Re: 1975 Mercury 115hp Tower of Power
Only trick for more power (enough that you will actually notice) = sell it and buy a bigger motor.
To get 15% more speed at the top end you need to increase your horsepower by 50% (on any given hull) - that's just the physics at work. It's not like the tires on your car rolling on a solid surface - you have to push infinately more water out of the way to increase speed. So if your top speed with a 115 is say 40 mph, to get 45 or 46 mph you need to "up" your horses to about 175. And you are never going to squeeze 175 out of a 115 - can't bore out enough displacement to do that.
Second, in 1975 outboard horsepower was measured in brake horsepower (bhp - at the crank) as opposed to shaft horsepower (shp which was widely adopted in the 1980's). BHP is roughly 80% (no quick conversion - varies from model to model), so in "today's" standards your 115 would be the equivalent of a modern 90 horse.
However, to muddy the waters a little more, Merc used to "under rate" the horsepower of their motors (before the days of EPA requirements). This allowed their 115 to be faster than the competitions. So a mid-70's 115 was more like a 125 in reality (which may make it approximately 100 horses "shp").
So to get back to your original question. There are 1000+ things on the market that promise you "more power". You may be able to fiddle (boyseen reeds, hydrofoils, retune the exhaust), emptying your wallet in the process and your net result will be "improvements" that you will never notice unless you lie to yourself. What you will get is an empty wallet and a 37 year old motor that a modern 90 Opitmax or ETEC will probably kick the snot out of (but it should still be quicker out of the hole that modern 4 strokes).
My recommendation - be satisfied with the motor and use all the money you save "leaving it alone" for gas to actually get out and enjoy it. But if your need for performance is overwhelming, sell it and repower...
Last edited by galamb; 06-26-2012 at 05:17 PM.
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)