I pick up 20mph with installing whipples and they say UPTO 50% hp gain with the system.Wasn't able to get it dyno before we installed them.I've heard you need 10 hp,16hp,etc. to 1 mph.Does anyone of the motor gods have any idea what i might have?

There is a very good treatment of this subject in the Jan. '06 Speedboat magazine. Many factors are involved as described in real tests conducted by the author.

My guess would be that you gained 350-400HP total.

This will be my first serious response to Rich....EVER!
After 70-80 mph I believe you need more HP per MPH than what is needed to go from say 40 to 50 MPH? A 50% increase would be 265HP additional. If you really jumped 20MPH then it seems you've gained more than 265HP to me. (Logic being you needed closer to 20HP per 1 MPH)

NOW, Get the thing on a dyno and stop guessing!!!!

Wouldn't the speed you gain per horsepower be contingent on how efficient the hull is?

I am hoping to go from 48 on gps to mid to high 60's with an increase from 200hp to 400hp. Mind you it is a light boat

My boat went from 67mph to 75mph when I swapped a 310HP7.4MPI for a 415HP/502MagMPI...

If you know you current hp you can simply input your weight(don't forget gas, passsengers and beer! ) and get a constant. Once you know your constant you can play with the other numbers. I've found it is extremelt accurate if you have the info to get a constant. Guessing at your constant doesn't give accurate results.

Speed Calculator

I would think you have gained approx. 360 hp

Thanks for all the info.I think the most i gained in hp is 300 hp per side but i've been known tobe wrong before.