pnichols wrote:
Andrew,
I believe that one of the big engineering pluses of an automatic transmission is it's ability to convert engine horsepower to higher axle torque via spinning the engine just a bit faster instead of, or for awhile before, dropping to a lower gear.
This kind of torque conversion of course makes for superb driveability but is not as efficient as using gears to do it, so it produces more heat (from less than 100% efficiency) in the transmission while this clutch slipping torque conversion is going on. Keeping overall longterm heat to a minimum in an automatic transmission contributes to a longer life.
With lower rear differential gears (as in the E450 over the E350), of course less clutch slippage torque conversion will occur when pulling weight (whether it be only the RV or the RV plus toad). Thus V10 E450 transmission life could be somewhat superior to V10 E350 transmission life when comparing the same weight Class C motorhome pulling the same weight toad or trailer.
I understand what you're saying now. I was confused mainly by calling the torque converter a clutch; it's not one, at least not in the traditional sense of the word, although modern ones do include a lock-up clutch (for efficiency) as part of the mechanism and have done so for many years now. The variable slippage is entirely due to hydraulic coupling in the torque converter, which causes the fluid to heat up, but isn't wearing friction material down. It's not like slipping the clutch in a traditional manual transmission vehicle in terms of wear.
Excess heat is bad in an automatic transmission, indeed, and towing would generally cause a little more heat than not towing. One hopes that the transmission cooler is sufficiently large to keep the temperatures at a reasonable level in any case; there's a good bit of load on the transmission of a motorhome regardless of whether or not one tows anything.
Some vehicles these days do have automatic transmissions that are more like automated manual transmissions (often a pair of them in a dual clutch transmission), without a fluid-based torque converter, and starting out etc. does have mechanical clutch slippage in these. So far as I know, they aren't used on many class C motorhomes; definitely not on the Ford E series chassis. (The diesel Ram ProMaster I think might have an automated manual, and I'll admit I'm not entirely sure about the Sprinter or Transit.)
