Why don't people use an FD diff? They already use a PPF, and they're an R230, i.e. a 9.1" ring gear. They should be indestructible... you'd have to fab a rear bridge to use Miata diff mount spacing, and probably a power plant frame, but it really shouldn't be rocket science...

 

As far as to answer your original question, I don't know how to quantify how strong either diff really is. I've heard of people breaking stock Miata diffs at stock power levels, and I've heard of 7.5s living in Mustangs up to 400+hp for a while. I've also heard of the exact opposite. I'd put the stock 3.90 diff back in your car and see what happens if it were me. If you go out trying to break it, you will, but if you drive it like a human being, you may never have an issue...

 

Again, i am most certain that the 1.8 diff is strong enough for 350 plus wtq, however I believe the worm gear start shearing in the torsen unit somewhere near 300 wtq. Its in a thread on m.net, I just haven't located it.

I would think you will have no issues driving it sensibly, even at much higher power levels, but it may only last one good launch, then blow she goes...

Hell, look at failflop, he's making a bajillion horsepower, with videos to prove it, and his is still intact.


Edit: found it http://forum.miata.net/vb/showthread.php?t=315963

 

Tim just uae the miata torsen. If i can do it so can you. Besides you know you will just part it out in 2000 miles nyways

 

failfloral isn't making impressive HP until we see a dyno. Otherwise, it's just noise. I'll check out that link in a minute. Thanks!

Oh heya Fai Theoretically I should make more torque than you, but I probably won't ever get there. Another reason to keep the 3.90.

But damn, I was looking forward to the 3.42 CTS diff. The ratios are already silly with the 1.8 powa.

Megan says do it right the first time.

Oh, and I'll probably sell it before I reach 2000miles. I don't think I'll enjoy daily driving a racespec 2618 piston/wall clearance. Or I will find another $500 junkyard engine (coincidentally what the head job cost), leave it stock internally and swap that in there. That would have been smarter.

 

Assuming equivalent exhausts and motors,


more noise is more power!!!!

 

True Fae! VTA EWG?

Thinking about how to locate the miata diff without a PPF. There's an idea from a F20c guy.

 

come on tim, you can weld aluminum. weld up your own PPF with custom mounting points on the drivetrain.

 

If you use a V6 F-body tailshaft housing, it will have the torque arm mount on it already. I imagine it would be pretty easy to build a PPF off of that...

 

This looks pretty easy.

 

is that strong enough? oh wait yeah, the wing mounts are still there right?

 

Yep, wingadig is still there.

Strong enough? A long torque arm/ppf is better but obviously not as simple at that heimjoint solution. i can make it more strong-looking than that pic. And by that I mean cleaner and Tigged. The lower bolt is still cantilevered though

 

when I first looked at it I thought that was holding up the entire rear section...I'm fail.

 

It's in compression. The body of the joint i'd do as one piece of probably some tool steel we have here, not that snot of weld there in the middle. You'd want it as short as possible too.

The heim makes it simple to adjust pinion angle too. Win.

 

But it has to be purple.

 

I think it's hood rich. Using a chopped up PPF as a diff mount just feels half *** to me. The subframe not being designed to be loaded in the middle of the front span, and the bolt being loaded in single sheer not withstanding.



Don't listen to this guy.

 

Cheerio Tim. We--of the Spear-Danes in the days of yore--say thou shall put a thick-*** plate between thee darn trans and diff. Thee shall henseforth welds that shiz up; thou shall make haste and throve in honours--bright fire ball in the sky.

 

B- for effort, B+ for idea Fae.

 

lawl.

 

I have essentially done something similar to that photo above albeit with a Ford Explorer housing (Ford 8.8" carrier). It works great, no issues.

I think what is shown in that picture would be fine. Personally I would use poly diff bushings. As far as single shear on the bolt, use a large grade 8 bolt, properly torqued, and it will be plenty strong. An aluminum doubler plate welded onto the cut PPF at the heim joint attachment point would help with load distribution, but I'll guess it is not strictly required.

I agree, I also like the easy driveshaft angle adjustment with the heim joint. That is a win.