Torque, horsepower and Displacement.

[rschnack]

Hi again,<br /><br /> I have a basic question that may not be so basic. I've noticed that several outboard manufacturer's have motors with different horsepowers but the same displacement produced by the same bore and stroke. From dealing with automotive engines I know that when you "hop up" a motor to produce more horsepower you usually raise the rpm where the usuable power is available, giving up some low end torque. From reading through these forums I learned that low end torque is very important for a boat engine for getting up on plane and pulling skiers up. Correct me if I'm wrong, but if top end isn't the primary concern wouldn't you be better off with an outboard rated for lower horsepower vs. one with the same displacement and a higher horsepower rating? Or do the manufacturer's compensate for the change in the torque curve with gearing?<br /><br />Best regards,<br />RCS

 

[JB]

Re: Torque, horsepower and Displacement.

Howdy, RSchnack.<br /><br />Good question, and you can expect a lot of input.<br /><br />Generally, the differences in "HP" of same-displacement engines are achieved by changes in breathing (intake and exhaust), compression ratio, port timing and area and ignition advance curves. Typically these differences allow the higher "HP" engine to safely run higher rpm, as you said, and develop more peak hp.<br /><br />You are correct that the engineers go to lengths to protect the low and mid range torque of the "hotter" engines so that they wont be outperformed in those ranges by the supposedly lower hp engine(s). <br /><br />Occasionally the higher tuned engines may have a different gear ratio in the lower end so that they operate at higher rpm per propshaft rpm and deliver better torque to the prop. <br /><br />It is also not unusual for the higher tune engine to give best performance with a shorter pitch prop for the same reasons.<br /><br />That's my input on the subject. There will be more.

 

[Hooty]

Re: Torque, horsepower and Displacement.

Basic boat set-up has a lot to do with where peak horsepower/torque are used too. For instance, a boat can be set up to come out of the hole like a bullet while the guy that had to climb up on his deck just to get the boat broke over will have several mph more top speed. Same boat, same engine.<br /><br />c/6<br /><br />Hooty