Diff a la mode
 

Running a fairly stock 90 NA in Chumpcar/Lemons. Previously had been running a stock 4.10 in 24 hour events with no issues. For this last race I found and put in a 4.30 GTU rear end. I replaced the seals and put in 75/90 Lucas synthetic gear oil. Car ran 7 hours with no issues on day 1. 4 hours in on day 2 got black flagged for leaking/smoking. Car came in and black oil was dripping out of both side seals. Each drop was smoking as it fell. I'm guessing things got a bit hot in there. After it cooled down, drove it around a bit and it was making no noises. My questions are:

1. Is the 4.30 rear end more prone to overheating?
2. Is it common to run some sort of rear end cooling?
3. Could it be that the oil I used was the wrong type?
4. Can I pop out the two hubs and replace the seals without removing the rear end?
5. Misc thoughts?

The other question I have is in regards to synchros. Does anyone make an uprated synchro for these NA cars? How difficult is it to swap in a Miata 2nd gear?

Thanks

Long races like that, I'd run a cooler. Even in some SCCA sprints, 15-19 laps, I know of cars that have coolers on the diff.

1. Don't know if the 4.30 is more prone to heating up, might be. The gears may do a little more meshing/sliding with the higher ratio.

2. see above

3. Oil is probably fine, wonder if a 95w 120 would be better?

4. Yes

5. Misc thought: In endurance races I've been on crews for(24hrs of Daytona) we ran a spool in the car. If we broke an axle/CV we would still make it back to the pits. The FC stub axles are poor designs for this though, if you look at the grooves in the stub you will see a "figure 8" wrapped around the shaft. The stub will fail right where the X is in the 8. On street tires it may not be much of a problem, on slicks you have to be careful.

With the higher ratio, you will have more heat. The cars that I have seen running coolers on the rear end have been running 5.12s, not 4.3s. My question is did you keep the stock GTUs viscous LSD? The way those work, they use a special fluid (within the differential, not the pumpkin) that changes viscosity based on heat generated by one wheel spinning faster than the other. It makes since in my head that just operating as normal, the diff would be heating the fluid. And over several hours, that could get hot!

Yes, I am using the stock GTU LSD. I see nothing in the FSM that indicates that there are two different oils for the rear end nor can I find a procedure for filling the LSD separately from the R&P. Can you point me to the procedure for filling/checking the LSD?

the diff is sealed. there's no check/service