depends on the manifolds but there would still be increases though

 

Seeing as I have never dyno'd one, I would guess it wouldn't be an extravigant amount, but I'm sure if you bumped up the intake as well at the exhaust, you'd have a significant increase. Your car has to breathe just as you do the better it breathes the better it performs.

 

just looked at an old MM, stock heads vs hi-pos only yealded 1 HP increase, however going to some headmens yealded 10, so I'm not worried. I'll keep stock.