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Valentine One Radar Detector

dom
11-16-2010, 09:55 AM
What is the difference between high/mid/low octane?

How do I know if my car needs high octane fuel?

MikeHalloran
11-16-2010, 11:46 AM
1. The octane number.
2. The owners manual will tell you.

1a. Octane number is formally defined as a percentage proportion of iso-octane in normal heptane that produces the same knocking in a standardized test engine.
See RON in http://en.wikipedia.org/wiki/Octane_rating

2a. The engine will tell you if it needs higher octane fuel, by knocking and running like crap.
Do not allow an engine to knock for any amount of time. Reduce the load, shift down, retard the spark, or tow it until you can get some higher octane fuel into it.

1b. Octane number is sort of the opposite of Cetane number, which is used to rate Diesel fuel.
Diesel must ignite instantly when injected into a high temperature air chamber.
Under the same circumstance, petrol must _not_ ignite until a spark is applied.

Eric
11-16-2010, 02:18 PM
What is the difference between high/mid/low octane?

How do I know if my car needs high octane fuel?

The way it was explained to me, octane is a measure of burn rate, or volatility. Low octane gas actually burns easier/faster. High octane, the reverse - which is what you want in a cylinder that has higher pressures (from higher compression ratios, etc.). If the gas explodes too soon, when the piston is not in the right part of its cycle, you get knock - the sound of uncontrolled/premature combustion (and it's bad, because it's trying to force the piston down when it's trying to come up on its compression stroke, etc.)

You should go with the octane that the engine was designed to use. Using high octane gas in an engine built for unleaded will just cost you money, while using low octane gas in an engine built for high octane fuel will cost you performance/mileage and may cause mechanical damage to the engine.