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Darkstar is right stop using the 100 octane. A rich air-fuel mixture contains a little more fuel mixed with the air. For gasoline, 8:1 (8 parts air to 1 part fuel) is a very rich mixture. A slightly rich mixture tends to increase power; however, it also increases fuel consumption and exhaust emissions. An overly rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust) A lean air-fuel mixture contains a large amount of air. For example, 20:1 would be a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel will be burned (leaving a white residue); however, too lean of a mixture can cause poor engine performance (lack of power, missing, and even engine damage). For gasoline engines 15:1 is optimal. This ratio will result in some black residue in the pipes.
 

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Discussion Starter · #26 ·
quote:Originally posted by RkyMtnWarrior


Darkstar is right stop using the 100 octane. A rich air-fuel mixture contains a little more fuel mixed with the air. For gasoline, 8:1 (8 parts air to 1 part fuel) is a very rich mixture. A slightly rich mixture tends to increase power; however, it also increases fuel consumption and exhaust emissions. An overly rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust) A lean air-fuel mixture contains a large amount of air. For example, 20:1 would be a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel will be burned (leaving a white residue); however, too lean of a mixture can cause poor engine performance (lack of power, missing, and even engine damage). For gasoline engines 15:1 is optimal. This ratio will result in some black residue in the pipes.




Correct me if am wrong, but how does changing the octane rating change the air-fuel ratio? The only way to change that is to reprogram the ECM, right? The same amount of fuel is being mixed with the same amount of air regardless of the quality of the fuel or the quality of the air for that matter. I'm not trying to be a wise guy; I would just like to have this explained to me.
 

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Okay here's the explanation because that's a valid question, and your right, it doesn't. I'm not trying to be a d**k, but it is confusing and long-winded.

Higher octane fuels burn less easily, and lower octane fuels burn much more easily. And since higher altitudes make it more difficult for fuel to burn and the reverse for lower altitudes, depending on your current A/F ratio, altitude and octane rating you establish your motor's ability to effectively burn fuel.

Now on an air cooled engine, like a Warrior, the extra fuel is an advantage as it assists in engine cooling. However, too much extra fuel can cause a rich burning scenario, and vice versa.

Since high-octane gas burns slower than low-octane gas, combustion is slowed with premium gas. Slower combustion prevents pre-ignition. Pre-ignition is ignition of the fuel/air mixture before the spark plug fires. When the air/fuel mixture is ignited by the spark plug, a combustion flame front spreads outward from the plug accompanied by pressure waves created by the combustion.

As an engine's octane needs are influenced by several factors, (i.e., cranking pressure, valve timing, ignition timing, operating temperature, plug heat range, carbon deposits, etc.) and each engine has it's own distinct octane requirements for optimum operation, if an engine only needs 93 octane to run properly without pre-ignition, the use of 100 octane will not increase performance! Conversely, the use of 87 octane, when 93 is needed, will result in pre-ignition, overheating and poor performance. Once you have established the actual octane needs of a particular engine for optimal operation, the use of higher octane gas is only wasted money.

Whew....welcome to the wonderful world of exhaust theory and the physics of wave dynamics.
 
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