[Putershark]

Originally Posted by glehav

good info putershark. you learn something every day on this site. but putershark i have a 150 e-tec and evenrude manuel states not to use highter octane than 87. and what you are saying is you might need to go to a highter octane in the summer months to get better preformance out of your motor. where does that leave me.and what is the best way to keep carbon deposits off my piston heads thanks for all your help

Great question and hopefully I can explain such...

We have to remember that your engine is a 2 stroke engine. The real time octane needs on such an engine design that is getting cooler (relatively speaking) oil injected into the combustion process already "boost" the real time octane numbers (like cool moist air getting pulled in). This since the oil is somewhat harder to burn than just the raw 87 octane fuel.

We also have to take into consideration, that "some" engine management systems are designed to allow the engine to knock (though you may not hear it) every so often... This so that the knock sensor can feed that data to the ECU so it can update (reprogram) the engine's timing curve in real time. So if you were to run a very high octane mixture (in theory) the timing advance curve would keep getting increased, and increased some more, and so much so, that the engine gets REAL hot and could suffer some real overheating effects. This and it could happen so fast that when under a load, pistons and rods could start to come all unglued

Now does this mean that higher octane fuels run hotter? Nope... It is just the effects of the higher octane fuel has the system "confused" (for a better lack of terms) and the timing keeps getting increased and that process (not the fuel) causes it to get hot. So as the link that I posted last night on octane explains better than I can right now and I quote...

Octane rating has no direct impact on the deflagration (burn) of the air/fuel mixture in the combustion chamber. Other properties of gasoline and engine design account for the manner at which deflagration takes place. In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating. Deflagration is the type of combustion that constitutes the normal burn. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not deflagration characteristics.

It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion).

So how do we help keep power robbing deposits from forming in your 2 stroke engine? Well the best way is a 2 fold process / attack to combat such in not only your engine, but pretty much "any" engine and they are...

1. Use a high grade (detergent) fuel. One that has a good additive package that both helps prevent the build up of deposits, this while cleaning up deposits that are already present...
2. Use a quality full synthetic oil that lubes better, burns cleaner, and as such much cooler... This is why on many such 2 stroke engines, that if you run a "full" synthetic oil the shop can adjust it to use less oil since it is much superior in both lubrication, burning and cooling properties.

This also applies to 4 stroke engines and the oil that is put into the crank case. Piston rings are the key here (which shocks many) for their primary job is to transfer heat from the piston's to the cylinder walls where the cooling water is flowing. If you do not transfer that heat from the pistons, they will fail... And when that happens, the whole thing just goes BYE-BYE and that short sucking sound you hear next, is lots of money flowing out of your wallet too fix it

Now some say that using such oils is expensive... No question that they can be! But if one cannot afford the oil, well I hope they can afford the repair bills that can happen later on. Besides, I think that the ROI from using such quality lubes and fuel pays off in the long run as my testing and tracking has shown me time after time after time

So you need to test, track, and compare... For I know of no other way too cut through all the marketing hype to ensure what we are spending our hard earned money on, is really worth it, or not

Dave