Trans Temp Important for Fuel Economy?

[jeff88]

Does the temperature of the transmission matter with regards to fuel economy? I know it's good to keep the fluid "cool", but is there a point where too cool is actually detrimental?

Thanks!

[silvermazda]

Good question. Going thru a little automotive education, I'd say to a degree. At least in automatics, fluid gets pump'd around. That fluid is thinner when hot. Cold transmission fluid can be anywhere from 40-60F and heat up to 200-260*F in normal operation (this is more of a manual fluid spec, auto's have a radiator compartment oil cooler that keeps temps more regulated). Colder fluid more resistance to pump thru oil passages in the transmission.

ATF is Wt10 oil. Pretty thin. but not 5w20 thin.
Manual transmission use 75w-90 which is more of a 10w30 (old honda spec)

Manuals dont pump fluid thus a little more efficent. (someone can add here, but off the top of my head a convention manual is 85-95 efficient and autos are 80-90%, newer pump less fluid and work better)

[Baltothewolf]

Originally Posted by silvermazda

Good question. Going thru a little automotive education, I'd say to a degree. At least in automatics, fluid gets pump'd around. That fluid is thinner when hot. Cold transmission fluid can be anywhere from 40-60F and heat up to 200-260*F in normal operation (this is more of a manual fluid spec, auto's have a radiator compartment oil cooler that keeps temps more regulated). Colder fluid more resistance to pump thru oil passages in the transmission.

ATF is Wt10 oil. Pretty thin. but not 5w20 thin.
Manual transmission use 75w-90 which is more of a 10w30 (old honda spec)

Manuals dont pump fluid thus a little more efficent. (someone can add here, but off the top of my head a convention manual is 85-95 efficient and autos are 80-90%, newer pump less fluid and work better)

Thanks for this extremely insightful information! I was thinking this on my way home from work myself yesterday, but didn't see it a big deal to ask.

[matt36415]

Many auto boxes will not shift into the higher gears untill they reach a set temp, heat is the most common killer of auto transmissions, the fluid breaks down, gets burnt... everything goes nasty.

[aardvarcus]

Good points made thus far, as the temperature rises typically the viscosity falls, but if the temp gets too high it can damage the fluid or transmission. Just to throw out some numbers, Amsoil posts the viscosity of their fluids at multiple temperatures. For example:

Amsoil Synthetic Manual Transmission and Transaxle Gear Lube 75W-90 API GL-4 (MTG)
Kinematic Viscosity @ 100°C, cSt (ASTM D 445) 13.9
Kinematic Viscosity @ 40°C, cst (ASTM D 445) 87.6

Typically for constant load (cruising the highway) you will usually reach an equilibrium temperature, because the lower viscosity fluid will generate more heat until the temperature rises and the fluid becomes less viscous. This balances out with the heat being lost by the cooling of the transmission, either active or air flow.

Note this same principle also applies to other gear driven parts of a vehicle, such as axles, transfer cases, portal hubs, etcetera.

[MetroMPG]

BMW was investigating transmission fluid heaters to aid fuel economy. I can't find the reference now of course, and I don't remember whether it was exhaust scavenging or electrical heating.

In short: the quicker you're at full operating temperature, the better.

[Daox]

Definitely getting them warm quickly is better. Cooler thicker oil takes more energy to sling around, and that energy comes directly out of your gas tank.

[mcrews]

Because I drove sooo many miles w/ the Q45, I ran an additional cooler w/ a fan blowing on it. I never worried about it being 'too cool' with a lot of short trips. I also ran a transtemp gauge. I was more worried about it getting too hot and ruining te tranny.

[silvermazda]

Originally Posted by MetroMPG

BMW ... I can't find the reference now of course, and I don't remember whether it was exhaust scavenging or electrical heating.
In short: the quicker you're at full operating temperature, the better.

Probs electrical unless they want to bake that fluid when driving their v6-8's. Also, can I bring up a point? How about heating the differential, suspension and tires? (from a durability point of view) On my vehicle I take it slow, but that's because I got a full 1-1/2 blocks between my home and the Blvd, so everything can warm up. ~30mph in 3rd going 2K rpm. And as the 2 full minutes are going the engine bay is heating up, drive train, myself.

Plus that piston to engine block clearance is getting closer to operating spec. So after all this discussion, we'll all do it F1 style and run 180*F water thru our engines to warm them up and allow them to go from seized in place due to tight tolerances to tight running engines. Capice?

[jeff88]

Thanks guys! I guess a little bit more info would be helpful...

I am getting a trans cooler for my 4runner to bypass the radiator. The 4R is well known in certain circles to have a 'pink milkshake' when ATF and coolant mix from the radiator going bad. To prevent this, I am going to install a trans cooler.

So what I'm interested in is first, does the cooler make the trans take longer to warm up or does it not 'open' until the ATF gets to a certain temp threshold? Second, if sized too large can the cooler actually over cool the ATF after it gets to operating temp.?