You said, "you might reasonably expect to raise the compression ratio approximately half a point from what it was." Does that mean that if the compression was 100, that it would now be 150??<br /><br /> Also, why would using a higher octane gas make a difference?<br /><br />Thanks,<br /><br />Shane Could you explain a little about the increase in octane when you get the time?<br /><br />Thought, my Seloc calls for the L20V anyway, so that's what I already had in it. Hopefully they will continue to work.<br /><br /> From all of this, I gather that it is really impossible to try and guess how much it will actually go up since we are not dealing with a perfect situation. That certainly makes sense, although it would of been nice if someone could have just said, "Oh yeah, it will go up 20 psi in each cylinder."<br /><br /> I ordered a new head gasket this past Tuesday, so hopefully it will be here soon so I can find out.<br /><br /> I always thought I liked to fish but if "time spent" is any indication, I must really like to work on outboards. <br /><br />Shane ). If there are hot spots (like sharp edges in the combusiton chamber) or if the compression is high enough the fuel may ignite prior to spark (like a diesel). Also if your ignition timing is too far advanced the mixture may ignite and detonate instead of burn and you get the same result (detonation ). <br /><br />Certain additives to gas reduce this tendency. Octane which is an 8 carbon chain hydrocarbon was arbitrarily set as the standard of 1 or 100% or "100 octane". Any fuel when measured against octane is either worse (less than 100), equal to (=100), or better (greater than 100) than pure octane. <br /><br />If you increase your compression ratio, you may have to increase your octane (it depends on how much you increased the compression ratio). If your engine needed 87 octane before it might need 89 octane now. It might not if it only needed 85 and you were using 87 but we don't know exactly what was required before. Detonation is hard to detect in 2-stroke OBs at WOT. You could retard your ignition timing and use 89 or 93 octane as a safeguard. Or you can find another cylinder head as I mentioned above. <br /><br /> Anyway, as soon as I got out of the water I took the compression readings.<br />#1- 140<br />#2- 125<br />#3- 150<br /><br /> So not exactly even but quite a bit better than ever before. I mixed more oil in with the gas than I normally do (45:1 instead of 50:1). The spark plugs looked fine also.<br /><br />Thanks a bunch everyone!<br /><br />Shane
