Yamaha 15hp or 20hp Motor Question.

Can someone answer me this?

Looking at the Yamaha 15hp and their spec sheet, the 15/20hp, the bore/stroke are the same as well as the gear ratio.

Why then can't one buy a 15 and bump it up to a 20?

Is the computer controlled module making or setting the HP of these two motors, or is it the carb jetting?

Can one buy a 20 module and thus have a 20HP motor?

Am I missing something cause everything seems the same as far as specs go.

Specs for both 15/20: http://yamahaoutboards.com/outboards...specifications

Displacement 22.1 ci (362 cc)
Bore x Stroke 63 x 58.1mm (2.48 x 2.29 in)
Prop Shaft Horsepower 15hp @ 5500rpm
Compression Ratio 9.3:1
Intake Carburetor
Ignition CDI Micro Computer
Spark Plug DPR6EB-900-000
Alternator Output 10A **
Gear Ratio (27:13) 2.08:1

Here is the link to their module: http://yamahaoutboards.com/outboards/Portable/benefits

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