Better to make more power from 3K - 5K, or 4K - 5500?
 

On an average sized single engine boat (Baja Force 235), if I have the choice between maximizing my power in the low and mid range RPM's or making more power from around 4700 on up to WOT, which is better?

A little background - I'll be running a set of AFR 265's on a 489 BBC, planning to spin it up to 5600 or so on occasion, but most of my time will be spend cruising from 2800 on up to 4200 or so. I came across this article on Car Craft from last November where they dynoed a 454 with 088 heads followed by the AFR's. Big power increase. Both of these runs were done with a Victor Jr single plane. They then ran the AFR's again with an Air Gap. The Air Gap made considerably more power and torque from 3300 to 4500 RPM, but began to give up ground after that.

Here's the article - look at page 2 for the dyno comparisons => Rectangle vs. Oval Port Big-Block Head Test

At first glance, the Air Gap would seem like a no-brainer, but I will be swinging .25 more stroke, so low end torque should not be a problem. I'm actually wondering if I might be a little better off making more of my power near my anticipated WOT RPM. I look at it this way - in a car, when you have a big increase in torque and HP at a given RPM (say 3500), it means you can upshift to a higher gear to transfer that power into more speed or acceleration. You don't have that ability with a boat. So theoretically if I am making 60 ft/lb more torque at 3500 RPM, I could swing a larger pitch prop to take advantage of it, but since I may be now making less power at higher (5K+) RPM, I may not be able to spin that prop without lugging the engine. Add to this the additional load I am putting on the drive with a larger prop, and the fact that I may have trouble with blowing out props (cavitation), and I start to wonder if it might be more favorable to shift some of that power higher in the RPM range.

I'm interested in thoughts that others might have on this, as this may drive my decision on what type of intake to run on my setup.

If it were me, I'd choose a intake manifold that will make max power at WOT.

A lot of times, dyno tests here can be misleading. For example, on a dyno pull, the engine is wide open, and loaded down. At 3500RPM, you may see additional tq and HP from the dual plane. But, when cruising at 3500, most of the time your at 50% throttle maybe, unless your accelerating to top speed. Personally, I would give up a little acceleration, say 25HP at 3500, for an extra 25HP at wot. We prop for full throttle trimmed out, everything else just ends up where it ends up.

Im sure if the engineers at mercury racing, felt a dual plane was better on engines such as the HP500, 420HP 454, 450HP 454, 460 cyclone, etc, they would have used it. Majority of boat owners here, lab their props for 2mph gains at wot, knowing they'll probably lose 1-2mph at cruise. We all like top speed numbers :)

I also would take the peak HP at WOT with XX prop.

Very well said and I agree 100%

This guy doesn`t even have a "magic room" !! You sure you wanna take his advice???

You guys obviously haven't seen my own "magic room" - actually, it looks more like a haunted house with all the projects I have going on. ;)

I don't know if you will see much difference. I switched from single plane to the dual plane air gaps and really can't tell the difference except maybe a touch more midrange. Wot speed and rpm are identical with same props. This was on a mild 502 though, maybe your combo could take better advantage of a single plane.

I told you earlier you need to put up a pole buiding .

If you build it, he will come"

I guess I'll just stick with the Dart oval port single plane that I have on hand, although I like the four corner water ports and provisions for the coil mount on the RPM. Thanks!