the reason for the secondary cooler on an overdrive tranny such as the 700r4 or the 2004r (or newer models) is because when the overdrive is in use, and the torque convertor gets into it's 'locked' position, it apparently generates additional heat (more heat than say a th350/400 in the same situation)...
I've done quite a bit of research on this over the years, and this site is a good starting point
http://www.bowtieoverdrives.com
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1964
K10 shortbox Stepside
383 sbc stroker under vortec heads
sm420 trans / t221 tcase / corp 10 front axle / corp 12 rear axle / 3.73 gears
rolling on 33x12x15 tires on 6 lug rims.
