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Originally Posted by 99-Boxster-MK
Hey all...I have been filling up with Sunoco Ultra 94 gas (the one with 10% ethanol), as this is the highest octane gas around me, but I recently filled up with Shell V-Power gas (91 octane) and the car feels faster and smoother????? Can it be that the boxster does not like ethanol additives in its gas?
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It's probably a combo. There is a warning in the manual to stop using ethanol fuels if performance is degraded.
For the octane, the higher the octane, the lower the volatility of the gas. When you get knock from a low octain gas it's because it ignites too early due to the compression, before the spark plug fires and before the piston is finished with its' up stroke (or out stroke in the Boxster's case). That causes the upward (outward) moving piston to have force shoved against it so it knocks and looses power in its' stroke.
On the other hand, if the octane is too high, it doesn't ignite on time. The spark plug fires and it takes a millisecond longer than it should to ignite so the piston is already on its' way back down (in). When the gas ignites it pushes the piston downward (inward), but not at the optimal time. Since the piston was already moving downward (inward), it's further away from the combustion than it is supposed to be so it does not create as much power in the stroke.
Multiply these by 3K-6K times per minute and now you know...the rest of the story.
