Towing Gear, what do you use?

[ShelbyRacer]

If lockup convertor is locked, good plan. If not, low rpms probably cause convertor losses/much fluid heating-bad gambit.
[/b]

Biggest issue here guys is this- torque convertor lock-up.

When not locked up, the convertor creates a lot of heat from fluid shear. Also, on many (GM and Dodge for sure, but probably most others) transmissions, fluid does NOT circulate through the cooler until the convertor locks. Essentially, you're making more heat and getting rid of less...

OD is OK so long as-
you don't constantly lock and unlock the convertor or keep it unlocked
you're near peak torque
your trans temps are ok

In my truck (Dodge 5.9 w/ 4.11s, 33" tires, auto obviously, a few bolt-on mods) I kick the OD off when I'm towing below about 65mph- basically anything other than cruising the interstate. I seem to get about the same MPG (about 10) no matter which way I set the trans.

[tom_sprecher]

When not locked up, the convertor creates a lot of heat from fluid shear. Also, on many (GM and Dodge for sure, but probably most others) transmissions, fluid does NOT circulate through the cooler until the convertor locks. Essentially, you're making more heat and getting rid of less...
[/b]

I don't doubt you, but what is the logic behind only cooling the oil when the least amount of heat is generated?

Why would it not use a thermostatically controled bypass like most other oil cooler applications?

Interesting...

[ShelbyRacer]

I don't doubt you, but what is the logic behind only cooling the oil when the least amount of heat is generated?

Why would it not use a thermostatically controled bypass like most other oil cooler applications?

Interesting...
[/b]

I would agree.

From what I'm told (my cousin owns a trans shop), when the convertor locks, the valving causes to flow to reverse (this is actually what causes the clutch to lock) and fluid flow changes. This opposite direction is required to get the fluid to flow through to cooler.

Makes no sense to me either, since I'd think that it would make more sense to only cool it when there's convertor slippage (if you *had* to make a choice).

I also remembered that part of the issue with constant locking and unlocking of the convertor is that it puts excessive wear on the convertor clutch mechanism that locks it up. Obviously everything wearable has a finite life, so the less wear, especially under heavy load, the better...