TOPIC: 15hp Evinrude/omc where did 5hp come from?

For the 1974 thru the 80's 15hp, I was wondering how they got a extra 5.1 hp from the 9.9hp motor. Was it from larger carb & reeds perhaps or??? Or was the 15hp detuned to 9.9? Both motors appear to be identical in size from the outside...I suppose this applys to other size and types of motors?

One on a forum said the 9.9 ran much smoother than the 15hp at slow speed especially for long periods of time.
Sounds to me like what ever they did to gain 5hp must have sacrificed the smoother slow speed running of the 9.9 a bit...

My thought is that fuel economy of the 15hp perhaps suffers more at the mid range than the 9.9? Thats a question!

I could search all the parts but takes forever with dial-up, yes its still around:(

The first 15Hp and 9.9 Hp were the same blocks, however the 15 hp had a larger Ex tube, a spacer for the reed plate, and the carb was diff.

Later they started to use the same ex tube on the 9.9 that was used on the 15 hp, so the spacer and the carb is the only diff.

I have change many 9.9 hp to a 12 hp to 14 hp by just changing the carb. (testing them beside my true 15 hp they run a bit slower same boat same load) and not installing the spacer on the reed plate

Being that the carb ports and venture are larger on the 15 hp they just will not idle as slow as the 9.9 hp does. Some what like when one hot rods a car.

As for fuel. If I run the 14 foot john at 15 MPH. On the 9.9 hp I use about the same fuel as I do running running the 15 hp at the same speed, however I can get on top faster with the 15 hp and still have more speed then I do on the 9.9 hp.

I think it is something like 1/2 open on the 15 hp and near 7/8 open on the 9.9 hp (at 15 MPH using the GPS)....I will need to check that out next time I am playing with them.