How efficient is an automatic transmission? How much heat does it produce. Trying to figure out how they size the tranny cooler.

 

with the stock tranny, id see right around 130 in stop and go traffic... id be alittle lower than that just cruising. with the goerend, i see 130-160 stop and go and 130 cruising..

 

I'm not interested in the temp, but what is the heat load (kW or BTU/hr). Another way of looking at this is how efficient is the transmission (including the slippage in teh torque converter). 50%? If so, then 400 Hp would have a 200 Hp heat load on a transmission if it were 50% efficient.

 

For the purpose of what you are trying to calculate, in lockup it is 100% efficient. In fluid coupling about 75%.

However, that doesn't help as the amount of heat generated will vary depending on the energy input. I would suspect the trans cooler is sized at the bottom end of efficiency for the max load, average resistance, average energy input.

In other words, a SWAG.

 

Even at full lockup, the tranny is not 100% efficient, but it also does not requrie a cooler. Heat generated will obviously depend on how much power goes thru it. what is worst case? At full slip, 100% motor power does not go thru. Also, at full slip, what is the efficiency?

Maybe for us a SWAG, but for the guy sizing the cooler, better not be.

 

When deciding what size cooler to use I have always used the term "Big is best, more is better". I buy the biggest one I can find and cross my fingers, that was until I stopped buying automatic transmissions to use for towing.

 

I was going to post same thing.
For us dummies that haven't a clue what a SWAG is we rely on the ole BIB* way !!!!

Besides, check how hot your tranny gets when you put it in reverse, you'll be VS **











* (Bigger is Better)
** Very Surprised

 

For the pupose of what you are asking the trans is 100% efficient at transferring the input energy, most of the energy is transferred thru, some is radiated out, and some is transferred to the fluid.

Back to my original answer, how much heat is generated is a function of input energy and a whole slew of variables. A formula to claculate it could be developed but each variable change is going to give a different answer. The best you can do will be an average.

Even the coolers ability to transfer heat is going to be impacted by a bunch of variables, flow, fluid temp, ambient temp, etc. Once again an average that will meet the most conditions at the least cost is going to be a determining factor irregardless of what a formula says.

If you are asking about efficiency in power transference then like I said, 75% in fluid coupling and probbaly about 90% in lockup after parisitic loss. You can use those figures to run a heat calc but it is still a SWAG.