From a test I did for a Paper:
Quote:
A 'spirited' down shift on a hill at 55 mph, with no towing load, the ATF bulk temp went from 185F to 230F in 4.0 seconds and then went back down to about 190F 12 seconds later, a total time of 16 seconds.
This data was taken using a Nissan PathFinder with a four-speed (step-shift) Jasco and a 3.5L engine.
For a beginning Sump temperature of 175F, the clutchpack (not the bulk fluid) will easily reach 335F at 0.5 sec. after engagement and then cool down rather quickly.
Quote:
High fluid temperatures oxidizes fluid. Oxidation alters
the Friction Modifier, causes sludging, degrades Anti-Wear
(AW) agent, and degrades seals.
High fluid temperatures varnishes clutch discs, plates, bands,
valves and actuators. Varnishing of clutch discs severely
modifies clutch disc material surface friction coefficients ui and
u0.
While there may be temperature spikes, I doubt the bulk (sump) temperature ever remains high enough to seriously degrade the fluid in normal driving.
With today's advanced fluids and additive packages, oxidation and degradation is not as serious as in past years.
When does a fluid begin to degrade? The first time the engine fires.
If you plot oxidation percentages verses temperatures, then a sump temp of 300F will certainly degrade fluid faster than having an average sump temperature of 185F.
So fluid oxidation is highly dependent on how long ( or dwell time) the fluid is at the elevated temperature.