Horsepower vs. Displacement
 

Often, one will see two outboard motors with the same rated horsepower
yet one may have 25% greater engine displacement (i.e. 68cc vs. 85cc)
yet both will be rated 2.5 hp.

(1) How does the smaller displacement motor achieve the same
horsepower rating as the one with a greater displacement?

(2) What's the bottom line advantage, if any, to the outboard that has
the same horsepower rating yet has a greater cc displacement?

Sorry if these two questions seem rather elementary to some but I'm
new to the outboard motor scene and figure the only thing to do if
you're not certain of something is to ask the experts.

Thanx.

-Jay


ANSWERS:

(1) _______________________________________

(2) _______________________________________

Horsepower vs. Displacement
 

On Wed, 23 Apr 2008 02:51:11 -0700 (PDT), in message

Jay wrote:

Often, one will see two outboard motors with the same rated horsepower
yet one may have 25% greater engine displacement (i.e. 68cc vs. 85cc)
yet both will be rated 2.5 hp.

(1) How does the smaller displacement motor achieve the same
horsepower rating as the one with a greater displacement?

That's a much more complicated question than can be dealt with in a
brief post. One simple answer is that it might run at a higher speed.

(2) What's the bottom line advantage, if any, to the outboard that has
the same horsepower rating yet has a greater cc displacement?

I don't think it is possible to answer that question in general
without comparing specific engines for specific applications. Try
treating the engine more like a black box:

What does it cost to buy?

What does it cost to run?

Will it run on the low octane fuel easily available?

What are the reported characteristics of reliability and durability,
or alternately the reputation of the manufacturer?

I have written papers on engine performance, yet those were the
questions I asked myself for my most recent small outboard purchase.

Ryk



Sorry if these two questions seem rather elementary to some but I'm
new to the outboard motor scene and figure the only thing to do if
you're not certain of something is to ask the experts.

Thanx.

-Jay


ANSWERS:

(1) _______________________________________

(2) _______________________________________

Horsepower vs. Displacement
 

On Wed, 23 Apr 2008 02:51:11 -0700 (PDT), Jay
wrote:

Often, one will see two outboard motors with the same rated horsepower
yet one may have 25% greater engine displacement (i.e. 68cc vs. 85cc)
yet both will be rated 2.5 hp.

(1) How does the smaller displacement motor achieve the same
horsepower rating as the one with a greater displacement?

(2) What's the bottom line advantage, if any, to the outboard that has
the same horsepower rating yet has a greater cc displacement?

....
-Jay


ANSWERS:

(1)
Runs faster,
has 2 stroke versus 4-stroke design
Has higher compression.
More effective piston ring sealing.
More radical publicity department
More ball races - for piston rod and crank.
Uses more gas (specific fuel consumption poorer)
_______________________________________

(2)

Lasts longer, though heavier
______________________________________

You're welcome

Brian Whatcott
Altus OK

Horsepower vs. Displacement
 

Sorry all, I should have been more specific since there are apparently
not too many generic answers to the questions.

Okay, here are the two motors I was thinking about when I posed the
question.

++2008 Tohatsu/Nissan MFS 2.5 hp with 85cc motor.

++2008 Suzuki DF2.5 hp with 68cc motor.

The Tohatsu has a 25% greater displacement yet the Suzuki is rated the
same horsepower (2.5) as the Tohatsu.

-Jay

Horsepower vs. Displacement
 

Deliberate detuning. Possibly smaller carb, different head etc.

The Tohatsu comes in a 2.5 and a 3.5. Probably using the same block on both
to save production cost. Several 9.9's use the same block as the 15's in
the same line.

Next size up in the Suzuki line is a 6HP. No savings there so they squeeze
all the power out of the smaller block.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Jay" wrote in message
...
Sorry all, I should have been more specific since there are apparently
not too many generic answers to the questions.

Okay, here are the two motors I was thinking about when I posed the
question.

++2008 Tohatsu/Nissan MFS 2.5 hp with 85cc motor.

++2008 Suzuki DF2.5 hp with 68cc motor.

The Tohatsu has a 25% greater displacement yet the Suzuki is rated the
same horsepower (2.5) as the Tohatsu.

-Jay