does horsepower essentially = horsepower

There are so many options out there when it comes to motors. I grew up on old clunkers that were never maintained well (my dad favoured bandage solutions over maintenance) and always had spotty performance.

But now with well kept older stuff, classic boat motors, e-tec, fourstroke, etc...it is a little mind boggling trying to figure out which motor to get.

But boats are all rated for a maximum HP. I always assumed that this had to do with the handling at certain speeds. Especially dealing with the hull shape. But if I slap a new e-tec 60 on my old '72, 14' runabout (rated for 60), will I see a much higher top speed? thus deeming it unsafe?

so if motors are not created equally (as far as performance) where does this HP guideline come from?


I'm not saying that I'm looking at buying a new $7000 motor anytime soon, but I'm trying to figure out where to start.

anything is better than my blown up '76 merc thunderbolt......

Re: does horsepower essentially = horsepower

the older motors hp was rated at the power head, the new motors are rated at the prop. i do not know the year of the change, off the top of my head. but yes you will get better performance out of newer motors. can be as much as 15%.

Re: does horsepower essentially = horsepower

tashasdaddy said:

the older motors hp was rated at the power head, the new motors are rated at the prop. i do not know the year of the change, off the top of my head. but yes you will get better performance out of newer motors. can be as much as 15%.

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holy smokes. I just checked out the specs. My old 50 hp merc had around 500 c.c.'s. A new 4 stroke one has almost TWICE that....

Re: does horsepower essentially = horsepower

psinatra said:

holy smokes. I just checked out the specs. My old 50 hp merc had around 500 c.c.'s. A new 4 stroke one has almost TWICE that....

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That's to be expected.
Two strokes will theoretically put out twice as much power as four strokes, for the same size.
2 stroke = combustion every 2 revolutions
4 stroke = combustion every 4 revolutions

Of course, there have been a lot of improvements in both types of motors since 1972...

Re: does horsepower essentially = horsepower

ridefst said:

That's to be expected.
Two strokes will theoretically put out twice as much power as four strokes, for the same size.
2 stroke = combustion every 2 revolutions
4 stroke = combustion every 4 revolutions

Of course, there have been a lot of improvements in both types of motors since 1972...

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Yes, but even the e-tecs have almost twice the c.c's

Re: does horsepower essentially = horsepower

I don't know the reason but if you compare e-tec to other 2 strokes available they are larger displacement and heavier than others of the same HP rating.

Re: does horsepower essentially = horsepower

psinatra said:

Yes, but even the e-tecs have almost twice the c.c's

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whats gonna happen is if you put a larger motor than it is rater for it can cause perement damage to the boat. You can rip off the transom or even wreck the boat.

Also ur gonna get a thing battle of engines going on in this thred. I learned that the hard way.

Re: does horsepower essentially = horsepower

Two strokes put out about 2/3 more horsepower than the equivalent displacement four stroke. The reason it is not double is increased pumping losses on the two stroke and since the ports take up about 1/3 of the total stroke, therefore volume, a one liter two stroke only has 2/3 liter effective displacement while the four stroke has more. NO engine has the same effective displacement as the actual cubic inches because of (as I said) port loss on a two stroke and valve overlap on a four stroke.

Net effect is that for equal displacement, a two stroke is usually lighter and delivers about 1 2/3 the horsepower of a four stroke of equal displacement. With improved four stroke design, this gap is narrowing. However, with one power pulse per revolution for the two stroke and one power pulse every other revolution for the four stroke it is difficult to argue. Note that with one power pulse per revolution, the two stroke has greater torque for a better hole shot and faster time to plane. That is why Mercury supercharged their four stroke Verado engines. Simply put, because supercharging forces more fuel/air into the cylinder, effective displacement is increased. Since supercharging is present at all speeds, versus a turbo, there is no lag, and low speed torque is increased.

It is possible to supercharge a two stroke but since the bypass and exhaust ports are open at the same time, it is useless to supercharge to more than about 2-5 psi. (this is the degree of crankcase compression usually achieved). This degree of supercharging will ensure full air flow through the reeds and full charging of the cylinder while not wasting too much to blow-by. Anything more will just blow out the exhaust ports.

Note that horsepower is horsepower and one horsepower will accelerate 1 pound 550 feet in one second (550 foot pounds per second). This is Physics and "etched in stone" However, the way the horsepower is developed will determine time to plane. Remember that when you nail the throttle, full rated horsepower is not developed until rated RPM is achieved. So while two equivalent horsepower engines, one a four stroke and one a two, would be expected to have ABOUT the same top speed on a given hull, the two stroke would be expected to get on plane faster because of its horsepower and torque curve. NOTE I said expected, not definitely will.