Recently upgraded my bbc with AFR oval port heads, slightly larger hyd roller cam & increased compression ratio by .5 . Now for some strange reason @ 3500 rpm cruising speed I'm +4 mph faster than the old setup. Really runs nice. My question is this. Traveling at 3500 rpm shouldn't the mph be the same weather the motor is producing 400 hp , 500 hp or even 600 hp???? Or does torque come into play? All my gauges are Gaffrig & the speedo is gps.

3500 is the same with 200 HP or 1200 HP I'm assuming that you are using a gps speedo so I would check the tach .

Steel to aluminum head?

Another variable is hull efficiency. It you trim position, weight distribution or total boat weight changes then the prop slip can vary....hence at a fixed 3500 RPM the speed can vary.

Those are really nice heads! I used them on my last build and was really pleased with the results.

Other than Heads & cam, all tests before & after were with same setup. Trim setting the same, 1/2 tank of gas and 2 people in the boat. Must be tach failure. And yes to gps speedo. Next is testing my tach.

Thanks Rick

If its a small light boat, its possible that getting rid of heavy cast iron heads, and going to aluminum, that the weight savings might have free'd up the hull a bit at cruise.

I have seen if someone goes from the front seat to the back will make a speed difference at lower speeds.

What type of boat are these motors in?
Single or twin set-up?
What prop are you running?