HP Rating, Displacement and Performance

[k13west]

How do outboards of the same HP rating perform when one is a detuned large engine with more displacement vs a smaller engine that is tuned to maximize it's output? All other factors being equal, will a 30HP outboard with 550cc perform like a 30HP outboard with 750cc? If not, how will performance differ?

My boat does fine during fishing season, but is underpowered once I add a duck blind and hunting gear. I'm looking to re-power and curious if a maxed out 30HP will give me that much more than the 25HP I currently have, or if I need to add the power(and weight!) of a 40 horse?

Forgive me if this has been covered before, I couldn't find much using the search function.

 

[GYPSY400]

I think what you should try is getting a lower pitch prop when you add the extra weight. Alot cheaper than a new motor.

 

[GYPSY400]

I think what you should try is getting a lower pitch prop when you add the extra weight. Alot cheaper than a new motor.

 

[RaisedByWolves]

The larger engine of the same HP rating will have more torque and will likely be able to swing a slightly larger prop.

With the same prop the larger displacement engine will get you out of the hole quicker, but if the WOT RPMs are the same there will be no difference in top speed as long as the gearing between the two is the same.

 

[turbotodd]

Having been in the performance industry my entire life, theory will tell us that a larger engine that is detuned will make more torque below peak power.

That ain't always the case. sometimes it is, other times not so much.

It's entirely possible with current technology, machining techniques, cylinder head flow, valves, everything else--to make as much or more torque with a smaller displacement engine as an older larger displacement engine would. And you will see this in the future, smaller engines with better machining, etc making incredible and impressive torque and power.

case in point is the yamaha 25hp 4 stroke. Only 432cc and it makes more torque everywhere except above 5000 RPM than the previous 498cc version. From 5000-6200 it's about the same, has to be in order to keep it a true 25hp. Remember, torque and horsepower are related, you can't have one without the other. How did they do it? The engine is totally different...less rotating mass (by far), less reciprocating mass, better cylinder head flow, EFI with a good throttle body and tuned intake manifold, exhaust it better tuned, etc etc. it all adds up.

 

[k13west]

Thanks, guys.

Todd, that's fascinating info and exactly why I asked. My current outboard is the 498cc Yamaha that you mention, and low RPM torque is part of what I'm looking to gain. My hull, gear and humans during fishing season totals 950lbs. Hull, gear, duck blind, dog and humans during hunting season can be as much as 1400lbs. That's asking a lot of the 2001 Yamaha 25, even when I go down to a 9" pitch prop.

So I'm looking at the Honda, Suzuki or Mercury 30HP, or the Honda or Mercury 40HP outboards. What I don't know, and have no chance to test, is if the 30HP will be enough to easily plane out, and top out in the 25MPH range when I have the heavy boat load. I'm currently planing out and running 20MPH at WOT with the 25 Yami, but it requires the 9" prop, and I know the motor is working hard to do it. All of the 30HP in production, with the exception of the Yamaha 30, are maxed out for the engine block. Are those 30HP going to provide the same low RPM torque as the 25HP on the same block? If so then I'll need to go to the 40HP.

The 40HP seems like a no brainer, but doing that adds at least 50lbs of weight to the transom and the boat in general.