I have watched a C6 and now 2 C4's with temperature gauges. I also have information about a stock C6 in an old Ford truck with big tires on it. In all cases temperatures have run over 200 deg F pretty steady.

I just opened up my other C4 that is in my ranger behind a 500hp 2.3. I have seen 250deg plenty on that one with the transbrake on. I had no color on the steal and did not have any more than normal clutch dust.

I have also noticed there is only one chart out there about the transmission fluid temps and transmission life, and everybody quotes from it like it is honest truth. All this temperature VS fluid life is from one source it seems, and that source sells transmission coolers.

The problem I have had is with getting consistent data as well. I have 2 different temp gauges that read 20 deg apart, I have taken readings from different parts of the transmission (pan, cooler line) and they are up to 30 deg apart. So when you look at a general chart that has been around in since the 70's and comes up in every transmission temperature thread I run across on the internet, then I wonder how much data backs it up, and how much variation is in that data.

Found some information from an engineer who works in the thermodynamics side of the Ford Motor company and he did not feel 220+ deg under server duty was a big deal. That was still within manufactures tolerances.

Also have information from towing with a 700R4 which was seeing 240 degs.

Also found a company that does transmission fluild breakdown testing which was running a 4l60E over 300degs for hours on end in a test stand.

Also note that fords E4OD's temp warning comes on at 300deg.

And consider that 90% transmissions only cool from the rad tank which is not as hot as the engine, but still only offers minimal temperature drop for a transmission to actually run at 165 deg.

I had all this information above before I started this post, but I wanted more real data and I am thankfull for that.

So i realize that I would be better off with lower temps, but I think it is one of those things that maybe is not a problem untill you have information to worry about. Maybe I should take the gauge out (J/K). I am not saying that I will not have to do a fluid flush sooner under these conditions, but the fluid in my C4 after a trip seeing lots of 250 deg looks and smells better than half the vehicles I work on. I could certianly not honestly suggest a flush to a customer if they owned my truck, based on the condition of the fluid. It looks and smells like new.

If my transmission does die in 25,000 miles I will rebuild it again and probably put a good converter in it. I think most of my extra heat is coming from an old tired converter that came out of a C5, as the transmission itself is not slipping to generate this heat. I wonder if this was a C5 lockup converter and that lockup clutch is slipping.