Let's talk about limited slip differentials
 

What's the truth behind all the different setups that are available and how can a person go about choosing the right one for their needs? Long ago I put together a clutch type lsd that had a higher than normal preload for drag racing. It performed exactly as I had hoped.

Now that I'm focusing on road course driving I'd like to know which direction to go. I'm sure others have questions about his as well since I've yet to get a straight answer when talking with people and searching gives me tons of answers but leaves me with even more questions. How does the factory torsen compare to a cheap RX7 clutch type lsd, compared to an aftermarket OS Giken, etc...and what are the merits to each one. Are there limits to them or is it going to be best regardless of the power level, etc...

FWIW when I was researching differentials for my LS1 Miata, most of the road racers (not drag racers) agreed that a Torsen type differential was worth the extra money over a clutch type differential.

After reading up on it a lot on several different boards, and talking to racers, the general order in preference (and in increasing cost, and in increasing torque ratio) seems to be:
Torsen T2
Allison TrueTrac
Torsen T2R

I went with an Allison TrueTrac (for a Ford 8.8 housing) which is very much like a Torsen. A lot of others used the Torsen T2 or the T2R. I could have saved about $200 if I had bought a clutch diff but I wanted to have good corner exit drive stability which as you can imagine is pretty key in an LS1 Miata. The corner exit behavior with the truetrac is great, totally controllable.

Now I know you are probably wondering about what can go in a stock Miata diff housing. What I am getting at above is the stock Miata TII Torsen should be great (at least over a clutch type) unless you are making too much power for one. People say the OSG differential is supermodel tits awesome on a road course, except for the price.

The question more is, what's your budget?

If you can afford it, get the OS Geiken and be happy forever.

If you can't well, just leave the Torsen in there. I've seen it used in a 350whp car with ok response. It makes the car a bit difficult to drive and much more tiring. The engagement is not as smooth and the lock/unlock points are not always quite what you expect.

That being said, the 250whp cars seem to have a blast with them.

That is interesting... opposite of what I'd lean towards purely from tracks have berms... get a wheel airborne and a Torsen looses its function there AND can cause some issues when getting traction again (impact on spider gears and boom?).

I'm going to clutch type rx7 (rebuilt) unit as my next LSD.

I ran a Tomei Traxx diff in my AE86. They are the same as a KAAZ unit, but seem to run cheaper in the market. They are setup out of the box with less aggressive lock up (more streetable), but if you switch the plates around you are right there with the KAAZ

changed from the torsen to a osgiken unit .. one of the best upgrades i have done for track use. now i am able to power out of corners much harder.

Torsens do not have spider gears so there is a lot more tooth engagement area than with a spider gear setup.

Another advantage of the torsen type is the bias ratio. That is, it can provide a greater torque ratio between the inside and outside tire. So in tighter corners it is less likely to cause understeer (push) when exiting the corner under throttle.

Also the transition from turning torque split (say 3:1) to straight torque split (1:1) is reportedly smoother with a torsen than a clutch type. That is it has better manners transitioning the torque split when turning into or out of a turn. Less 'chatter'.

And there is much less change in performance over time. A clutch diff will wear from day one and change its behavior, like a shock absorber. A torsen degradation curve is not nearly as steep.

I would rather have the three above characteristics than whatever the benefits are with a clutch type when curb jumping. That should be a relatively short period of time anyway if the suspension and driving is decent. I've done it and it helps get the car rotated, but I've used it more on really tight tracks personally. From what I have heard/read most going from clutch type (not OSG) to torsens for road racing are happy with the results.

Pros and cons to anything. Pick your poison. Keep in mind the comparison I am making is to a 'normal' type clutch diff, with spider gears and clutches/springs in a standard type carrier. The OSG (which also uses clutches) is much different than this. The OSG is really nice, but 3-4 times more expensive than a true-trac, and 6-7 times more expensive than a LSD.

Again I am not saying you can get a truetrac in your Miata housing. You have to do a diff swap, which for parts and hassle factor gets you up into OSG territory.

The torsen spins the inside wheel when there's little to no traction on the inside wheel. Like I state on my sig, I hate my torsen with a passion.

This was my mindset like two years ago. There's a reason I have a Torsen back in my car. Rx7 clutch LSDs are absolute trash, regardless of the preload.

When considering bang for the buck... still the same opinion? Did you ever run with one that was rebuilt?

I've never had a torsen in a miata but I have had them in 3 camaros that I've had and have also had the trutrak diff as well, comparing them to my RX-7 diff I would say that I find the RX-7 diff more stable, I always would find the torsen type diffs getting confused as it where when ever you have more than a moderate slip angle, it seemed like the power would always jump from one tire to the other making a slide more unpredictable than any other clutch type, locker or spool that I have ever driven. But maybe its just me, but I'd rather have a dinosour that just always does the same thing.

Forgot to say that my RX-7 unit is rebuilt, w/ oversized discs (tighter than stock). I think it all comes down to driving style.

I don't remember a Miata with that model name.

Interesting opinions. Does anyone feel there's a power level at which one setup would be preferred over another or even a certain type of track configuration that one particular setup would shine? Or do all these thoughts rule the world in every configuration?

Didn’t you have an OS Geiken in your car at one time?

Haven driven a very tight time attack on a go cart track with curbing on it I really feel that having more lock up in the diff with one wheel very lightly loaded should make me faster.

When making a hard turn on smooth pavement without the bump on the inside wheel however the torsen hooks up just fine.

Bob

I'm kind of curious, who is running a rear swaybar and having problems with the torsen?

At cal speed last year. He also had the Xidas on and a Fastback. It spoiled him!

Sav is.

I mention the sway bar because there is a really tight banked hairpin at my local track that was causing me to lift the inside rear tire. I would just spin that tire on corner exit. Pulling the rear sway bar solved it for me. I assume it helped increase droop travel and let the inside tire get some grip. Worked for me on that particular corner. I think some of the spec drivers found the same thing. May have zero relation to what Sav is seeing. YMMV

FWIW I have my rear bar off right now.

Sav is getting the same thing. That's the nature of Torsen, trutrac, etc.

I don't run a rear bar on the Volvo. I hate my car with one on. Apparently a lot people here shake their head at a lack of rear bar though.