I've been meaning to ask this question for a while.

Last September I was driving my '14 Z51 on the highway at 90 mph in 7th gear. I noticed the trans temp getting close to 300*. It did not throw any codes or warnings. It runs cooler in 6th gear.

The highway is relatively flat. Air temp was probably in the 90s.

Anyone else have this problem?

 

I'd almost suspect something wrong with your temperature sensor for the transmission. I have driven my base C7 (no transmission cooler) at all kinds of speeds, particularly hard in lower gears, and never really seen my transmission temperatures much over 200F. Of course, that is likely because when driven hard my engine oil temps would sky rocket to 290F while the tranny is still at only 200. At that point I just back off and shift up. I see no reason for having 300F temps in 7th gear at only 90 MPH.

 

What is strange is it is lower in 6th gear than in 7th gear. The only time I have noticed it getting that high is cruising on the highway for a few hours at 90 mph or more in 7th in warmer months. I didn't have any problems driving it home from KY to West Texas after NCM delivery (June 2014) or during the last caravan.

I know the exhaust running on the sides of the transmission transmit heat to the transmission. Maybe a combination of that, with 90+ degree day is causing increased heat.

I will add that it is a convertible so it doesn't have the vents on top of the fenders like the coupes. The vents are smaller openings underneath and just ahead of the rear tires.

 

base coupes don't have functional vents either.

 

I put header wrap on mine in the back part of the exhaust all around this part of it. Plus I put wrap around my headers up front on the tubes at runs close to the z51 cooler on the block.

When it was running around 100 deg temp this last summer I drove the car for 2 hr drive came home pulled the car on my 4 post lift for sure the header wrap on the exhaust in the back that I put on really does help. I could put my hand onto it and it was to bad at all as far as the temp goes. But the uncovered pipe before the wrap will burn your hand. Robert

 

The taller the gear the harder the load on the clutch and the least torque at the axle. Torque division in OD ratios causes higher op temps.

 

The torque at the axle will be the same. The HP transmitted by the clutch will be the same although torque will be higher but RPM will be lower. Bottom line is HP is what determines how much heat is input into the "system" (transmission in this case), not torque. While there is more torque on the input shaft/countershaft, the torque on the output shaft is exactly the same. There is no more HP being transmitted in 7th vs 6th and no extra heat is generated so basically, you're wrong about OD causing higher operating temps.

 

The first thing I would do is check the oil level then see if the oil pump for the oil cooler is working.

Also consider this: while this seems counter-intuitive, given equal speeds in 6th vs 7th gear, there is a higher relative velocity between rotating parts in 7th gear than in 6th gear. Every gear set (with the exception of 4th gear) has two gears meshed together at all times with one of the two gears free to rotate on its respective shaft. The free rotating gear has needle bearing to make it "frictionless" but there could be a problem with the needle bearings on one gear. It may even be 6th gear because the gear you're in has no relative rotation with that gear respective to its shaft. If the oil level and pump check out, I would suspect the needle bearing of one of the gears. It gets a little involved but it's possible to check the "resistance" of each gear set needle bearing without disassembling the car although you would have to jack it up and remove the rear wheels.

For 4th gear, there are actually no gears...the input and output shafts are locked together making it direct drive. It's also why 4th gear is the most efficient gear to perform a chassis dyno on because there is no power loss to gears. There is a roller bearing between the input shaft and output shaft meaning there is relative rotation between those shafts in every gear except 4th with the highest difference being at redline in 1st gear. As you go to 2nd and 3rd, the difference decreases. As you go from 5th to 7th, the speed difference increases but never to the point you see in 1st gear at redline because you'll never redline 6th and 7th. I wouldn't suspect the bearing between the input/output shaft.

Whatever the problem is, it isn't going to get better on its own...don't stick your head in the sand and ignore the symptoms, it's screaming loudly that it has a problem. Good luck!