised me it wasn't the first choice for hard-core drag racing, but for high performance street, autocross, or track use it came very highly recommended.

My personal opinion, which is probably worth just what it costs you, is that in a 2500 lb. Miata with D.O.T. tires that will fit inside its wheelwells, and running good, clean lubrication, there is little chance of over-stressing the T-2R. Any potential load great enough to seriously wear on the unit will slip the tires first. The heavier cars with big, wide, sticky race tires would have the ability to put far greater loads into the differential.