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      09-17-2012, 04:08 PM   #19
ajm8127's Avatar

Drives: '07 335i
Join Date: Aug 2012
Location: Pittsburgh

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Originally Posted by N54_Fan View Post
I have tried to estimate how much BTU thermal loss we should shoot for in an appropriate tranny cooler. There are 2545 BTU per HP. Therefore if we are running 200 HP over stock (lets say 500 hp) then we should want a tranny cooler that is rated to 500,000!! It does not exist! When I looked at the various Corvette and Monster truck type of forums they generally ran as big a tranny cooler that would fit and have reported giid results. Some of the best ones out there are the Hayden Rapid Cool and the B&M Racing Universal Supeecooler series. I have narrowed it down to these two. They are both bar and plate designs which is most efficient.
I don't think that is the proper way to size the transmission cooler.

BTU's are energy, and horsepower is power. Power = energy / time. Most transmission coolers are rated at a BTU/hour value, which is also power (energy / time). Now your conversion factor of 2544 is correct for converting HP to BTU/hr, but your fault lies in your reasoning that all of your horsepower in excess of stock is going to be dissipated in the transmission.

The transmission has a certain efficiency rating, and this is how much of the engine's power that is lost in the transmission. This assumes you are in a gear and the clutch packs are not slipping. When the gears change, the clutch packs disengage and re-engage and in doing so slip. This generates a lot of heat very quickly. I think its a rapid change of gears that is causing the heat buildup in the transmission, and forcing the ATF beyond the critical temperature of 130 C. The toque converter also produces a significant amount of heat, but I am not sure when it is locked in this car. A locked torque converter should produce far less heat (maybe none) compared to an unlocked one.

We need to know how fast the fluid changes temperature over time to be able to calculate the size of the cooler. The only way to do this would be to measure the fluid as it exits and enters the transmission. This would depend on how you were driving. So if you wanted to size a cooler for a road course, you'd want to do a couple laps and log the temperature values. We know the specific heat of ATF and also the volume of ATF, so we should be able to calculate how much heat flows into the ATF over time once we have logs of how the temperature changes.

I personally do not have the means or opportunity to take these measurements. Also, looking over the calculations, I see things I remember from school, but my specialty is electronics, not thermodynamics. I would sooner say use the cooler size referenced in the link I posted in post #5 because it seems to fit reasonably well.