stock c6 in a 1981 f150 w/302 and 3.25 gears; when i start my truck to go to school (about 30* give or take) i throw it in drive and she runs perfect... shifts from 1st to 2nd at 25 and 2nd to 3rd at 45. perfect for 3.25 gears. when this c6 gets hot the shift points drop to 15/25 and it really bugs me because im not running a kickdown (swapping in an inline from a 302, why bother?). also what do slipping clutches feel like? i think 3rd is toast on this old slushbox

 

AWFUL hard for a stock 302 to cook a C6. You'd have to try. It was designed to take 429 and 460 torque and hp. Double or so the power you apply from a 302. If you have a vacuum modulator, that is probably involved in the changing shift speeds.
A slipping 3rd would feel like the engine rpms are increasing while the speed of the vehicle is staying constant. You could not accelerate, as the engine would be turning faster and faster, but the truck would not.
More likely, 1st is the gear you'd lose 1st. (no pun intended) Most trans have a higher line pressure in reverse, (no idea why), so the weaker gear in comparison is 1st. You get 3rd by locking up both clutch packs, sending the power straight through a 'single' shaft, rather than through the sun/planetary reduction gears. If 3rd slips, another gear will slip, and probably slip worse because of the increase in torque being carried by a lower gear.
tom

 

well then, 3rd gear isnt slipping then, thats good

what about the shifts? i replaced the modulator and screwed it all the way in, made a slight improvement but it still shifts too early to my liking.

 

You can use a longer pin behind the modulator.

There are 4 different lengths.

Shortest will give the earliest shifts.


1/ 1.601 - 1.611
2/ 1.634 - 1.644
3/ 1.650 - 1.660
4/ 1.667 - 1.677

I use a chunk of metal coat hanger.

 

Cold fluid is more viscous. It will affect how the trans shifts as all the valve body does is respond to fluid pressure. It is a hydraulic computer. It 'knows': throttle position (kickdown...!!), car speed (governor), load on the engine(vacuum modulator) and 'internal pressure'.
It will ALWAYS try to upshift as soon as it can. That is the design. It has to have xx pressure from the governor -- on the driveshaft, telling it how fast you are traveling, it has xx pressure it builds from its pump - telling it how fast the pump(engine) is turning over -- RPM, and it has throttle pressure(demand kickdown) and it has the modulator telling it load on the engine. When cold, the vacuum is different than when warm, idle rpm are higher for a while, etc.
The design balances the rpm/speed/demand such that it will always upshift if the demand is not there, and the engine load is not heavy. the governor will tell it velocity, and if fast enough, if no kickdown and no 'low vacuum' signal(wide open throttle) it will upshift.
You could adjust the modulator. There is a screw (in most) right in the middle where the rubber tube attaches. It adjusts the spring balance against the vacuum from the intake manifold, making the spring stronger or weaker pushing against vacuum. That makes the pin that moves in the valve body react differently, raising or lowering 'low demand' upshift speed.
tom

 

thanks for the help guys, i'll either go get a different modulator for my new one or wait till the inline to get the kickdown