2007 9.9 yamaha to 15hp conversion

[edzzed]

hi, just bought a detuned 15hp so it is now a brand new 9.9 2stroke. what do i need to do to retune it to the 15hp it should be. shoulda seen the look on the dealers face when i asked hm. said i should be able to simply spend 35 for a new carb linkage. he says more to it. so how much more ?????? regards ED

 

[deejaycee_2000]

Re: 2007 9.9 yamaha to 15hp conversion

Why did you buy a detuned motor?

 

[WillyBWright]

Re: 2007 9.9 yamaha to 15hp conversion

Who told you it was nothing more than a linkage? That's a new one. Most think it's just a jet. Wrong again. There are many ways to get more Hp out of the same displacememt. Carb throat size, exhaust tuning, reed valves, timing curve, porting more. Most use at least two of these. Changing just one gets you partial results and could throw things out of balance and leave you with scrap. Remember, we're talking about a motor that's run full throttle most of it's lifetime. It doesn't take much tampering to destroy a motor that's run like that.

To know what you'd need, you'd need to compare the 15 to the 9.9 to see which parts are different. I have a link to Yamaha for work, but I think it's valid for anybody. Go to www.yamaha-motor.com and click around. Unfortunately they don't have all of the 2007 models up yet.

 

[Silvertip]

Re: 2007 9.9 yamaha to 15hp conversion

What decals are on the engine cover? Are you sure you didn't mistake something the dealer said? On some smaller engines (don't know about Yamaha) it is not difficult to change a 9.9 to a 15 or back again. However, there is usually a substantial cost increase from 9.9 to 15 when new so your scenario doesn't make sense. If one needs a 15, the price to up-convert a 9.9 may make sense if the total invested is less than the cost of a 15. However, going the other way is not cost effective.

 

[JB]

Re: 2007 9.9 yamaha to 15hp conversion

You are looking at a 50% increase in hp, Ed. That is substantial.

The change in linkage is probably to alter the ignition advance and curve. I see no chance that could produce a 50% increase.

I would put my money on what WillyB says. It would take several expensive modifications to get that much change and they would almost certainly cost more than the difference in original price between the 9.9 and the 15.

 

[edzzed]

Re: 2007 9.9 yamaha to 15hp conversion

we bought the 9.9 as the 15hp has to be registered with boat decals. being that the 9.9 and 15hp share the same block, i figured if the 15hp is 400.00 more then maybe i could modify the 9.9 cheaper with off the shelf parts. when i say the 9.9 is detuned it is simply because the corvette and camaro shared the same motor. the only difference was in computer programming because it was easier for an engine builder to make one motor and detune one for lower hp than to uptune the same motor for more hp. does that make sense???

 

[Silvertip]

Re: 2007 9.9 yamaha to 15hp conversion

OK -- so you really bought a brand new, unmodified 9.9 right? Then you also know that the $400 difference between the 9.9 and 15 is not to cover the cost of a set of decals. You question has been asked over and over and over on this forum and the answer is always the same. Rarely is it a matter of swapping jets or carbs. In other cases it is a matter of removing a restrictor plate/gasket (if you watch Nextel cup racing you know what that does to horsepower). Until you compare the parts lists for pistons, exhaust tuners, carbs, jets, reed blocks, etc, you won't know what to change. Although the block determines displacement, two different part numbers would indicate porting is different for the two. These changes are easy for the factory to do, but expensive if you do them after. If you look at the specs for every manufacturer (two stroke or four stroke) there are generally two, three and even four horsepower ratings for a given displacement.

 

[JB]

Re: 2007 9.9 yamaha to 15hp conversion

Corvette and Camaro do not share the same engine, Ed. The blocks are similar, but there are many, many differences.

When I raced Karts, way back in the dark ages, we always started out with 2 stroke chainsaw engines. Then, WITHOUT changing the displacement, we changed some or all of the following:

* Larger venturi carb or multiple carbs.
* More and/or bigger reeds in custom reed blocks.
* Added 3rd port for added motorcycle style intake.
* "Stuffed" the crankcase to increase scavenging pressure.
* Modified size, shape and timing of ports.
* Added ports for combined loop and cross scavenging to cross scavenged engines.
* Increased compression ratio.
* Advanced initial ignition timing.
* Installed "maverick" ignition for a 20* advance past about 6,000rpm.
* Added motorcycle style extractor exhaust geometry.

Then we fueled them with methanol and nitromethane and turned them 3 to 4 times stock rpm to get 3-5 times original hp.

They made astronomical horsepower and lasted two or three hours, enough to collect another trophy or two..

I am not suggesting that Yamaha makes more than a few of those changes to get 15hp from the same block as the 9.9, but it should give you an idea of differences in tuning of 2 stroke engines. That is TUNING, not detuning.

 

[ebsworj]

Re: 2007 9.9 yamaha to 15hp conversion

I see your point Ed about it being easier to de-tune that to tune. I agree that Yamaha detunes it's 15hp to get 9.9 to be within the under 10hp law on some lakes.

I also think it is a combination of many parts that will work together to give you an extra 5 hp. If it were just a linkage, there would be tons of people doing it and lots of people would write about it. I bet it will not be cheap but it could be worth tinnkering with...

Talk to a good mechanic and maybe he can do it for you.