2018 GT PP2 Rear Differential Overheating Issue

[sonicc]

I think something like a "air curtain" that was mentioned previously could work if there's enough space...or some kind of a low profile scoop to direct air over and around the trans/diff. I'm actually looking into adding a gauge to the diff and transmission so I can keep an eye on temps while trying out different things.

Now there's really no substitute for a rear diff cooler...there's just no way of getting around that. Nothing else can match the heat exchange. But for some of us that occasionally track the car, diff cooler is a bit pricey.

On a side note, is there a reason why 2016-19 rear diff cover wouldn't work on a '15? Besides the sensor port?

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[sigintel]

Coupke weeks back I jacked all four corners up and spent several hours under the car.
It is very tight under there.
I know this could be solved simply and cheaply, but so massively busy w work.
Would love to have time to design an “air box” or air curtain for anything putting heat into the pinion, half shafts and case...

Wrap exhaust from cats back to 18” past diff.
Cheap aluminum flashing from Home Depot cut to a pattern to allow a scoop or high temp silicon line feed from front bumper.

I know... tall order.
Time is the enemy.

There isnt that much heat created in the diff, its just bathed in hot air that puts it at +100F over ambient just cruising at 85.
I am more confident that a cold air curtain could be enough.

The geto washer line solution is...
Geto
But its fucking hilarious w puffs of steam coming out the back on demand and the temps dropping.
Refilling the washer bottle with water every 20 min session would be .. uhhh geto....

 

[Norm Peterson]

There isnt that much heat created in the diff,

If you can show me where the 2% to 4% or so that represents lost efficiency in the R&P goes, and prove that it's not just heating up the diff lube, I'm all ears. OK, yeah, a really tiny amount goes into making gear noise.

its just bathed in hot air that puts it at +100F over ambient just cruising at 85.

What that hot air is really doing is limiting how fast heat can be transferred into it from the diff housing, which depends directly on the temperature difference between the hot thing (diff housing) and the fluid flow (air) going past it. It's not adding heat to the diff, not when you're out there on the track (where the diff temp problem rears its ugly head). Just not letting the diff dump its heat as fast as it keeps making more of it.


Norm

 

[sigintel]

Norm

Not sure I follow. Not looking to prove anything to anyone: all about testing and learning from others testing so a diversity of opinions and results will move the ball forward and others can reach their own conclusions.

Yes generally convective heat flows from hot to cold. Heat is energy.
Get yourself a harbor freight or amazon infrared temperature gun or thermocouple. Dont take my word.
From my actual measuring:
on a cold start, the engine, exhaust and transmission AND the air flowing off them reach a higher temperature than the diff in just a few minutes.
During that time energy flows INTO the cooler diff case from radiation, air convection, and conduction via the pinion/driveshaft and the half shafts.

Additionally there is some heat generated in the diff itself.

Since energy must flow from hot to cold, the diff temperature MUST rise ABOVE the air temperature around it to flow energy out of the diff.

“Its not adding heat to the diff” eh... dunno
“Not letting a diff dump its heat(energy)” <this yes

I think you convinced youself it actually is being heated which I agree with:
Heat is energy. The coldest object at the end of the trans tunnel 5 minutes after a cold start is the diff and it has to receive the energy from the hotter objects around it.

Temperature is not heat. Temperature is the measure of average kinetic energy.

Until the diff reaches the temperature of the air coming off all the 200F block, 200F trans and 300-800F exhaust: I am pretty sure the diff is absorbing energy like a cheese covered jalapeño on a nacho under the broiler. Mmmm cheese....

Keep in mind the exhaust temperatures are so high that the 4th order radiation heat flow cant be ignored. Worse yet is that radiation heats the most outward surface that air is flowing over on any object that can “see” the infrared emissions off the exhaust surface. Basically, all the surfaces in front of the diff that can “see” the exhaust.
Leave the oven door open and put a top rack load of nachos in the oven. Mmmm fried cheese.

Again, I am all about setting up tests that anyone else can repeat to draw their own conclusions.

About 30-40 minutes of cruise control set at 85 gets me +100F over ambient in the diff and usually pretty close to 190-205 or roughly 30 under the head temps.
This is mostly straight line driving out to the ranch up 183 out of Austin. Very little heat is generated in the diff cruising at 85 compared to max limit road course work.

I can pull off highway and into the shop and put a leafblower pointed direct at diff from back and Im +30 ambient in 15 minutes.
Driving 135 for the last 15 minutes does NOT lower the diff temp.

Anyone can test this themselves.

Anyone can also take a thermocouple and hang it in the air in various locations within/around the front of the diff near the pinion.

My conclusion is the diff is bathed in +\- 190F hot air off the engine, trans(bolted to engine block), drive shaft(bolted to trans), exhaust and resonator.

There is a badass thread where someone experimented with a different/finned diff cover. The result sure seems to support the hypothesis that the air around the diff is all hot as it had limited gains.

I am confident if you make an airbox for the diff and cut a hole in the trans tunnel and point in a 2stroke leaf blower mounted in back seat : you will easily stay below 190F diff during a 40 minute track session. Likely below +60F ambient, or 40F less than what we currently experience.

I am NOT confident on how to get an air box and much cooler air flow around the diff in a cost effective and cheap manner.

I am thinking the resonator has to go in lieu of straight pipes to make room up the middle for a pair on 1-2” high temp silicon hoses. Also thinkn 2x 1-2” hose from back of diff air box routed to dump into the low pressure area behind the rear wheels or at rear hubs.

When I get ahead of work, I will test more and aim to provide others with info to test and draw their own conclusions.

*edit:OMG. Facepalm. Somebody send me a better edit of this to replace it?

 

[Norm Peterson]

Sometimes the air will be warmer than the diff housing, other times the diff housing will be warmer. It's probably safe to assume that the main concern is the "diff housing is hotter" scenario, in which case it's not the air heating the diff.

The concept of introducing cooler air from inside the cabin crossed my mind a while ago. Hopefully it wouldn't open a structural can of worms. You'd probably need to use an electrically driven fan/blower, though. Fuel/flammables not firewalled off from the passenger compartment is generally regarded as a big no-no.


Norm

 

[mustang5o]

Just completed a track weekend after wrapping the exhaust around the diff and switching to Royal Purple 75W140. Temps each day were 80-85 at the highest and pretty humid. I never received even the first warning. Most sessions I ran the full 20 minutes and ran hard for most of it as I was running RE71R tires and they were holding up well for almost all sessions. I ran with Advance Track off. I may still consider the FTB cooler eventually but for my purposes I may be OK without it.

Side note: took second in my class (stock cars running 200TW tires and higher...stock at this event allows quite a few mods short of tunes, power adder, coilovers or big aero and of course tires. My car is stock except I had ST45/43 pads and Motul 600).

 

[JohnD]

What he said. I wrapped the rear sections of the exhaust where it goes around the diff and I did not get a single axle overheat all this year. Last year on any hot day I would get 5 - 6 laps and get an overheat. Nothing else changed, except the addition of a Race Louvers center hood vent. We had plenty of hot days this year but I had no issue. When I wrapped it I figured I was wasting my time so very pleased that it actually helped. I've abandoned any idea of installing a diff cooler, who needs the extra weight, I'm trying to get rid of weight not add more.

 

[Champracerj]

Just completed a track weekend after wrapping the exhaust around the diff and switching to Royal Purple 75W140. Temps each day were 80-85 at the highest and pretty humid. I never received even the first warning. Most sessions I ran the full 20 minutes and ran hard for most of it as I was running RE71R tires and they were holding up well for almost all sessions. I ran with Advance Track off. I may still consider the FTB cooler eventually but for my purposes I may be OK without it.

Side note: took second in my class (stock cars running 200TW tires and higher...stock at this event allows quite a few mods short of tunes, power adder, coilovers or big aero and of course tires. My car is stock except I had ST45/43 pads and Motul 600).

I'm now running the same set up including tires (except in track mode with the advanced track on) and at WGI last weekend I had no warnings no heat issues.

 

[hlh1]

This sounds like a great, easy, fix.

Question though: We used to wrap stainless motorcycle exhausts like this and it was great for keeping down the heat, but we found that this actually caused the exhaust to rust and corrode where the wrap was. Seems it holds water against the stainless steel from driving in the rain. Has this been seen with our Mustang exhausts?

 

[shogun32]

motorcycle headers or made of cheap and thin material and even crappier cladding. Not saying the factory pipes can't accelerate their corrosion but why does it matter?

 

[hlh1]

motorcycle headers or made of cheap and thin material and even crappier cladding. Not saying the factory pipes can't accelerate their corrosion but why does it matter?

I just saw where aftermarket, expensive, race headers would rust when they were wrapped and I was concerning that possibly this could occur on our Mustangs. Guess it's no big deal as I would just replace the rusted parts. I wanted to know if anyone has this happen on our Mustangs.

 

[JohnVallo]

Next time you drop/remove your exhaust, paint the exhaust with Hi-Temp header paint.
This will help keep heat in as well as control any rusting/corrosion.

 

[Norm Peterson]

motorcycle headers or made of cheap and thin material and even crappier cladding. Not saying the factory pipes can't accelerate their corrosion but why does it matter?

Would kind of suck to have to consider these pipes as being consumables on a periodic basis. Or to guess that some aftermarket alternative might be more durable and find out that you were wrong.


Norm

 

[mustang5o]

I could see that being possible but my car doesn't see much rain. I may consider painting that area as mentioned but I also think if you're not driving in the rain a lot it should dry fast enough with the hot exhaust.

 

[shogun32]

rain on a hot exhaust is unlikely to be the problem. it's the retained moisture when at ambient that keeps fluid against the metal and exacerbates corrosion.

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