Sure, the stock converter is weak, but the transmission itself eats up power. A good converter will help make the automatic/converter combination more efficient, but it still will eat up a lot of horsepower over a 5 speed. A higher stall does not make horsepower?
I have an AOD with all above mentioned mods, and I feel like my car would pick up at least .5 second in the quarter with the 5-speed swap. You can really feel the AOD holding back the motor, at least this is my experience with my combination.
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1993 GT/AOD
'93 Mustang GT