2018 GT PP2 Rear Differential Overheating Issue

[dickeb]

I've read this thread and the other (really long annoying troll version) on the general forum and can't find much...

Can someone point me to where I can find out about why overheating the differential is bad, and whether those effects are cumulative (wears out seals, internals of the diff, or something like that), or we are just trying to avoid some catastrophic type of failure (locked-up diff)? I suppose if manufacturers are putting sensors and coolers on there it's for good reason, just trying to understand the underlying issues so I can figure out what the most appropriate solution is for me.

Also, what's a good heat wrap for the exhaust? Been a long time since I messed with that stuff.

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[Mountain376]

I've read this thread and the other (really long annoying troll version) on the general forum and can't find much...

Can someone point me to where I can find out about why overheating the differential is bad, and whether those effects are cumulative (wears out seals, internals of the diff, or something like that), or we are just trying to avoid some catastrophic type of failure (locked-up diff)? I suppose if manufacturers are putting sensors and coolers on there it's for good reason, just trying to understand the underlying issues so I can figure out what the most appropriate solution is for me.

Also, what's a good heat wrap for the exhaust? Been a long time since I messed with that stuff.

When it comes down to it, the thing affecting people is the factory warnings and then limp mode, preventing people from fully completing and enjoying a track day. Behind that, and why Ford has the checks in place, is to keep the car from failure.

At rear axle gear oil temperatures of around 250-270F, you aren't going to suddenly fail a rear end. However, you are operating in an instance where you are breaking down the gear oil at a very accelerated rate, decreasing the hydrodynamic film layer, overloading the oil's capability to cool and begin creating excessive pressure inside the axle housing. This point is more about sustainability and not all-out failure. Once you get to around 300F, you get into an operating condition where the gear oil is simply not able to perform to it's intention, breaks down at a significantly elevated rate and you risk failure of the gears, bearings or seals and/or pushing gear oil out the seals (loss of fluid and fluid on the track/tires).

The word is that there are two limits: an initial warning of too hot gear oil, somewhere around 260F, and a final warning/limp mode, somewhere 280-300F.

 

[aleccolin]

Also, what's a good heat wrap for the exhaust? Been a long time since I messed with that stuff.

I'm trying heat shields as shown in post #82 above, should be easier and more effective than header wrap, not much more cost if any.

Been slammed with work, as soon as I can get these on and figure out how to access the diff temperature sensor output (going to try a BDX first), I'll post some results.

FYI based on a conversation I had with a few other guys who track their cars with Torsen diffs, and two who did Formula SAE when we were in college and used a spec Torsen diff in their FSAE cars, I'm pretty confident the real issue is heatsoak from the exhaust and not heat generated by the diff. To that end, anything that helps keep heat off the diff should improve the situation.

 

[GT350Brakes.com]

I did a feeler a year ago for a billet finned diff. At 1k a piece no one was interested.

It’s a bit more than say a S15 or other diffs since the cover is structural.

 

[Norm Peterson]

FYI based on a conversation I had with a few other guys who track their cars with Torsen diffs, and two who did Formula SAE when we were in college and used a spec Torsen diff in their FSAE cars, I'm pretty confident the real issue is heatsoak from the exhaust and not heat generated by the diff.

Not convinced. Hypoid gearing is only around 95% efficient, meaning that 5% of the power you're using ends up as heat. At the 350 HP level (as a guesstimated average), that's around 16 HP, or like having a 12,000 watt space heater running full last. That's about the half again as much heat as all four stove top elements turned all the way up, combined.

Nobody is arguing that heat added from the exhaust doesn't hurt. Even if it doesn't actually add any heat to the diff it still gets in the way of heat being dumped into it from the diff. You can't dump as much heat from 275°F aluminum into ambient air that's already been heated by the exhaust. Heat flow needs a temperature difference for it to happen.


Norm

 

[aleccolin]

Not convinced. Hypoid gearing is only around 95% efficient, meaning that 5% of the power you're using ends up as heat. At the 350 HP level (as a guesstimated average), that's around 16 HP, or like having a 12,000 watt space heater running full last. That's about the half again as much heat as all four stove top elements turned all the way up, combined.

Nobody is arguing that heat added from the exhaust doesn't hurt. Even if it doesn't actually add any heat to the diff it still gets in the way of heat being dumped into it from the diff. You can't dump as much heat from 275°F aluminum into ambient air that's already been heated by the exhaust. Heat flow needs a temperature difference for it to happen.


Norm

Torsen differential overheating issues aren’t a well known or widespread problem across motorsports and multiple platforms, it’s not like you’ve ever heard someone say don’t use a Torsen it’ll just overheat, so that made me think initially it wasn’t the design of the rear end or diff itself. You make a good point about the hypoid gearing, but the vast majority of the time that’s only going to apply to the pinion and ring gear interface which is always working therefore always making heat, the gears in the diff are only making heat when they’re actually differentiating, so some small fraction of that 5% you referenced.

Relevant post from Torsen:
https://torsen.com/667-2/amp/

 

[Norm Peterson]

Agreed. In comparably hard usage I doubt that the fluid in a clutch or cone style diff would be running all that much cooler.

I did want to put some numbers out there to show that the amount of heat we're trying to get rid of is a whole lot more than, say, what a 150-watt incandescent light bulb or even a 1500-watt space heater makes.

Thanks for the Torsen discussion link. Could be timely for more reasons than just this thread.


Norm

 

[Mountain376]

Torsen differential overheating issues aren’t a well known or widespread problem across motorsports and multiple platforms, it’s not like you’ve ever heard someone say don’t use a Torsen it’ll just overheat, so that made me think initially it wasn’t the design of the rear end or diff itself. You make a good point about the hypoid gearing, but the vast majority of the time that’s only going to apply to the pinion and ring gear interface which is always working therefore always making heat, the gears in the diff are only making heat when they’re actually differentiating, so some small fraction of that 5% you referenced.

Relevant post from Torsen:
https://torsen.com/667-2/amp/

Just to make sure, and im just basing this off the way your post comes off:

Torsen = differential
Hypoid = design of ring & pinion
Rear End = the assembly of the rear housing with the differential and ring & pinion installed

Maybe someone can try running their car on track without the rear mufflers/disconnected and the cross-under pipes gone. Then, same day-same track, connect it all back up?

 

[aleccolin]

Just to make sure, and im just basing this off the way your post comes off:

Torsen = differential
Hypoid = design of ring & pinion
Rear End = the assembly of the rear housing with the differential and ring & pinion installed

Maybe someone can try running their car on track without the rear mufflers/disconnected and the cross-under pipes gone. Then, same day-same track, connect it all back up?

Yes that’s correct.

As far as the exhaust, testing a car with side exit in front of the tires vs a stock rear exit design would be helpful, but I don’t think I’ve even seen that on the 6G Mustangs

 

[Mountain376]

Yes that’s correct.

As far as the exhaust, testing a car with side exit in front of the tires vs a stock rear exit design would be helpful, but I don’t think I’ve even seen that on the 6G Mustangs

I was thinking that, too, but the results would be more meaningful in A-B-A format.

 

[t44tq]

I'm curious- can you just bolt up the GT350 diff cooler setup? The parts are mostly available except for the diff cover. Cost around $800 for the whole thing.

 

[ihasnostang]

i would say the exhausts are completely responsible for the overheating. a f150 running 75w-85 moving 17k lbs (5k truck +12k trailer) steady state under full load for a long time is equally as demanding as a track day. data logging and mounting a thermocouple half way between the exhaust and diff would be interesting to see.

 

[Hack]

I'm curious- can you just bolt up the GT350 diff cooler setup? The parts are mostly available except for the diff cover. Cost around $800 for the whole thing.

That's what I did in my car. You are going to want to buy the diff cover, though.

 

[johnson.ba]

a f150 running 75w-85 moving 17k lbs (5k truck +12k trailer) steady state under full load for a long time is equally as demanding as a track day.

I'd argue that for the diff, that's not as demanding as a track day where your LSD and traction/ESC are all working together along with the heat generated in the exhaust. The f150 trans in that scenario is likely more taxed that one in a track car in that scenario.

 

[Hack]

i would say the exhausts are completely responsible for the overheating. a f150 running 75w-85 moving 17k lbs (5k truck +12k trailer) steady state under full load for a long time is equally as demanding as a track day. data logging and mounting a thermocouple half way between the exhaust and diff would be interesting to see.

How would a truck ever be running under "full load"? If you are talking the weight of the load that it's hauling - sure it could have full load.

However, a track day is either (nearly) full throttle or full brakes. That is a true heavy load on the components with respect to heat.

There's really no comparison between a truck on the highway and a car at a track day, unless your theoretical F150 is always running either up or down a steep hill at full throttle or full brakes.

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