Canadian_CD9A

With no other options, I had the diff replaced AGAIN with a low mileage unit, and got the matching axles to go with it. It drives very nicely now. She was locking pretty damn hard near the end, the harshness coming from it was unbelievable. It simply wasn't driveable anymore.

So, here's the answer, I'm not surprised nobody had the answer to this. Here's what was "wrong" - basically nothing.



My guess was correct when I first drove the car. The shady JDM imported diff was actually an aftermarket diff (bonus: it's the Cusco RS I always wanted), and I'm guessing that the fluid wasn't compatible, so it locked progressively worse with usage. Or, the plates/disks weren't happy to start with. I'll take it apart to inspect the discs, maybe do a rebuild, but I think this one will be going back in someday. I appreciate those who gave their input.

Punter

I can't comment on the diff, but I had the swaying under deceleration - especially when I engaged the clutch. An alignment fixed it, but it crops up when my tire pressure is low. These things are really sensitive to rear tire pressure.

CMK

Wow, that's quite an ending.

DanielB

Just seeing this thread for the first time so taking advantage of hindsight ...

Isn't the stock diff a torsen? The symptoms of the OP's JDM sounded like a clutch LSD. In fact I put a Cusco in my old Z car and it did the same thing right after the install. I was told to add some friction modifier (I think that's what it was called) and do a series of low speed figure 8s in a parking lot. Things calmed down a lot after that.

However, the problem with a clutch LSD is that it's hard to tune to only achieve partial lockup hence the car would push (understeer) badly in high speed sweepers as soon as I was on the throttle and then get loose as I let off. I think for a car driven on road courses (and the street) a torsen is a better choice. Clutch is probably better for drag where there's little turning while on throttle.

DefSport

I was going to post that you got a clutch type diff. That's all normal behavior. Nothing "not compatible" with the fluid or anything. You can deactivate some clutch plates and put in friction modifier, but all you're doing is reducing the effectiveness of what is essentially a "race diff."

The stock diff is a helical/torsen type that is very street friendly. A 1.5 way clutch diff (what it sounds like you had) is not the nicest on the street, but will offer superior performance on the track.

S2KIWI

What DanielB said. Sounded like a plate type diff. Torsens usually either work or they are broken lol. I upgraded mine to a Z/28 torsen so will be interesting to see the difference.

DefSport

Clutch plate diffs are 100% faster on a road course than a torsen, but they pay for it in slightly rougher manners.

I've only noticed understeer on initial turn-in with a clutch diff, they always seemed to provide great balance on power when things lock up.

DanielB

Hmm, not sure I follow that, and my own experience with both clutch and torsen in my previous 240Z didn't bear that out.

Isn't the primary purpose of the LSD to ensure that the less-loaded wheel doesn't spin so that the maximum power can be applied? On a road course, this is most important when exiting a corner under acceleration and the inside rear wheel is unloaded relative to the outside. With a torsen LSD, some slip between inside and outside is allowed which is beneficial under acceleration to ensure that both tires remain hooked up with the pavement with minimal slippage. With a clutch LSD, it's going to lock fully meaning that the tires must be slipping in order to deal with the fact that the car is still turning. Wouldn't this limit the amount of power that can be applied?

As I mentioned above, I ran both types on my 240Z with a boosted L28 and found it easier to apply power exiting corners with the torsen. The clutch was more entertaining in that it was easier to rotate the car when the rear end would lose traction, but I don't think that was the faster approach. Disclaimer: there were other enough other changes made when swapping to the torsen that I could not compare lap times before and after.

Not trying to be argumentative, just trying to understand if there's something else that I'm missing.

DefSport

I went from an S15 helical diff to a 1.5 way Cusco RS in my S13 (Cusco was setup to 80% lock, stock preload). The car immediately went 1.5 sec/lap faster on a 2 min/lap course, and that's with an experienced driver behind the wheel (been instructor level for 8-9+ years, tracking for 15 years).

A helical diff will absolutely SPIN the inside wheel when it's unweighted. It can only transmit some multiple of the inside torque to the outside wheel. If the inside wheel slips (essentially little resistance), all the torque goes to it, and it acts like an open diff. Off the power it acts like an open diff (no biasing across the axle).

A clutch type diff will absolutely transfer power equally to the wheels in relation to the input torque as well as braking force (for a 1.5 or 2.0 way diff). The 1.5 way Cusco RS was also more stable under braking by biasing across the axle during braking, and this also allowed one to adjust the attitude of the car more with the throttle vs. the helical as there was still biasing across the axle off throttle. The helical is also limited in how much it can bias across the axle which is limited by the geometry of it. It's typically some multiple of the lowest loaded tire and the input torque - the clutch type is essentially "completely locked" under hard throttle.

A novice or intermediate driver will not like how a clutch type diff feels, as there is no "safe space" that you can possibly go to like with a helical diff. If you go off the throttle and put in too much steering input, many cars will tend to understeer a bit (stockish S2000 does this generally). You do that with the same balanced car with a clutch type diff and the car will turn in MUCH harder.

The clutch type diff did have more understeer on turn-in because the preload made the car want to go straight more than a helical diff that is unlocked then, but on throttle, the clutch type diff promotes MUCH more stability, and the high degree of locking will make the car generally rotate better under throttle.


The diffs drive differently, but if you're skilled enough to exploit it, you can absolutely go faster on track with a clutch type diff vs. a helical diff. The helical diff has far better street manners though, and is a much better day to day diff for a street driven car.

Canadian_CD9A

The last diff (Cusco) basically acted like a welded diff when it was removed. There was nothing normal about it.

I'm starting to hear a low level groan/grind coming from the rear on start-off again, so something might yet again be wrong back there. I think my replacement is yet again a clutch type diff, since it engages the right side wheel abruptly, not smoothly, with low traction. I think I found the culprit though, the left rear brake caliper is seized. This must give a LSD fits, and probably ruined my last 2. With the Torsen, most of the torque would be going to the wheel with most resistance (a dragging caliper would always be the case), so the LSD would always be active and heavily biasing torque, which leads to the common failure. With the clutch type, it would always be engaging the right side when torque is applied, and since the dragging wheel would always be resisting, the plated LSD would flip out, too. I just hope to get this fixed before diff #4.

fwiw, clutch type diffs pretty much always beat the stock helical front diffs in Evos, and serious Honda racers almost always switch out their helical diffs for clutch types. The Torsen is nice and quiet for regular usage and doesn't wear out, those are the benefits. It does, however, become an open diff if a tire lifts off the ground (as mentioned), or in deep snow. The clutch type is otherwise superior all around. Setup is key, at full engagement, the clutch type can be a power-on-oversteer nightmare. I have the Ralliart 20-plate in my Evo detuned to 80% to prevent this. MotoIQ did a good article on setting up a Cusco diff in a lapping 370Z if anyone wants to learn more.