does anyone know what would be the difference in performance if i repowered my 454 / 310hp carb's to 502 / 415hp mpi's (both stocks) would be on a 34 foot speed boat with bravo 1's. i get upto 48mph @ 4500rpm with the 454's trims up .

jack

It takes 12 HP to increase 1 MPH so figure it out that way.

I think one MPH for 12 HP is optimistic. I think closer to to 15 to 20 HP for one MPH depending on the hull. Just a rule of thumb.

15 Hp on a stepped hull = 1 mph

20 Hp on a conventional hull = 1 mph

since i will change my engines from 454/310hp's @ 4400rpm to 502/415hp's @ 5000rpm. with the additional 600rpm's do i still need to change my props?

that will also be hp increase per engine on twins.

You don't have to change props, however when you get the new engines in, the tell all will be in what rpm range your running. Not sure what pitch your running now, but if they are on the low side, you might be bouncing off the rev limiter at full throttle..... which means that you could gain more speed with a higher pitch prop.... so if you keep your current props, you'll pick up speed by the added rpm difference of 4500-5000 only, which isn't going to be that much since you are adding a lot more HP..... putting that extra HP to a bigger prop will turn you more speed.....

just some thoughts..... check out the BAM website and play with numbers in the prop slip calculator... http://go-fast.com/Prop_Slip_Calculator.htm