I'm looking to see if anyone has an idea of how much horsepower is lost do to accessories added to an engine after a dyno run.On the dyno my 502 pulled 631hp.No accessories and dry exhaust.Now in the boat I have alternator,water pump,raw water pump and Revolution Marine wet exhaust.I'm trying to get a base number for a prop selection but I'm not sure how much HP and TQ would be lost by the time I get to the prop shaft.I'm thinking 75 to 100 horse,is that a close figure?And what happens to the torque curve,does it drop equally across the board?

Thanks Nordic95

Borrow props from friends and you'll zoom in a lot faster on what you need than counting numbers on a napkin...

That's the problem I have no friends and just used my napkin. I have a base line already but I just wanted to back it up with some HP loss numbers.I was wondering if anyone has dynoed before and after accessories just to see what the loss was.

Nordic95

depends on a/f ratios too. If they are good (boat runable numbers) then i would say 65-75hp loss.

I took hp500s,ported the heads, bumped the compression, added a slightly bigger cam, carb work and spacer and new ignition and PROPSHAFT dyno'd it at 509.5hp.

Other then hp, your boat it self has a lot to do with what prop you use. Before you slap down some big bucks for a prop, I would look for a seller that lets you try them out before you buy. Go to the prop section and ask some people on there.

e-bay is the best place to get props. I went through this last year. you can ussually get a set of stainless steel props for about 300 to 400 a set. then when your done dialing in the prop size put the ones you don't use back on ebay

Its not the prop I'm having a propblem with,Its the horsepower numbers after the accessories are added.I just looking to get an idea of the power loss.

Nordic95

Nordic95, Not sure you'll get a good answer to this question. My educated guess would be at least 50hp. However, your torque curve should remain about the same. Several years ago I had a a 540 that I dynod at 608 with all accessories and dyno headers. I installed stainless manifolds and lost 25 HP, and 60 ft. lbs. of torque, but the curve stayed the same. That was running dry tail pipes, so mixing water will hurt even more. I have a new prop shaft dyno and engine dyno that I've been doing some testing along these lines. I should have a lot more data in a few months to help answer some of these questions. I also believe that you drop additional HP once the engine is in the boat, depending on how it's installed. Examples would be from heat and airflow to the engine and exhaust routing. Everytime that exhaust (with water) makes a turn it has to add more restriction. As I build engines I plan on dynoing them on the engine stand, then again once installed in the boat. So far I'm seeing some pretty low power numbers when measured at the prop.

FYI, the 540 mentioned above was installed in a 2004 28 Heat. I ran a 26" Bravo 1 prop and spun it 5600 RPM at 75 MPH.

Throttle up has a program for you. Call Julie or Matt. They will send you different test props as long as you purchase your final set from them. Great deal for you.

Keep me informed of the progress with your shaft dyno results.Sound very interesting.
Thanks Nordic95