Increasing Mercury 6 Horsepower?

[kenelson]

I just bought a Mercury 6 HP engine S/N 0D192887, Model Number 1008211ND. I think it is about 15 years old. What can I do to increase the horsepower output? I believe the 6, 8, 9.9, and 15 HP models were quite similar. Is it as simple as increasing the main jet diameter from .039" to .056" to get to 9.9 HP? What else has to change? Can I get it to 15 HP?

 

[skier]

Re: Increasing Mercury 6 Horsepower?

Simply put, NO!! You would have to open the exhaust ports as well and that is not an easy task. Sorry to be the one to tell you.<br /><br />Craig

 

[JB]

Re: Increasing Mercury 6 Horsepower?

Hi, Ken.<br /><br />Merc tells it true.<br /><br />If you want or need more hp, you should buy a bigger engine.<br /><br />

 

[oldboat1]

Re: Increasing Mercury 6 Horsepower?

interesting thread -- think there is a common idea that you can increase hp from, say, a 9.9 to a 15 by changing carbs (think that is thought to be the case with OMC engines). Would that work? I'm thinking there might be some engines that are nearly identical except for carburation (?)<br /><br /> Curt

 

[skier]

Re: Increasing Mercury 6 Horsepower?

Sorry oldboat:<br /><br />The way that motors of the same cid have different hp ratings is deternined by how much air/fuel can get into the motor. If the exhaust ports only let so much out, then on the intake stroke only that much will be let in cause you cannot put more in than it let out.<br /><br />Craig

 

[oldboat1]

Re: Increasing Mercury 6 Horsepower?

Thanks, Craig. What determines size of exhaust ports -- piston displacement size? size or type of block? That is, the 9.9 and 15 are probably not really identical, right?<br /><br /> Curt

 

[kenelson]

Re: Increasing Mercury 6 Horsepower?

Thanks, Craig, for your answer. The reason I thought the HP conversion from 6 to 9.9 was possible was that the replacement powerhead, 811686T97, appears to be the same for 6, 8, and 9.9 HP, only the carb main jet diameter appears to be larger. Got this info from parts lists on www.mercurypartsexpress.com. The 15 HP does appear to be different. Would the conversion up to 9.9 be possible?<br /><br />Ken

 

[Franki]

Re: Increasing Mercury 6 Horsepower?

I can't speak specific to outboards.. <br /><br />but essentially, the exhaust is as or more important then the intake on 2 strokes..<br /><br />amazing amounts of power can be gained by good exhaust.. <br /><br />so the answer is simple, yes you can upgrade your motor.. intake and exhaust..<br /><br />The answer to "is it easy" is unfortunantly no...<br /><br />If you find someone wrecking a 15hp.. and you know the powerheads are otherwise the same, then grab it and use the exhaust and carbs to "upgrade" yours...<br /><br />rgds<br /><br />Frank

 

[kenelson]

Re: Increasing Mercury 6 Horsepower?

Thanks Frank for your thoughts. Temporarily, I have given up on the 15 HP change, but wonder if everything except the main jet is the same on the 6, 8, and 9.9 and increasing the main jet diameter would increase HP.<br /><br />I guess for $3.91 I can find out, but thought someone else would have already tried it. Having worked as a designer in a different kind of industry, I know how hard we worked to keep parts the same when performance differences were small. I just wonder if Mercury did the same thing and just re-sized the main jet for the three different HP values. Looking in the parts list, I could not find the exhaust port differences for the 6, 8, and 9.9.<br /><br />I'll bet there is someone out there who has already tried this.<br /><br />Ken

 

[Franki]

Re: Increasing Mercury 6 Horsepower?

Well, let us know how you go..<br /><br />but how do you plan on telling one way or the other? do you have a dyno?<br /><br />rgds<br /><br />Frank

 

[kenelson]

Re: Increasing Mercury 6 Horsepower?

Whoops, Frank, now I have to tell more of the story. We are in the process of getting permission to construct a pier in front of our coastal house. When its done (next fall?), I will put the 6HP engine on a 12 - 14 foot aluminum boat, yet to be purchased. I will test the larger main jet by measuring my speed with my GPS before and after the main jet change, but I won't have the answer for possibly a year!<br /><br />But, I still figure someone out there already knows the answer … to me, it is such an obvious question.

 

[skier]

Re: Increasing Mercury 6 Horsepower?

Again I hate to burst your bubble, but it's the size of the exhaust ports in the cylinder that makes the difference. Putting bigger jets will only get you a rich running motor and bad economy. You can't put more air/fuel into the cylinder than the exhaust ports will let out. It is physically not possible without putting a turbo or blower on it.<br /><br />Craig

 

[Fouled Plug]

Re: Increasing Mercury 6 Horsepower?

I don't know about your 6 to 9.9 idea, Ken, but my Force 9.9 differs from the 15hp version in the size of the carb jet only....all other part #'s are identical. You need to research the part numbers of the different motors to determine what changes were made to boost output. I have not converted mine yet, as it is a kicker motor only, but if I move it to a skiff I'll have to try it out.

 

[FlyBoyMark]

Re: Increasing Mercury 6 Horsepower?

GUYS, GUYS, GUYS..listen to Merc........bigger jets only run richer and carbon up the engine..you must also increase the air flow to maintain proper air fuel mixture ratio..such as improved exhaust ports and/or a bigger carb. If an engine uses same carb with bigger jets to get more horsepower then the engine has been "ported" for greater flow. Also keep in mind some of the reed valve ports, reed valves and reed valve retainers are different. Or in some few and far between instance such as the merc 20 to 25 conversion using a bigger carb only.

 

[flatskiff]

Re: Increasing Mercury 6 Horsepower?

Ya going to a bigger jet will not increase power it will decrease it and the engine will sputter and probably not run at all. If the only part# changes are the jet then the only difference between the engines is the porting. Though there should be a difference in the powerhead #'s even if they are the same displacement and only the porting is different to distinguish the difference. Given the power heads are the same and only the porting is different between the engines dosen't mean all is lost. You could have the powerhead ported or do it yourself though I wouldn't reccomend the latter without being very comfortable with engines. I plan to possibly port a 6hp chrysler I got this fall. It will require complete engine teardown and using a computer program to reconfigure the ports degrese and size. My first mod will be fitting a motorcycle carb to it good for maybe 1hp but the oem one dosen't allow for main jet adj so I'll be able to fine tune my engine as well as increase air flow. To have someone port your engine you would be looking at around $600 guessing and maybe less if you tear it apart yourself. But don't just take it anywhere I don't even know anyone that does outboards but if someone knows someone that does good work you may want to look into it. Then you'll have to goto the bigger jet size and if your card dosen't have a main jet adj you'll probably need a motorcycle carb like Mikuni, or Keihin.

 

[Franki]

Re: Increasing Mercury 6 Horsepower?

This brings up some interesting points for me..<br /><br />I suspect why outboards produce so little power compared to a high perf 2 stroke motorbike motor.. is the exhaust.. on a single cylinder motorcross motor, the expansion chamber on the exhaust is huge. impossible to do with 4 barrel outboard engines, so whatever they do is a compromise..<br /><br />So, my question is, how many of the standard hot up tricks would be off use to an outboard??<br /><br />Stuff like <br />full balancing.. <br />port matching..<br />Port work, (reshape/resize)<br />Combustion chamber reshaping.<br />head and crown polish...<br />Aftermarket Reed valves. and possibly cradle reshaping..<br />An outboard version of what we called "aftermarket powerpipe" exhaust.<br /><br />Stuff like that, if done yourself, would be relatively cheaply, but you have to know what you are doing for most of it.. and a flowbench is handy as well.<br /><br />I have a spare powerhead here, that I am toying with rebuilding and hotting up.. <br /><br />I even want to try and adapt a Holley fuel injection kit to fit the 4 barrels of carb.. <br /><br />rekken that with fun and perserveriance I could get alot more power then this usits rated 85HP...<br /><br />and the balance would probably have it reving further saftely as well...<br /><br />I wouldn't do this sort of stuff unless you have time and some cash to spare.. but I hardly payed anything for my spare powerhead, so why not try??<br /><br />now I just have to rig up some sort of cheap comparison method.. (poor mans dyno) I might base it on a car torque converter, a machanical tacho and a computer program to tie it all together..<br />(I am a programmer so the last bit shouldn't be to hard for me.)<br /><br />anyone got any tips for this exercise??<br />I'm actually sort of looking forward to getting started.. (motor gets a complete rebuild in the process.)<br /><br />rgds<br /><br />Frank

 

[flatskiff]

Re: Increasing Mercury 6 Horsepower?

Goto www.macdizzy.com and post your question there they have all your answers. Post your question under the "watercraft" or "other two stroke" sections. I wish I knew more about outboard exhaust an expansion chamber to match would help so much. Maybe if you want to get crazy have some made and run them through the side if the cases

 

[FlyBoyMark]

Re: Increasing Mercury 6 Horsepower?

On a side note...some outboards DO have tuned pipes....in the motor leg....usually a stainless shaped insert instead of nothig to increase ponies up to 20% for stock models......