Yamaha Horsepower 20hp vs 15hp Identical Specs?

TheWaterDog

Recruit
Joined
Jan 19, 2017
Messages
2
Hey All,

Hoping someone can clarify something.
Please see specs below. Specs were taken from Yamaha website.

How can specs be identical between a 20hp and 15hp motor? Where is the extra 5hp coming from for the 20hp motor? http://yamahaoutboards.com/outboards/Portable/comparisons


Yamaha Portable F20Yamaha Portable F15
Engine Type2 Cylinder2 Cylinder
Displacement22.1 ci (362 cc)22.1 ci (362 cc)
Bore x Stroke63 x 58.1mm (2.48 x 2.29 in)63 x 58.1mm (2.48 x 2.29 in)
Compression Ratio9.3:19.3:1
Prop Shaft Horsepower20hp @ 5500rpm15hp @ 5500rpm
Full Throttle RPM Range5000 ~ 6000 rpm5000 ~ 6000 rpm
Fuel InductionSOHCSOHC
IntakeCarburetorCarburetor
ExhaustThrough PropellerThrough Propeller
CoolingWater/Thermostatic ControlWater/Thermostatic Control
IgnitionCDI Micro ComputerCDI Micro Computer
Spark PlugDPR6EB-900-000DPR6EB-900-000
Alternator Output10A **10A **
Starting SystemManual or ElectricManual or Electric
LubricationWet SumpWet Sump
Engine Oil Capacity1.8L/1.6L w/without filter1.8L/1.6L w/without filter
Recommended Engine OilYamalube[SUP]?[/SUP] 4M (See owner's manual)Yamalube[SUP]?[/SUP] 4M (See owner's manual)
Recommended Fuel FiltrationYamaha Mini 10-Micron Fuel/Water Separating Filter (external)Yamaha Mini 10-Micron Fuel/Water Separating Filter (external)
Recommended FuelRegular Unleaded (Minimum Pump Octane 87)Regular Unleaded (Minimum Pump Octane 87)
Ethanol Blend Limit10% Maximum10% Maximum
Gear Ratio(27:13) 2.08:1(27:13) 2.08:1
Gear ShiftForward, Neutral, ReverseForward, Neutral, Reverse
Shaft LengthS=15" L=20"S=15" L=20"
Degree of Tilt67?- 71?67?- 71?
Degree of Trim4 positions4 positions
C.A.R.B. Rating3-Star3-Star


Thanks,

TheWaterDog
 

GA_Boater

Honorary Moderator Emeritus
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May 24, 2011
Messages
49,038
Welcome aboard.

Could be cam, carb, timing or anything else to hot rod a 15 to a 20. The only way to be sure is to compare part numbers.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Jul 23, 2011
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47,297
the 15 is a detuned 20 for the reasons GA_Boater mentioned
 

ajgraz

Lieutenant Commander
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Mar 1, 2010
Messages
1,858
This brings up a question in my mind...

In the poster's example, the same block, etc. is used between a 15hp and a 20hp. Another example might be a manufacturer who uses the same 1.5L block for a 75, 90, 115 and 125hp rating. (Obviously this keeps manufacturing costs down--but also improves parts availability for us!)

So the question: is the manufacturer really "hotrodding" a "lower-spec'd" block into a higher hp block, OR are they actually "detuning" a "higher-spec'd" block into a lower hp?

Seems to me like the former might be cheaper for the manufacturer, while the latter would surely be safer and more durable for the user...
 

TheWaterDog

Recruit
Joined
Jan 19, 2017
Messages
2
Thanks for your posts! Sounds like it is worth the research to find the real answer. Buy a 15hp and save $1000. Maybe swap out a part for $250 for the upgrade.
Any mechanics in the forum have the resources to do the research?
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
Sometimes the conversion makes sense, but most times it doesn't, you just need to examine the part numbers very closely to know. You also have to look at the issue of voiding the warranty, some people want the warranty, others don't care.

​The other thing to look at is there's a fudge factor in HP ratings of + or - 10%, so the true difference in HP may not be as much as you think.
 
Last edited:

jimmbo

Supreme Mariner
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May 24, 2004
Messages
12,944
Might be different cylinder head/valves, and/or as mentioned before, carb, cam.
 

ronaldj

Senior Chief Petty Officer
Joined
Apr 1, 2013
Messages
655
Just so you know
The Suzuki 9.9, 15 and 20hp EFI 4 stroke outboards all use the same block and have the same weight
It appears that's the way the market is headed
Of course that makes one wonder why you would buy the 9.9 over the 20 unless the difference in price was huge or you fished someplace where they were actually savvy enough to tell the difference between a 9.9 and a 20

It also makes you wonder if you void your warrenty if you swap the 9.9 controller for a 20
 

ondarvr

Supreme Mariner
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Apr 6, 2005
Messages
11,527
Yes you would void the warranty.

​A manufacturer can easily change the HP on an EFI engine buy changing the programming, no other part differences may be needed, but might be.

​The market has been doing that since the 50's or so, same block and 99.9% of the same parts can cover a large span of HP ratings.

​People already try to fudge on HP in regulated areas, if they decide to enforce the rule they check the model # on the motor....busted! No difference here.

Modifications like reprogramming would be a little harder to detect.
 

ronaldj

Senior Chief Petty Officer
Joined
Apr 1, 2013
Messages
655
Yes you would void the warranty.

​A manufacturer can easily change the HP on an EFI engine buy changing the programming, no other part differences may be needed, but might be.

​The market has been doing that since the 50's or so, same block and 99.9% of the same parts can cover a large span of HP ratings.

​People already try to fudge on HP in regulated areas, if they decide to enforce the rule they check the model # on the motor....busted! No difference here.

Modifications like reprogramming would be a little harder to detect.

I am in no way advocating this but
Based on what you said
It seems like you would get the 9.9 to use on lakes and reservoirs where they check and then replace or reprogram the chip so you could gain your 10hp for use in saltwater
 

ondarvr

Supreme Mariner
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Apr 6, 2005
Messages
11,527
If, and I say if because I have no idea, the only difference is the programming, then you would probably need to swap computers, they may not be reprogrammable to that extent by the owner. You would need to pull up the parts list for each motor and start comparing #'s to know if programming is the only difference.
 
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