Re: how can i make my 1975 2 stroke 15 hp johnson go faster
FIRST GET A TACH! You can't know where you are going until you know where you are! And even with a GPS for speed, the tach gives you a direct reading on engine performance. Then make one modification at a time. If you have more than one variable, you can not know which has helped or which has hindered. ALL racers are in a constantly improving state--and it takes thought and time. You can NOT just throw a bunch of modifications at an engine and magically it will become a barnstormer--just doesn't work that way. You need to work smart--not hard.
Mill the head for higher compression--research FIRST to be sure there will be enough clearance for spark plug electrode. Don't go too far or you will have a hard time pulling it over and you will need premium fuel. 140 psi is a good high number for a small engine.
Pack the block by filling the bypass area and other areas where the rods and crank don't reach--better volumetric efficiency. Get a bigger carb and fine tune it to your engine, open up the reed stops a little so reeds will open wider.
Get a smaller diameter, higher pitched prop so the engine can rev a little more and yet take a bigger bite out of the water. Only way to go fast is high revs and high pitch prop. Big 140 racing Mercs used a tiny 10 inch diameter prop with about 25 inch pitch with 1 to 1 lower gearing or even 1.2 to 1 (like overdrive)
Raise the engine on the transom--need to experiment here with best height. Experiment with timing. Get the engine "on the pipe" (tuned exhaust pipes)
Vent the exhaust gas out above the cavitation plate for less back pressure--a few 1/4 inch holes on each side of the leg will help some--noisier though and not quite legal for recreational use!
"PORT" the bypass and exhaust ports. This usually results in significant gains on older cross flow engines and if done properly will not affect engine life. You must disassemble the engine to do it though, so it is a winter project. If you want to see an example of porting, go over to Chrysler crew, join and go to photo albums. Look for Frank's boats. There is a 3 cylinder ported Chrysler in there
RUN ALCOHOL/NITRO OR NITROUS.
I'm just spitting out ideas here. You pick and choose. Example: if you have tuned pipes, you don't need to vent the leg.
ALL THESE MODS (with the exception of raising the engine and a prop change) WILL HELP TO MAKE MORE HORSEPOWER--ALL WILL HELP TO SHORTEN THE LIFE OF THE ENGINE. Some cost quite a bit. Speed is always expensive so it depends on how serious you are.
When you start modifying an engine for speed, you need to experiment and sometimes things go wrong -- sometimes you lunch an engine--then you need to start from scratch.
Go over to hydroracer.net, join and pick their brains. I'm sure that although they run standard classes of engines, there is still a LOT of knowledge out there on souping-up these small stock engines.
