I have a Th400, 4000 Phoenix Racing Converter, trans brake, 3.73 12 bolt car. The car has ported heads, and cam. Leaving off the brake it seems lazy. I have been leaving about 3600 off the t-brake, however the car is not launching near as hard as it was with stock heads and cam and a 3800 stall on the 4l60e. It was cutting 1.57 stock motor trim. Any ideas as to what could be causing this. It the 400 just that much harder for the motor to turn.?

 

No the th400 isnt that much harder to turn...

Could be:
difference in trans gear ratios
trans/converter issue
or just simply your setup doesnt make much for power down low compared to the stock setup