This question comes by way of www.mustang6g.com forum user Brent Dalton. This came from a discussion regarding managing axle temperature on the S550 Mustang and preventing overheating of the axle during track use. For reference, 2016 and newer Mustang GT models with the Performance Pack (PP) option (which includes a Torsen® differential) have a temperature sensor on the rear axle cover to monitor axle temperature and warn the driver if temps become high.
|Originally Posted by Brent Dalton
…So it’s only really on 3.73 torsen diffs… so it makes some of us wonder… is it overheating due to the torsen and the regular type diff’s don’t suffer the same? A few guys … are working on the answer.
This is a worthwhile question, especially as more people take these cars on track day events. A couple of comments – all limited slip differentials (LSD) are friction devices. The higher the TBR/locking effect is, the more friction it generates. So, the Torsen® in this car – with a TBR of around 2.8:1 (versus 1.8:1 for base clutch plate LSD) – does create more friction than the base differential does. However, with all of that said, friction only creates heat when you force slippage of the friction surfaces under high (torque) load. I think that if you could actually watch wheel speeds during track events, you’d be surprised by how little differentiation occurs, at least under load. At that TBR level, the Torsen® isn’t allowing the wheels to change speed much, even if it continues to bias torque side to side as warranted by traction conditions. In general, in track use under high torque loads, the TBR level is sufficient to prevent most differentiation. If you have a plug-in device that’s interacting with the vehicle CAN by way of the OBD2 port, see if you can monitor the rear wheel speed sensors and see what actually goes on during a hot lap. You might find it interesting.
Anyway, the differentiation level is low, and even when it does, the differential rate (referred to as delta-N) is also quite low – on the order of 15-20 RPM difference from side to side. So the frictional heating component that the differential contributes to the axle oil is pretty low, in relation to the system as a whole. You get a lot more heat generated by the ring and pinion due to the hypoid mesh, which is running constantly, and at much higher speeds than the differential gears operate at. Past that, most of the heat probably comes from external sources – like the proximity of the exhaust, which passes quite close to the axle carrier on this car. Combine that with the fairly small volume of oil available to absorb heat, and you get the situation you’re in.
In my opinion, likely the biggest reason that Ford monitors axle temp on Performance Pack cars isn’t because the Torsen® creates more heat, but rather because they see a greater need to pay attention to axle temp on those cars, due to the perceived (track) usage. I expect that in their minds, the non-PP GT models are much less likely to be on the track, so they (Ford) doesn’t need to spend the extra couple of bucks to fit those cars with the sensor. It isn’t that the non-PP axle is likely to run cooler, but instead they are less likely to be in a situation where excessive heat is generated.