Just had a 700r4 rebuilt, and looking to add an external trans cooler. I'm going to be running about 2k stall speed with improved internals for quicker shifts and Dex 6 fluid. My main use will be cruising and road course. What size and/or gvw should I aim for with a cooler? Also, am I fine to use the factory cooler lines and just add a fitting to the end to connect to the trans cooler, or should I just buy new line all together? (I plan to bypass the radiator completely)

 

I would not bypass the radiator cooler entirely because it helps take alot of heat out of the fluid when you are really beating on it. Direct the fluid to the in-radiator cooler first followed by your additional cooler.

 

Doesn't that heat up the coolant unnecessarily? Hence the idea of getting a large enough trans cooler.

 

Study these. My opinion - run the largest fan-cooled cooler you can fit.

 

Agree w/ both of the above; except to the extent that, it's possible to have "too much" cooler. Not in the sense of, too much cooling; rather, just a waste of space, money, and whatever else.

The ones with a fan are actually quite small physically, but move A SHIPLOAD of heat. Install per the above diagram - hot fluid to the rad, cooled fluid to an oil thermostat (some of the coolers have these built in: pay attention), then to the cooler, add a thermostatic switch for the fan on the line between the rad and the other cooler. Set it for something like 180°. Remote filter is optional but certainly desirable.

Trans fluid needs a certain amount of heat. It does not work right if it's less than 165°. Plus, the heat helps keep any moisture that infiltrates in, from accumulating. Anything between 165° and 180° is great. Start getting above 200° and trans life begins to go down. Above 250° or so the trans fluid becomes as acid as lemon juice and begins chemically attacking the other contents of the trans. Above 275° or so you can expect the trans to last no more than acouplahundred miles more.

I would also add, DO NOT put the trans cooler in front of the condenser and rad if you can help it. With a fan, it doesn't depend on "ram air" to the extent the rad and AC do; you can put it just about anywhere, as long as air can get to it and away from it, and its heat doesn't heat up something else (gas tank, engine intake air, or your SO's feet for example).