How much horsepower can the 904 trans handle?

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Go for it, You live this life only once.

In stock form with the 3 disc clutches, not much.

With the V-8 clutches and 4 pinion planetaries, a lot.

None of the Slant racers use a 727 if that tells you anything.

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Official Cookie and Mater Tormentor.

Everyone that I know that runs in stock class runs them.
Most all run low 12's to mid 11's so you are looking at 400 HP.
Some guys use a JW bell housing and put them behind big blocks too.
They have well over 500 HP.
IF the car is heavy (4000 lbs) I might be leary,, but who knows.
All of mine worked just as good as the 727.