Rear differential heat
 

Has anyone run into excessive rear differential heat while endurance racing? My team lost a fairly new rear diff last weekend due to overheating. We're not sure what the cause was but it may have been 5/30 motor oil mixed in with the 75/140 syn gear oil. It drained like water when super hot. We've been racing our FB for ten years without issue but the car is much faster now through development over the years. I think were at around 180hp from a street ported 13b 6 port. We're thinking maybe it's time to add a diff cooler but don't want to jump the gun if that's overkill. Anyone else running one? Thoughts, ideas and opinions welcome....
https://cimg0.ibsrv.net/gimg/www.rx7...fdb63e9b3d.jpg

https://www.youtube.com/user/conekiller13

paging @Carl
if anyone knows, it will be him :)

make sure you use a metal-head recirc pump and not a plastic one, on an RX8 with 5.125 rear gear a cooler and pump are mandatory just for track, let alone endurance.
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I appreciate your confidence Alberto, but I am probably not so helpful on this. I have never had gear problems in my car. The bolts holding the third member have come loose a couple of times, but even then no gear damage was incurred. I resolved the loose bolts by installing studs and using metal locknuts. I generally have run 4.3 gears (from Miata), Redline 75-90, 140-150HP at the wheels and 30-40 ftlb LSD break-away. Having said that, my buddy has blown 3 diffs in the last 4-5 enduro weekends. He is running 4.77 Kia gears and has had both Torsen and clutch LSD's. Not sure what gear oil and break away, although I think he has set them up pretty tight. He is convinced that it is overheating, but I am not so sure. I guess it is possible that the lower gear heats up faster since the pinion has to turn faster. Certainly once things to fail overheating seems likely as parts that weren't meant to rub together start doing that. I suspect that his problem has been set-up/installation. In any case, he recently switched from FB to FC with 4.3 gears and is installing a diff cooler to be extra cautious (since 4.3 gear sets are apparently nearly unobtanium for FC's). Dave Lemon of Mazdatrix apparently strongly recommended it for FC's too, especially for even lower gears.

I did blow up the diff on my spec miata the first time I raced it. That was my fault. I ignored a minor (or so I thought at the time) gear oil leak. Turns out the capacity of the NA Miata diff is only 0.6 liters compared to 1.7 liters in the FB RX7, so it didn't take too much oil leaking out to leave too little inside. It blew a big hole in the diff housing and according to the course worker it expelled quite a large fireball out the back of the car.

Sadly for me, now that I have stated that I have never damaged RX7 diff gears, it will probably happen next time out :-0 Dang!

I suspect this wasn't very helpful, other than maybe providing some additional data points.

Good luck,

Carl

Also meant to add that adding 5/30 motor oil might not have been such a good idea. Specs for motor oil are way different than for gear oil.

Carl

Trans and Diff life
 

Offering this from the “for what it’s worth” department. I’m in TX where ambient today will be 105F.

Ive a dedicated track FD generating~375 whp with an OE transmission and diff. This card has used two used/unknown quality PO transmissions in less than 60 track hours and the diff pinion trailing edge is pitted.

Engine is now out also and I’m building it for ~450 whp. Hope to run with early model GT3’s.

The trans is being built with Libertys Gears Chromalloy input shaft and OE bearings and synchros AND a Derale fanned cooler and metal-geared pump. If I had the money I’d have Libertys build me a dog box transmission, but I need to see if this will significantly extend trans life without it.

im building a 4.11 differential to match corner exit speeds and engine torque better for my home track. Planning to use stock LSD. Installing a simple finned cooler and metal-geared pump also.

Going to run Neo Synthetics 75w-90; TA cars here in TX prepped by pro’s swear by it. You can get it on Amazon.

Can we be friends? haha. Pursuing a similar transmission rebuild from Liberty myself and I recently ordered an OS Giken diff. In the midst of building the car up for ~400hp.

@TeamRX8 got a link to your diff cooler setup? I need to do that on an RX8 race car I've been rebuilding...

https://www.rx8club.com/rx-8-racing-...2/#post4654897

two posts below the link is a picture

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So, if we had the motor oil in the diff it was by accident. I had a minion at work fill containers for me, one gear oil and one motor oil.

WE now have conformation that the failure was due to the ready to run third member I bought was set up too loose. The crush sleave was over crushed and then shimmed to try and take up the gap. We're going to run it at the next race in Two weeks at P.I.R. with a temp probe inserted through the vent in the rear end to get an idea of what the temps are now with new gears set up correctly. That should tell us if a cooler is a good idea or overkill.

Ran an FC with a comp prepped clutch type diff for years in sprint racing without issue with 4.10 and 5.12 gears. Gear type diffs will generally generate more heat. We run a Wavetrac gear type in a 9” in our road race car currently with no cooler and temps stay in the 200-220 range.

When you put it back together, put a gauge on it and then you’ll know for sure how hot you’re running it and won’t be guessing.

Looks like your diff survived the Portland enduro and seemed like a successful weekend.

Carl

Yes it did. We put a sensor on it and were seeing temps around 275 degrees. My understanding is that is too hot for longevity. So we are looking at cooling solutions both passive and active. We almost had a successful weekend. Saturday while in 4th place in class, having been bumped up to Group B due to low car count (27 Group C cars, 5 Group B and about 10 Group A), the 4th gear synchro exploded leaving the trans stuck in fourth with around 2hrs to go. We pulled in, swapped the trans and got back out with about 45min left. We finished 7th. Sunday, they left us in B. We got up to third in B mostly due to attrition as we had to manage lap times to keep engine oil below/at 230 degrees. Coolant temp never got over 205 degrees. Ambient air temp was around 98 degrees by that time. They had shortened driver stints to 1hr due to heat so we all had twice as many pit stops to take. At the last stop for the final hour when the last driver got in the car something broke in either the clutch or the input shaft. We haven't pulled the trans yet to inspect but there was no longer a connection from engine to transmission. So from 3rd to a DNF. Defeat snatched from the jaws of victory.

You should come up for the ORP race at the end of next month.

Interesting. Race Hero somehow didn't show the 1:45 needed to replace the tranny on Saturday. Just saw it on Speedhive. Anyway, sounds like everyone got plenty of seat time.

With the oil temp that seems kind of high. I only have about 150 HP, so less than you, but my oil temp stays around 200 degF even when ambient is well over 100 degF. I did have problems when I initially moved the cooler out in front of the radiator. I just used the old Pro7 config of hanging it out front. In that config I was getting around 230 degF. After a little more investigation, that sounds like about what the Pro7 guys were getting in SoCal during sprint races. Not really a problem for a sprint race, but very concerning to me for endurance. I ended up making a diverging duct for the cooler and that dropped the temp by 20-30 degF. There are pics of the modification in Scott's build thread. You might think about something like that to keep air going through the cooler rather than around if you don't already have something forcing air through the cooler.

Cooling your diff does sound like a reasonable idea at this point. I did just text a friend who is a Mazda engineer and as part of his job works with their vintage race cars and also with an enduro RX8.

Probably won't make ORP, since I have a couple of long drives up to Laguna in the next couple of months. Sprint racing up there this weekend and spectating historic races in August. Will definitely be at Thunderhill in November. Am planning on PIR next year tho. Convenient opportunity to visit my brother up there too.

Cheers,

Carl

Cool, we will definitely see at Thunderhill then.
We still run the oil cooler under the radiator as stock but the whole assembly is tilted forward about 45 degrees. We have a duct based on Scott's thread with a bottom that goes under the oil cooler to just passed the cross member. Oil temps were at 200 on Saturday when it was "only" 90 degrees so a possible issue is the undertray was pushed up a bit by bad jack placement during the transmission change. We may have compromised air flow. A separate scoop to the oil cooler like you have would be good to do as well.

I'm curious what your engineer friend thinks on the diff temps. Were exploring adding heat sinks to both the diff and the trans or using one of the extra engine oil coolers we have with a pump as a heat exchanger mounted at the rear behind the fuel cell.

https://cimg1.ibsrv.net/gimg/www.rx7...3fc7517a51.jpg

Nice picture!

He felt that 275 degF was too high. Probably 250 degF max with a target of 220 degF for longevity.

Carl

Can you ask him if he thinks passive heat sinks attached to the housing would drop the temp that much or if we really need to pass the fluid through a heat exchanger?

One of the cool things with Lucky Dog is the two professional photographers Cathy hires for each race that supplies free pictures for us all.

I will check with him and see what he says. It might be a couple of days.

Carl

I guess I should ask first. Do you have a stock FB solid axle rear end. My understanding of passive cooling would probably require a diff housing where the back side is removable, like the FD and RX8. I might be confused though.

Carl

https://cimg4.ibsrv.net/gimg/www.rx7...51e64102ce.jpg
Yes, were running a GSL-SE rear end. One of my guys had the idea of attaching cooling fins(heat sinks) to axle tubes to draw heat out as the fluid sloshed back and forth on corners and also on the back of the diff housing.

In order to get much heat transfer, the fins would have to be welded continuously, which could be difficult to do without warping the axle housing. Even installing something like in the pictures won't be very effective unless some kind of heat transfer "goop" is installed between the finned piece and the housing.

I will ask though and see what he thinks and if he has any experience with something along those lines. My own heat transfer experience is with large industrial heat transfer systems, so not exactly the same, but the engineering principles are the same.

Carl

That all makes perfect sense though and along the lines of what I was thinking.

how about some kind of air duct? if you had a duct to blow some air on the diff maybe it does enough?

I think what we are going to do is plumb in a cooler. Using a Tilton gear oil pump, thermostat switch, filter and -10an lines to an FB engine oil cooler ( we have a couple spares) mounted behind the fuel cell with a duct made under the cell to guide air up to the heat exchanger. I'll post some pics in a couple weeks when we get it together.