What dictates horsepower?

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What dictates horsepower?

Postby Slack Tide » Wed Jun 04, 2014 7:05 am

Obviously the bigger the engine the more power....but I was thinking about it....
Is HP a determination of piston push? RPMs at the prop? Torque?
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Re: What dictates horsepower?

Postby beretta24 » Wed Jun 04, 2014 11:17 am

Hp is torque x angular speed.
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Re: What dictates horsepower?

Postby T Man » Wed Jun 04, 2014 7:05 pm

1 HP is equivalant to the amount of work it takes to lift 550lbs 1 foot
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Re: What dictates horsepower?

Postby mudpack » Thu Jun 05, 2014 10:44 am

Horsepower is a calculated number, not a measured value. It is arrive at by using the formula:

HP = torque (ft.lbs) times rpm divided by 5252
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Re: What dictates horsepower?

Postby radfordc » Thu Jun 05, 2014 12:25 pm

Slack Tide wrote:Obviously the bigger the engine the more power....but I was thinking about it....
Is HP a determination of piston push? RPMs at the prop? Torque?


It's a function of rpm and torque combined. More rpm with the same torque equals more HP. More torque at the same rpm equals more HP.
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Re: What dictates horsepower?

Postby Jimmy82 » Thu Jun 05, 2014 1:17 pm

So the most simple answer to your question is "yes!" :hammer:
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Re: What dictates horsepower?

Postby toolmaker » Thu Jun 05, 2014 1:18 pm

so Slack.....what this all means is...DONT bother bolting on anything less than 100 hp!!!

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Re: What dictates horsepower?

Postby marsh water » Thu Jun 19, 2014 12:59 pm

merc has always measured hp at the lower unit(rpm), and most others at the head(hp by power head displacement). My dad always bought a merc cause according to him the hp at the lower unit, foot, creates more power. I DONT KNOW! Like the man said, it means bolt on at least 100 hp.
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Re: What dictates horsepower?

Postby Rhock19 » Thu Jun 19, 2014 1:03 pm

Now ive heard the opposite that jap motors are measured at the lower unit :?:
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Re: What dictates horsepower?

Postby goldfish » Thu Jun 19, 2014 4:52 pm

If it's measured at the prop or motor, then why is a jet drive of the same power plant sporting a lower HP rating?

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Re: What dictates horsepower?

Postby Boatman » Fri Jun 20, 2014 8:03 am

Rhock19 wrote:Now ive heard the opposite that jap motors are measured at the lower unit :?:


All motors are rated at the propshaft these days. And mid 80's and older they were all measured at the powerhead. That was the type of dyno.s they all had.
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Re: What dictates horsepower?

Postby Boatman » Fri Jun 20, 2014 8:05 am

goldfish wrote:If it's measured at the prop or motor, then why is a jet drive of the same power plant sporting a lower HP rating?

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Less efficient propulsion system.
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Re: What dictates horsepower?

Postby Boatman » Fri Jun 20, 2014 8:17 am

marsh water wrote:merc has always measured hp at the lower unit(rpm), and most others at the head(hp by power head displacement). My dad always bought a merc cause according to him the hp at the lower unit, foot, creates more power. I DONT KNOW! Like the man said, it means bolt on at least 100 hp.


Not true, merc has always under estimated there hp rating for side by side comparison. Hp was not calculated by power head displacement, it was crankshaft pressure hooked to a dyno.
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Re: What dictates horsepower?

Postby mudpack » Fri Jun 20, 2014 2:57 pm

marsh water wrote:merc has always measured hp at the lower unit(rpm), and most others at the head(hp by power head displacement).

Not true. Until a certain date (not sure when, possibly sometime in the 1990's) ALL outboards were rated at the powerhead. After that date, ALL outboards were factory rated at the propshaft.

Engine displacement is not a part of the horsepower calculation; produced/measured torque and rpm are the two variables.
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