Johnson 15 (Want to make power)

Hi guys.<br /><br />I got a 89 johnson 15 that i dont use anymore. I want to play around with it and make some more power. My goal is about 30hp. <br /><br />Can i shave the heads (How much) <br /><br />Can I port and polish<br /><br />Bore the carb<br /><br />Rejet <br /><br />If anyone knows how to make power with this little engine please help me out. Its all in fun and even if it blows up i dont care. <br /><br />So please tell me how to get every hp out of this thing.

Re: Johnson 15 (Want to make power)

The 15 hp is already rather high performing (only 216 ccm) - so it would probaly not be capable of producing much more. Why not sell it and buy a 30 hp if that is what you need?

Re: Johnson 15 (Want to make power)

Thanks Enn.<br /><br />Like i said I dont need it. I bought a 30 yamaha for my boat and now this thing is just lying around. Im just looking for somthing to do and theres always a way to squeeze more power out of somthing.

Re: Johnson 15 (Want to make power)

Yes you can do all of this for educational or racing purposes......You need to do a search on the web for outfits that do outboard racing work. It WILL cost some money though, the crank will have to be magna fluxed as the connecting rods too. You may find that you will have to go thru several rods or even cranks to find one that's perfect and will stand up to operational 10,000-12,000 RPM. You will have to have port work done to the block and probably total replacement of carb, reeds and induction manifold. There are also exhaust plates that replace the standard removable one on the side of the engine, if it is equiped with one, to adapt external pair of tuned pipes(20% gain in power alone). There are also racing lower units made too for certain engines with specific gear ratios for racing. In addition you will probably have to run a cupped prop with the lower unit jacked so high that the prop will actually run out of the water at speed. On the extreme end 60 HP is attainable but reliability could only amount to a few hours of running time at these performace levels. A simple quick fixup, convert to alchohol fuel but your fuel consumption will almost double, and get next size up carb(much larger jets needed too). Fuel system must be converted to silicone based products to keep from dissolving.

Re: Johnson 15 (Want to make power)

Thank you for your responce. <br /><br />I dont think i want to mess with the crank and pistons. I was just hoping the head could be shaved. I can grind it my self. I can also do port and polish myself. This would not cost me anything to do. <br /><br />If this can be done to this engine please let me know. <br /><br />Do you also have any websites about all the info you stated. <br /><br />Thankyou.

Re: Johnson 15 (Want to make power)

You can mill the head, but you MUST THEN ALWAYS use a higher octane gas. Look up in the manual for this particular year motor and it will tell you the compression ratio. Milling the head decreases the cubic inches but increases compression ratio. Make sure you don't mill so much that the spark plugs hit the pistons. You should have at least .060" clearance(more clearance would be better between the spark plug and piston). Going beyond 10 to 1 compression will creat a series of problems all interlaced together. At this compression ratio you will need 100 octane gas to be safe and you will have to run cooler spark plugs. Raising the ratio more will require gasoline that is not presently available to consumers unless you have access to racing fuels. You can though, convert to alcohol which has a natural rating of 120 octane. If you do this with the stock carb, you need to up the jet size considerably. And again convert the entire fuel system to silicone compounds to avoid the rubber melting. Hp gain based on the compression ratio alone will only give you about 5 to 10(personally, I think only about 5 HP) more ponies at this point and you will HAVE TO increase prop pitch to utilize the existing port timings and torque band. Polishing the ports on this particular engine will have very little effect. What REALLY has to be done is the ports SIZE and TIMING of when the piston allows them to open.(professional mill work...)

Re: Johnson 15 (Want to make power)

Thanks i will keep all of this in mind. <br /><br />You wouldnt happen to have any web links for this kind of stuff. <br /><br />You know products, Pictures, Diagrams, Prices.<br /><br />The only reason i wanted to do this is because i saw on tv a boat racing episode with this type of engine making huge speed and power. Wanted to know whats involved with abtaining that.

Re: Johnson 15 (Want to make power)

This is not exactly what you want, but its about a 2 stroke and they all work on much the same principles...<br /><br /> http://www.macdizzy.com/atc.htm <br /><br />Do you have a dyno or something so you can test your changes???<br /><br />I ask because in alot of cases, this is trial and error... and being able to easily compare various configs is pretty handy.<br /><br />There are some other links of that guys site that are pretty handy..<br /><br />rgds<br /><br />Frank

Re: Johnson 15 (Want to make power)

Talk to Scott at chine walkers place.<br /> www.pfs-ware.com/scott . His father raced outboards and he races outboards.He should know some tricks.

Re: Johnson 15 (Want to make power)

THe OMC 15s are already a pretty high revving motor. Shaving the head won't give you what you're looking for. If you increase compression, you have to increase other factors to take advantage of it. It's all interrelated - fuel, air, exhaust, compression, timing. You can't change just one and expect dramatic results. If you want to really have fun with it, find a 'A' class racing lower unit (bolts right onto your motor), a small light hydro and go have fun! Those little 13 cubers will push a light hydro over 55mph! Now that's performance!!<br />- Scott

Re: Johnson 15 (Want to make power)

Also, seeing as you're from TO, try a web search for TORC (Toronto Outboard Racing Club) or the CBF (Canadian Boating Federation). Andrew Fralick runs the TORC site and he's quite successful in class A and could probably point you in the right direction for parts.<br />- Scott

Re: Johnson 15 (Want to make power)

Fly Boy<br /><br />Try as you will, you were not able to discourage zzzz1. <br /><br />You probably need to clarify something. Guess I'll make an attempt. Shaving the head does not decrease cubic inches (CID)of the cylinder. It decreases the volume in cubic inches of the combustion chamber. Cylinder CID divided by the combustion chamber volume is how the compression ratio is determined. <br /><br />Billy Bob