how much horsepower could a stock th400 handle thanks

 

Depends what year it's from. The 64-71 versions are stronger however the weak internal parts in the later models can easily be upgraded to the stronger earlier ones.

Even the weaker models can easily handle 500 hp.

 

With the early units pre 1971, you could add a 34 element intermediate sprag, Trans-Go Performance Shift kit, and Borg Warner Hi-Energy clutches and band, and this will hold 700-800 horsepower, in a 3,200-3,400 pound vehicle pretty easily.

 

The main weak link is that intermediate sprag/roller clutch. All the TH400's went to a weaker int. roller clutch in '71. You'll need to change the direct drum to an earlier version to use a sprag, since the roller clutch uses a cammed inner race, and the sprag uses a smooth race (the inner race is part of the drum). Most stock sprags had only 16 elements, but you can easily upgrade these to the 34 element versions (direct replacement). Around the 800 hp mark, you'll want to upgrade to a good aftermarket input shaft and forward clutch hub, as these are prone to fail at this level. Also, many internal parts from the 4L80E trans will fit, and are stronger "stock" replacements.