Come on guys Really..hows it done?

Hey guys,

I got a 03 yamaha 4 stroke 15 hp motor. Mint condition just picked it up from a old fella our family known for some time for a hell of a price. I have it installed now on my 1552 WELDBUILT boat. Took it to the lake here in SC and I am no speed demon but would like some extra power to get across the big water faster.

Heres the deal,

Should I call yamaha and ask the techs?!

done google searched it and searched the forums here too and no one seems to have any knowledge on "how to accurately and safely make the FOURSTROKE 15 THE 20!?" Everythings seems so similar right down to the powerheads and all but whats the change? I have read the TWO threads that discuss this same problem im having and noone has spoke up with the solution with a "definate" answer.

Would me buying a 20HP CARB AND INTAKE DO THE TRICK? Heard something about exhaust too on the other two threads but how would yo go about switching over that? PLEASE ALL YOU MOTOR HEADS WITH ALL THAT KNOWLEDGE ANWER THIS QUESTON!(itd be very much appreciated guys!

Thanks,
Jeff

Re: Come on guys Really..hows it done?

Welcome to Iboats.
I have no idea how to turn a 15 into a 20hp but if your boat is a 15 footer and you have any wieght to yourself + some gear the boats not going to plane out. Your going to be needing 40-50+HP to get any kind of speed.

Re: Come on guys Really..hows it done?

Come on. The guy wants to know what needs to be changed and how. He doesn't want to here all the other stuff about oh this is better and that is better, he wants to know what needs to be changed. If you know tell him, if you don't leave it be.

And you can plane a 15' boat with people and gear without going to a 40hp + motor. I do it all the time with a 25hp and am very happy with 26-27mph. We're talking aluminum fishing, not fiberglass ski boats.

Re: Come on guys Really..hows it done?

Ahoy, killa.

Welcome to iboats.

Seems to me that you have unrealistic expectations. If the 15 cannot plane your boat with a normal load it is very unlikely that a hopped up 15 would be any better.

I suggest that you look into your rigging and propping to maximize performance of the package or try to swap the 15 for a 25.

Re: Come on guys Really..hows it done?

If you do enough searching on line you may find what you are looking for, but I think it will be easier and cheaper to trade up or buy one that you want.
You can also look at the part break downs of both the 15 and the 20 to see what all is different.Only gaining 5 HP is not going to help much and then you will still have to find a different prop to match boat and motor.

Re: Come on guys Really..hows it done?

Four stroke engines rely on cam, compression, carburetion, intake and exhaust tuning to gain power. I doubt the cams are different and I doubt compression is different. But I would bet carburetion and ignition timing are different. Perhaps the intake and exhaust are affected as well but the only way to know that is to compare the parts list, item by item as was suggested. Even if it's just carburetion, intake, and exhaust, you will find those parts to be expensive and not worth the benefit. On EFI/DFI engines the engine management system (ECU) controls the engine so it would likely need to be changed and you don't even want to ask what those cost. You would have been better off buying a 20 or 25 to start with. The way to determine what HP you need is to never power a boat with less than 70% of its maximum rated HP. 70% will push the boat but performance will not be stellar by any means. Taunting by the way does not work here. The answers you are getting are obviously from those trying to help you understand the situation. Because someone did not answer in a short time does not mean the answer is not forthcoming. Patience is a virtue.

Re: Come on guys Really..hows it done?

the F20 wasnt released unitil about the 07 production run and was different than the earlier F15 motors.
while the F15C and the F20B motors were based on the earlier F15 the only way to tell what fits what is to sit down with the parts list for your current 03 model F15 and compare cams,carbs,ign CDI units,exhaust tuners and carb and intake manifold.
a call to most techs or even yamaha will likly result in a blank stare as its against US law to modify an EPA motor unless the EPA recertification is done.