View Full Version : Differentials, temp and lube capacity.

Simon TR4a
04-26-2006, 09:43 AM
There has been a bit of discussion recently about Spec Miata diffs running temperatures up to 300-325 degrees F, because like the TR diff, they hold less than a litre of fluid.

I know there is a finned alloy rear cover available for TR2-TR6 with slightly greater capacity, but no idea how much of a problem this is for a "fast road" car used for lapping days.

I do know my old diff developed a whine over a period of 9 years so I have replaced it, and made a drain at the bottom of the rear cover to make fluid changes easier. Is this enough?

04-26-2006, 12:05 PM
I can't answer your question directly (since I don't have a TR) but I do know that the sythetic gear oil (like "Redline) is supposed to be able to "hold up" better under heat.
I have been running it in my Spridget (which has notoriously leaky bearing seals) and it doesn't seem to ooze out any quicker that the regular stuff.
I have heard of a number of Miata rears that have failed, but almost always in very high-mileage cars.

Matthew E. Herd
04-26-2006, 12:18 PM
And I'm sure those rears never had the oil changed before they were acquired by spec racers. Even then, they might not have been changed!

04-26-2006, 02:53 PM
Simon, you probably won't have a problem with the diff, although I agree that using Redline might give you a bit of an edge if it does heat up too much.

You could also consider a trick I saw on one of the TRS twincam Le Mans Triumphs - they took the back diff cover and welded on a section of oil cooler - looked like a VW piece, maybe, extending backwards, oriented up and down so that when the axle went up and down there would (hopefully) be a bit of air going through it.

No pump to move oil, but it would add a bit of capacity and I suppose they thought that there would be some circulation from thermosiphon effect or something.

Or just ignore the problem, as you probably won't come to grief anyway! /ubbthreads/images/graemlins/crazyeyes.gif

Matthew E. Herd
04-26-2006, 04:03 PM
Yes, if you got one with larger fins and/or a cooler on it, it'd cool the oil better. The heat from the hotter portions of the oil would quickly dissipate to the fins. Plus, all the thrashing of oil by the gears will definitely circulate everything.

04-26-2006, 05:06 PM
Or, if it's going to cause you to be concerned, go whole hog and put a pump, cooler, and temp gauge on the diff. You can get either electric pumps, or ones that are pulley driven from the pinion flange. The cooler and plumbing are super simple, and the extra gauge in the dash will awe everyone! /ubbthreads/images/graemlins/grin.gif
Both the Sprite and the Spit race car diff. hold less than a liter, and I've never cooked one.

04-26-2006, 05:20 PM
They're a pretty stout unit. I've not seen one come apart unless run dry for a looong time.

Matthew E. Herd
04-27-2006, 09:48 AM
In the case of the sprite rears, the live axle gives it an enormous heat capacity and even two very long 'fins' to cool it all down. It's a great design for cooling, at least.

As to the TR/spitfire rears, I don't really know. They must have reduced mass, but I don't know if it's a problem.