I am putting a Torsen unit in mt FB and I was wondering if I can use the 90w I have with limited slick addaitive in it? I just have a couple extra quarts around and it wiould just be easier.
Thanks JEB

 

Don't need limited slip additive with a Torsen ,but it wouldn't hurt anythign either.

 

Thats what I am looking for. I would prefer to use what I have instead of buying gear oil w/o LSD additive if I could just as well use oil with an additive.
Thanks

 

What will this swap end up costing you, and how easily was it accomplished? I need to upgrade mine, but am undecided on which way to go...

 

You can pick up a Miata Torsen for $350 to $500. Then you pull your axles, the third member, swap in the Torsen and put the axles back with so 90w and you are done. The swap takes less than an hour.
JEB

 

Wow, that might be an option for me then. What year Miata do I need to look for? And are they all the same, or is there some special one that I have to find other than the differences in years?

I've got a nice 5 speed tranny I was going to try to trade for a lsd rear end, so maybe I need to update my planning. Have you driven it yet? Any drawbacks to it?

Thanks for the info man...

 

Mine's from a 1999 5 speed Touring.

With the 1999 and later years, you geht the added bonus of 4.3:1 gear ratios.

Earliers have the 3.9...

 

How does the Miata torsen unit compare to the FB LSD?
Also, what about the driveshaft flange. Does it need the larger 83-85 driveshaft, the smaller 79-82 driveshaft, or do you have to swap the FB flange onto the Miata pumpkin?
I would like to upgrade to an LSD, but if the Miata torsen unit is not as durable as the FB LSD, I think it would be a secondary option in my book.

 

So if the pre-99 miatas had 3.9 rear ratio Torsens, then if I swap one of these into my diff housing I shouldn't have to worry about a change in speedometre readings? (according to http://www.mazdatrix.com/faq/ratios.htm the '84 12a rear diff has a 3.9 ratio)

Jon

 

Yes, if the pre-99 Miata's have a 3.90 gear, your speedometer *should* be unaffected. The stock 12a ratio was also 3.90.

I still want an answer on the driveshaft flange, though....

 

wrong post...

 

Torsens are great for AutoX,road racing and mild to moderate powered lightweight cars.....I.E. the Miata or 1G.

What they are not great for is turbocharged,drag racing, clutch drops.
Not that you couldnt put a Torsen in a turbo car,its just that they dont take kindly to shock loading abuse.The stock clutch type LSD can handle that stuff because of its direct design which grips under load.The weak point becomes the axles and gearset.
The Torsen is more complex and has to transfer power a few times,in various directions thru numerous small gears before it makes it to the axles.Heavy abuse tends to tear them up.

 

Well put Steve!

 

Still no answer about the &%#@$ driveshaft flange......

 

Just the LSD and gears not the whole 3rd member so no on the driveshaft flange...

 

That doesn't make any sense at all. Even if you just use the Miata gears, you still have to use a driveshaft flange. I have heard the Miata 3rd member will bolt into the 1st gen axle housing, and the axles will fit as well. The only item I haven't been able to determine, is if the driveshaft flange from the miata unit is the same as the 1st gen large (83-85) flange, or if you have to use the flange from the old 3rd member.

 

I think what he's saying is you use your own 3rd member, your own driveshaft flange, you just rip the guts out of it and replace the innards with miata torsen.

So whatever flange you've got (and whatever it connects to just inside the third member) would stay.

Jon

 

Now that made sense.
This brings up another issue. I know the 79-83 axles use a smaller bearing than the 84-85, but are the splines also different? I have heard conflicting info about this.

 

As I understand it...
You just take the whole Miata third member and bolt it in to an 84 or 85 housing. The drive shaft is supposed to bolt right up.
I will let you know...

 

I have the Miata torsen in my basement and a driveshaft in the shed. I'll post about it when I get the two together later.

 

Ok, I measured and the drave shaft will NOT just bolt up to the third member.

So, what do I need to do to change the drive shaft flange?
Will I be changing the backlash, since I am using a part different from what was there?
Do I need to pull the whole thing apart and get a new crush bearing?

What are your thoughts?

 

If you change the driveshaft flange, you *really* should change the crush sleeve. It's easy to wreck things if you change the flange and try to re-use the old sleeve, which is a "use once and throw away" item. The gear meshing and backlash wouldn't be affected in any way, that is adjusted with shims on the pinion gear and the adjustment collars on the differential bearings.

I take it you are married to the idea of using the Miata gearset, so swapping the differential into your RX-7 housing isn't in the cards. That would be easier in a sense, in that case you'd just need a dial indicator to re-set the backlash.

Generally, I like to keep a given gearset in the case in which it was set up. The main pain in the *** is setting the pinion depth, which involves a certain understanding of hypoid gears, a knack for the job, plus possibly some voodoo. Backlash is comparatively very simple.

 

Would it be simpler and less risky for the gearset to change the driveshaft flange to the Miata one? Assuming the shaft diameters are the same.

 

Yes, I am married to the Miata gear set and I too like keeping the gears with the case they came with. I like it becuase I am just not up to setting the pinion lengeth and backlash.

All the first gens use a drive shaft flange with 62mm spacing right, and the second gen car use a 67mm spacing. (Right?) So, I could take an earlier drive shaft with removabel U joints. I could change the drive shaft flange to one from a second gen car with 67mm spacing and I am set.

 

I thought you take paint or chalk and rub it on the pinion, then give it a turn and see how it marks up the ring. The marks should be at the center of the tooth.

I forgot who told me this, but there's a spacer that sits in front of the pinion in the diff. It's available in like four or five different thicknesses, each one a fraction of an inch off. Keep that with the housing when you transfer gearsets. The spacer is to correct tolerances in the casting of the housing. The gears are usually made pretty accurately.