Inspector24 is also running an OPM diff. For his transmission (G-type) it was Quaife @$1300 or Phantom Slip. The OPM was an excellent alternative.
The road racers love the OPM and other clutch diffs because there is always some percentage of drive to either wheel when you bump a curb and go two wheels up. Helical gear diffs tend to revert to open diffs if one wheel goes up and then you have no forward drive. That said, there shouldnt be a reason for a FWD car to have a front wheel go off the surface in autocross.
One thing I have noticed though is that clutch diffs and helical gear diffs feel differently. The clutch diff (SM civic with a Kazz) felt very agressive. The steering feels heavier but with little kickback. There is a ton of corner entry turn in off throttle. Im guessing since the diff is always loading both tires so some extent, when you turn in there is a bit of drag on the inside tire. Its kind of like "diff braking" for the inside wheel. That seems to tuck the nose in faster in the corner. On the other side, getting out of the corner I had some on throttle corner exit push (probably again because the inside wheel is loaded). Given time I probably could have driven around all of that. Although I dont know how aggressive the owner of the car set up the plates in the Kazz, I imagine all clutch diffs will have some traits like this. It Felt really nice though.
The torsen is a lot more smooth/seamless. The steering is a lot lighter. Ive heard that some torsens, on uneven surfaces, can "hunt" for traction sending torque to one wheel and then the other quickly. A lot of torsens now come with preload shims to mitigate this issue I think. I havent felt this with the diff I have (Guru motorsports out of Oz) and they preload their diffs. Off throttle turn in isnt as aggressive as the clutch diff but on throttle corner exit is better. Get on the gas as fast as you can and the nose grabs and pulls you out of the corner. Seems as long as theres a little power going to the torsen its great. Off throttle.....not so great(mental note....more LF braking). Again, just takes a while to get used to the differences.
xelderx said:
There are 5.5" twin disc clutches available for Miatas. Hmmmm.....
When I had to reshim the diff bearings in my car I looked at upgrading my clutch as well("since its out....."). I was surprised at how fast I got that done considering I had to build it from scratch(no flywheel options in 116t size).
I was thinking about the 5.5"ers too, but my car isnt 1900lbs of CSP weight. I had concerns about how durable and manageable that size would be for a heavier vehicle. I worked with Quartermaster and I went with the 7.25" twin disk with the rally clutches(a bit thicker so they will be able to be slipped a bit more/wear longer). Your cars weight is between mine and the CSP miatas, you may be able to get away with the 5.5". If not the 7.25" is a viable option. Yeah, Its heavier than the 5.5"....but depending on how its set up (I used a
3lb "flywheel") the whole assembly can weigh the same (or less) than a 5.5" with aluminum flywheel. Oh, and it was cheaper to boot
Either way...get one! It transforms the motor like you wouldnt believe.
Gavin (loves his GURU torsen and his <14lb clutch assembly...even though he stalls @ 3 times getting to the start line :s )
