Hello everyone, im kicking around the idea of ls swapping my 99 miata. Its mostly a track car, with power goals being 300rwhp. (L33, valve springs, headers ls6 manifold) has anyone used the diff and axles off an 04-05 mazdaspeed miata? It has bigger axles and a 3.9 ratio. What do you guys think?

 

I would also like to know more about this. Pictures would be cool too

 

Great question and welcome to the forum! Are you running an LS1? If so they usually turn more than 300 rwhp even stock.

I would think the mazdaspeed diff would cost way more than a Tbird diff and not be near as strong. I paid 100 bucks for my 8.8 diff and have pounded 400 hp to it many times and it doesn't seem to notice.

What do you see as the benefit to using that diff over the GM or Ford diff guys usually use?

 

I sold a type 1 torsen for $450 and a type 2 for $600. I imagin your diff is worth more. Why not sell it and end up with a stronger diff, better choices of gears, and put some cash in your pocket?

 

Biggest reason I can see for using the Mazdaspeed parts is that it's a bolt-in deal, no splined hubs or custom diff mounting or axles. You would have to deal with the PPF, but mounting the front end to the trans crossmember has been done. The downside, specially for a track car, is the unknown strength factor. There probably are a few 300 rwhp Mazdapeeds out there.

Try it, be sure and let us know how it turns out

Mike

 

I don't think I would do it. Its a 4.11 diff ratio, btw. I would imagine it would break. The diff itself is pretty much the same, the axles are bigger though, but the hubs are the same. I'd say it will still break. So I sold mine to use the ford. Its really, really easy to install with the kits out, and it will cost you less. Even has a torsen availiable.

 

I've been considering using the stock diff in my mazdaspeed if I go the V8 route rather than a K swap (as I've been considering lately), I imagine that mechanical empathy would go a long way to preserving its longevity, but is that really the point of a v8 swap?

 

Differentials are broken by torque, not horsepower. Aye, there's the rub, as Shakespear might say. The little diff doesn't care if you are turning 7000 rpm and making 300 HP, but stomp on a V-8 at 3500 rpm with 400 ft/lb. of torque and sticky enough tires to hook...

Alas, poor Yorick; I knew him well.

 

There are plenty of high hp MSMs, as you mentioned, enough between 300-400 hp to easily prove the diff/axles at that rating, for a turbocharged 1.8l engine. However, Sunshine hit it on the head. What kind of torque do they make down low, or even from a 4krpm launch? Not even comparable to a near stock LS1. When you are talking about all that downlow torque and shockloading the rear diff, something has to give. I.E. the axles.

I'd really give up on trying an MSM diff in a V8. All one needs to do is search LS1 RX7 Axle snap, LS1 Miata broken axle, and I'm pretty confident in my assessment. Now, on the one hand, it does seem even with the aftermarket axles, if they use OEM miata outers, this is what fails, correct me if I'm wrong. In that case, maybe you could build some very strong axles for an MSM diff and give it a try. Myself, I prefer to have 0 problems after its all put together, so the 8.8 and FFR axles should be sufficiently overkill.

 

This. Who builds a V8 Maita to drive reasonably?

 

And in general, who spends the time building a car so they are always worried about breaking the rear diff instead of enjoying the drive. I'd be always worried about babying it so as not to blow it up. (BTDT with turbos, btw)

 

Besides the big torque difference, there is one more drive train difference here that will sneak up and bite you: delta in gear ratios.

Builder A does a conversion with an L33, a T-56 with 2.95 low, and an 8.8" rear end with a 3.27 ratio. When he/she romps on it in low gear at the engine's (stock) torque peak, the engine makes 338 ft./lb at the crank, which gets multiplied by 2.97 low gear in the T-56. 1,004 ft./lb. exits the tranny tail shaft and heads for the differential. The 3.27 gears in the diff multiply that torque to 3,283 ft./lb. twisting on those big, fat, conversion kit axles.

Builder B does a conversion with an L33, a T-56, and the MSM rear end and axles. The same 1,004 ft./lb. enters the differential, but now it is multiplied by the 4.10 gear instead of the 3.27. Those spindly little axles are suddenly getting twisted by 4,117 ft./lb. of torque.

You can counter that there's probably a 10% or so loss of torque as it travels down the drive train, so it's probably a little less than 4,000 ft./lb. Or you can look at those little four cylinder axles and just think, "eh...no."

 

Seems like OP checked out 6 months ago...