Yep delta T btu exchange is limited by air supply(flow) and liquid supply (Flow)
You can increase them to the limits of the piping/pump power etc but there is a limit for sure..

Bigger cooler = more capacity to exchange btu's between the air and oil.

You have the same limit in the cooler tube in a radiator, its a few inches long and a single fluid pass.

 

Good catch. It does belong here. I'm not sure how I posted that on a 9 year old thread. Doh!

As for the cooler bypass thermostat, it goes in the lines to and from the cooler. When the fluid is below the thermostat temperature, the fluid is bypassed from the inlet to the outlet without flowing through the cooler. When the fluid temperature rises above the thermostat temperature, the thermostat closes the bypass passage and the fluid has to flow through the cooler. Derale makes a few of them. Here's a diagram of how it works:




That's from the Derale 15719 "fluid control thermostat" listed at https://www.summitracing.com/parts/der-15719

When you put a cooler in the line from the stock cooler back to the transmission, you really need one of those thermostats. That keeps the fluid at the thermostat temp, and prevents it from getting too cold. As @vorteciroc mentioned, there's not much danger of damage from cold fluid, but cold fluid can cause performance issues. Actually in sub-freezing temperatures, it's possible to cool the fluid to the point where it doesn't flow very well. That's less of a problem with synthetic fluids, but with Dex-III, it's an issue in areas with extreme winter cold.

Also, there are some coolers (mostly "stock" ones) that incorporate the thermostat into the cooler itself. The best example I know of is the oil cooler on the 2nd generation RX-7. That thing is huge, and it is a dual pass cooler, with a big thermostat in the tank with the fittings on it that opens and bypasses the core when the oil is below temperature. If you see a dual pass cooler in a parts yard with an additional plugged opening on the end tank with the connection fittings, it's likely there's a bypass thermostat in that cooler.

 

The thing with the plate cooler in the radiator is that it's heavy (thick) brass, which transferrs heat much more efficiently, and that it's submerged in engine coolant, which is a much more efficient heat exchange medium than air. The plate coolers in the radiator may seem small, but they have the capacity to exchange a lot of heat into the coolant. Also, the transmission cooler plate is always located in the "cool" end of the radiator, so if the engine cooling system is operating as designed, it's typically 30°F or more cooler than the engine coolant thermostat temperature.

And if you're using and external fluid-air cooler, a thermostatically controlled electric fan on the cooler is a really good idea℠. Medium duty vehicles with automatic transmissions (think large RV's and "school" buses with Allison automatic transmissions) almost always have a large external transmission cooler with a fan built onto it in addition to the plate cooler in the radiator.

 

I don't have any pics right now but my parts list is in the Sticky at the top of the transmission section.

I have that cooler mounted where my a/c condenser USED to be. About 1/2 in. off the radiator in the front. Braided steel lines bypassing the SILLY stock "cooler" in the radiator.

My trans temp guage sending unit is in the pan.

 

Which direction do you mount plate-style coolers? which direction is best for the fittings to point--up, down or sideways? and if sideways, inlet on top or bottom? I see the big long plate-style coolers, some that were in the $109-$139 range, came down to $75-$85! Is something wrong with them that the bottom fell out on the price? I'm thinking of those or the plate style with built-in thermostat row for ~$50... Thanks.

 

When vehicle manufacturers use(d) air-fluid plate coolers for transmission fluid or for engine oil, they generally mount them next to the radiator, in clean air, with the hot oil/fluid inlet at the top and the cool fluid exiting the bottom. Look at late 1980's Volvo turbocharged engine bays for an example, or early to mid 1980's Mercedes-Benz turbo diesels.

For two pass coolers, it doesn't much matter (like the oil cooler from 2nd generation Mazda RX-7), although there might be some thermo-siphon advantage to mounting those crosswise, with the upper fitting on the end for inlet (hot), and the lower fitting for outlet (cooled). The only time that direction really matters is when there's a bypass thermostat in the end tank, like in the 2nd generation RX-7. Then it's fairly important to get the direction correct. You want the thermostat sensor "pellet" on the hot side (inlet), which dictates which line is which.