Miata diff power limits?

At what power levels does the miata diff begin to let go? It wont be in a drag car but it will go down the drag strip now and then.

I'm not looking for an exact number just a range. Just wondering if I should start with a stouter diff from the begining.

Thanks Jake

MKI_GT6 wrote: At what power levels does the miata diff begin to let go? It wont be in a drag car but it will go down the drag strip now and then. I'm not looking for an exact number just a range. Just wondering if I should start with a stouter diff from the begining. Thanks Jake

What kind of Miata?

It's for my triumph. So say the strongest unit out there (still shopping for one), how much could it handle?

MKI_GT6 wrote: It's for my triumph. So say the strongest unit out there (still shopping for one), how much could it handle?

Ah ok... was just making sure you weren't starting with a 1.6 unit or something.

The guys at MT.net seem to be ok in the 300-350whp range. (Not sure what that typically means in terms of torque, which is probably the actual issue.)

They aren't doing a ton of drag racing, though.

The 400whp range is where things start to be a crap shoot from what i understand.

Keith would probably be able to explain more in detail if he pops in here. I know he's had a 2.0 litre stroker with a turbo in on their dyno that's made a lot of power.

Breakage seems to be directly related to wheel hop. It's not the weakest part of the drivetrain, usually the transmission acts as a shear pin and lets go first But I have seen a 400 rwhp Miata take the diff apart by wheel hopping.

Something to consider: the Cadillac CTS-V rear costs about the same, brand new from the factory, as a 100,000 mile Miata Torsen. The CTS rear has a really nice clutch-pack LSD in it too. Just watch out for the cheap 1st gen ones on eBay, I would spend the extra money and get a slightly later one.

What final drive ratio are you looking for?

Thanks for all the info.

3.73 or higher (numerically lower). It shouldn't take much to get the car out of the hole.

I'll check out the CTS-v diff. What's the going rate for a torsen miata diff? Are there after market limited slip units that can be installed?

I've been looking at the for 7.5 diff. any input on it?

Thanks again Jake

If you want a Miata diff, you're stuck with a 3.636 as the highest (and most expensive) option. Best you'll do from a junkyard is going to be 3.9. Going rate is $500-800, ignoring outliers. There are aftermarket options such as the OS Giken, but they're in the $1500 range for the diff alone.

$900-1100 (new) CTS comes in 3.7, 3.4 and 3.2.

Can't help with the 7.5, it's not one I've got any experience with.

If you want a pretty robust and super inexpensive setup, it's really hard to beat the Ford 8.8 IRS units. They came in 90s T-birds, Cougars, and ones with aluminum housings came in Lincoln MkVIIIs. They have a good 6-disk clutch LSD, lots of gear ratios available, huge aftermarket support, and can be purchased for peanuts in pretty much an junkyard in 'murica.

If you're planning to use a miata rear subframe, you can get axles and hubs that will fit up to this setup too.

We've got an 8.8 in one of the Miata V8 cars here right now - the first one we've installed ourselves. New aluminum diff. It's making a spectacular amount of noise on closed throttle, we just had to yank it out. Lemme say, right now I'd so much rather have the GM unit in that car. It's missing two magazine tests because of that rear end...

i hope by the time my torsen lets go, the road to installing a CTS-V diff will have become well documented.

It's one of those projects that I know I could finish in about 3 hours if I was given the chance, but there are various reasons why I can't do that. If I wanted one in my own car, no problem. One custom driveshaft, one custom bracket and some off-the-shelf halfshafts and diff mount: voila.

If I ever decide to sink the money into it, I'd be tempted to do this to my street car along with a 6-speed.

"I'm not looking for an exact number just a range."

There is no real answer. It all depends on how you drive on which surface.

Drag race starts or smokey burnouts? It will take far less to break a diff with this behaviour vs. "adult driven."

Having said that: my turbo car (250-300whp) never broke(94 torsen). A few accidental "sideways under boost in 2nd gear" events, and the car would bury the tach in gears 1-4 at Bonneville (salt). The v8 car has a 7.5 which some folks claim is unbreakable. I get too much wheel hop to trust it much.

YMMV.

Having owned a CTS-V, I wouldn't recommend putting that diff in anything. They're weak and prone to exploding. Though I admit that could have more to do with the rear suspension, and might not be as big a problem in a car that's half the weight.

Depends on which CTS-V, too. The earlier ones weren't as strong, and there were a number of running changes. I don't recommend junkyard ones unless you check part numbers.

They've been bombproof in the Miatas so far.

JohnyHachi6 wrote: If you want a pretty robust and super inexpensive setup, it's really hard to beat the Ford 8.8 IRS units. They came in 90s T-birds, Cougars, and ones with aluminum housings came in Lincoln MkVIIIs. They have a good 6-disk clutch LSD, lots of gear ratios available, huge aftermarket support, and can be purchased for peanuts in pretty much an junkyard in 'murica. If you're planning to use a miata rear subframe, you can get axles and hubs that will fit up to this setup too.

The 'Monster Miatas' use the 7.5 diff. That's the reason I was looking at them. The 8.8 is a pretty big, physically, unit. I have a 8.8 solid axle in my 500WHP dart, so it seems a bit of over kill.

Keith Tanner wrote: It's one of those projects that I know I could finish in about 3 hours if I was given the chance, but there are various reasons why I can't do that. If I wanted one in my own car, no problem. One custom driveshaft, one custom bracket and some off-the-shelf halfshafts and diff mount: voila.

Custom driveshaft could be done semi locally, as could the bracket. Could you share the cost of the half shafts and diff mount?

In reply to MKI_GT6:
Basically the Spit/GT6 chassis is built around the envelope of the diff.
Once you decide to upgrade, you will be doing some major surgery.
The only exception to this is the Subaru/Nissan R160/R180 swap.
It's possible to fit one of these using only adaptor brackets.

Of course, if you have a swing axle car, you will still have to replace all that with some custom rear suspension pieces, as no modern diff is equipped to handle the thrust loads from swing axles, and frankly, they are already known to break with hot Spitfire power levels.

OTOH, if you have GT6+, you can keep the stock rear suspension, but the rotoflex axles must be replaced with custom CV axles that uses a GT6 compatible joint and stub from a Triumph FWD Sedan on the outboard side, and a Subie/Nissan compatible joint on the inboard side.
And of course that allows you to replace those awful heavy lower wishbones (made of depeleted Uranium?) with simple tubular steel ones.

But if you have decided to abandon all of the Triumph rear, then the size of an 8.8 is no longer a constraint, as you will have to fab both new rear chassis section and new suspension.
I've trial fitted an 8.8 aluminum cased IRS diff into my Spitfire project, and it will fit, with suitable chassis modification (ie Sawzall the stock section, and graft in new pieces).

But I've decided to use a Subie R160, to avoid the time and trouble of chassis surgery.
My time is better spent with other issues, there is plenty of challenge.

FWIW, this link shows some pix of the 'Jango' Spit/GT6 rear subframe, using a Sierra diff (used in USDM Merkur), about 1/3 of the way down.

http://retrorides.proboards.com/index.cgi?action=display&board=readersrides&thread=92009&page=2

HTH

Carter

Carter