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  #1  
Old 01-10-2017, 09:49 PM
clawman clawman is online now
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Do all 250hp motors perform comparably? Not talking displacement here but horsepower, why would one perform better or worse than another?
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  #2  
Old 01-11-2017, 08:48 AM
PJM PJM is offline
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Clawman

You would think there would, but there are other factors. Two stroke, four stroke different power curve and there is the HO and Pro series motors that can add 10 percent more horsepower.
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Old 01-11-2017, 10:10 AM
winchester jack winchester jack is online now
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Displacement and weight also play a factor. Fuel efficiency, noise, ect.... are also factors. It is nice to have so many good choices.
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  #4  
Old 01-11-2017, 10:44 AM
REW REW is offline
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Clawman,
If you put a dyno on all of the motors under test and find 5 different motors that all put out identical HP - they will power your boat in a similar fashion.

However, to extract that equal power from each motor, you may need to use a different prop, you may need to have a longer run with one motor compared to another to achieve top speed because of different torque curves - buy HP is HP is HP>

If, when tested on a dyno at the prop shaft - all of the motors are equal - all of them will have identical power and when propped to match the gear ratio in the lower unit will have quite similar performance.

Note:
I stated - that if each of the motors have equal measured HP at the prop shaft.
This is where the rub comes in. Due to manufacturing tolerances and allowable deviations, you might find one motor stronger in HP than another motor - even though it has the same HP rating.
But, that does not mean that the motor with the lesser HP is defective in any way, it is just at the low end of allowable HP. Conversely, you might find another motor that is significantly higher in HP than its rating. Again, not a defect, just the variations in manufacturing from one unit to another.
Also, the BIA ratings allow for a certain % of HP deviation in individual motors and still have the same rating.

If you want to purchase a given motor and have doubts about it - have the motor tested on a dyno before you pay for it to insure that the motor is developing the HP that you wish to have in the motor.

++++++++++++++++++++++++
From another source - a copy of the standards of manufacturing for motors:

ttp://continuouswave.com/whaler/reference/ICOMIA28-83.html

The standard is a bit hard to understand, and even very smart and very well educated readers tend to misunderstand what it says. There is a limit on horsepower rating of six percent deviation from maximum. There is a tolerance of ten percent variation allowed for production variation. These two are often confused.

Good luck

Last edited by REW; 01-11-2017 at 10:53 AM.
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  #5  
Old 01-11-2017, 11:17 AM
MadMan MadMan is offline
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+/- 10% on the represented horsepower. (Example) a 250 horse motor can have a power output of 225 - 275. But to my knowledge, there are really no weak 250 hp motors in this class. Like stated above though, 2S / 4S, gear case ratios etc, can all play a part in making one 250hp motor different from another.

Where you really notice the difference in power is Mercury & Yamaha's 200hp I-4 vs. there 6 cylinder counterpart. Example, 200 I4 Verado is a weak 200 (Say 180 hp), while the 200 Verado Pro might be a 220 hp motor.
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Old 01-12-2017, 10:02 AM
REW REW is offline
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To get the answer that you really want - have all candidate motors put on a dyno and check the prop shaft hp of the motor in question.

Be safe
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Old 01-12-2017, 02:29 PM
Uncle Paul Uncle Paul is offline
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Clawman Do all 250 hp motors perform the same in a short answer no, there can be quite a big difference in performance. First thing a Dyno does not measure prop shaft or powerhead hp.What the Dyno is measuring is the Torque of the engine. Horsepower is nothing more than a mathematical forumla.to get HP you take Torque x RPM divided by 5252 to get HP However most printouts of the Dyno will show both but its actually measuring the Torque of the engine. But back to your question if you ever see the printouts of the power curve of a 4 stroke and 2 stroke engine of the same hp generally the 4 stk will have more hp at a lower RPM and the power curve will gain hp at a slow but steady rate as the rpms go up. The 2 stk will have a lower hp at the lower rpm but jump up quickly as the rpms go up. So the engine manufactures will gear each differently and have a recommended operating rpm range to keep the engine in the highest torque point for best performance
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Old 01-12-2017, 05:34 PM
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Thunderbucket Thunderbucket is offline
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Some manufactures don't rate their HP at the prop shaft, just the power head, be very aware which ones those are and do your research. 2 strokes will always have a better hole shot and bottom end torque.


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Old 01-13-2017, 08:16 AM
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Bobby Winds Bobby Winds is offline
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Quote:
Originally Posted by REW View Post
To get the answer that you really want - have all candidate motors put on a dyno and check the prop shaft hp of the motor in question.

Be safe
I can't believe you keep saying this.......

It's tough enough to get a test ride in a boat similar with the same HP from a dealer yet get a Dyno test on one. PLEASE name me the dealers in his area or any area for that matter that have outboard Dyno's........I went to a mechanical tech college and we had ONE Dyno machine for testing auto's, yet never seen one yet for outboards........not saying manufactures or specific companies that just test outboards don't have them, but a dealer, really and why would they have one in the first place ? ? ?
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Old 01-13-2017, 08:18 AM
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Bobby Winds Bobby Winds is offline
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Quote:
Originally Posted by Thunderbucket View Post
Some manufactures don't rate their HP at the prop shaft, just the power head, be very aware which ones those are and do your research. 2 strokes will always have a better hole shot and bottom end torque.


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That was years ago, now all modern day outboards are rated at the prop and I believe it was government mandated.
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