^^^ what he said! HP= top end , torque gets you from point A-B faster

MC, you didnt tell us we would have a pop quiz...I didnt study.

A question- It was stated to cruise at peak TQ but you cruise at 30-40% throttle(3000-3500rpm) and peak TQ is measure at WOT. What does the TQ curve look like at part throttle? Is it the same as WOT just lower or is it a different curve?

This example may help:

The 455 inch Buick engine in a GS Stage 1 makes 360 hp @ 4600 RPM.

It makes 510 lbs. of torque @ 2600 RPM.

After 2600 RPM the torque drops off and the horsepower continues up.

Where they cross is the "sweet spot". If this was a boat engine it would be close to the ideal cruising speed depending, of course, on about a zillion variables.

Torque and Horsepwer always cross at 5250 RPM in the power curve.

Wish that was my "cruse" RPM!!!!

Mark

No, my point is that all dyno curves are done at WOT. You cruise at much less than WOT. What would a dyno curve look like at say 50% throttle. Would it have the same peak rpm points?

When you guys say "most effecient cruise is at peak TQ" You don't really mean I'll get better engine life and milage at 4600rpm do you? I usually cruise around 2900 and 45Mph. Sometimes lower. My motors seem very happy from 2500 . The carbs run on the primaries up to around 3000 so I figure this is my best cruise. All the test I've seen show best mpg on most boats at around 2800-3000. I'n running 29 Hydros on a 29'fountain,twin 454s 1.5 gear. Haven't got the power to go more than 4700(79mph) with those props but I have a set of 27 bravo 3 blades I guess I'll go back to for more top end. peak TQ is right there but peak HP is around 5500. Guess I'll shoot for 5200 with the 27s.

Good point.
No, the peak points would be lower in the rpm range, is my guess. Somebody run desktop dyno on an HP500 config, then run it again with a 325cfm carb. That'll tell us.

So yeah, I see what you're saying. You'd have to prop for WOT operation at peak torque to match the peak on the dyno sheet. And that would be a suck thing to do.

Torque peaks and BSFC numbers are an indication, though, of how efficiently the motor is converting fuel into power.

I guess if somebody was entering a "fuel economy range competition" with a given hull/motor combo, then the peaks at certain throttle openings would be necessary to baseline your optimal speed and proper prop selection, but in recreational boating, we almost always prop for WOT at peak hp, and the rest just ends up wherever it falls.

And cooltoys, your point is valid, too.
While the motor may INDEED be most efficient at 4200rpm, your planing hull requires an exponential increase in power to raise the speed. This makes it impossible to match cruise speed to peak torque.

But it does pretty much work that way in displacement hulls. My bad for crossing it over to planing hulls. Too much information is worse than not enough sometimes.

And engine life is directly related to increased rpm. As long as a motor isn't lugging, it will always last longer at lower rpms.

That can't be right. What engine has a curve like that?

Hp is not measured. It is a product of a mathematical equation, using torque (which is measured) and rpms. As part of this equation hp and torque always cross at 5250.

it has to be equal at that point, or the universe will collapse....