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, Fri, 18 Feb 2005 18:46:26
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Just to clear this up for those who might not be up to speed, a few points on what octane ratings mean.
When running a gas-powered, spark-ignited IC engine, you want the spark to ignite the fuel, since it does it at the precise moment when best power is produced (this is an oversimplification, but basically that's what happens). However, forces within the engine - heat and compression - can cause pre-ignition of the fuel, or knocking. This is bad for a variety of reasons, and can be more common in older engines with carbon build-up on the piston tops that causes hot spots. All things being equal, engines with higher compression (or boost) and/or engines that run hot will be more prone to knocking. Knock sensors in modern engines sense this, and retard the timing to stop it. This can result in a loss of power.
Higher octane fuels are more resistant to pre-ignition, as they contain compounds which reduce their tendency to ignite. Yes, that's right, REDUCE their tendency to ignite. So, again all things being equal, higher octane fuels contain marginally less potential energy than lower octane fuels. From a design standpoint, there is absolutely no reason to use higher octane fuels if your enigne will run properly without them; in fact, you wan to run the lowest octane possible that allows you to make full power. In older engines with carbs and distributors you could tune them to work best with different octane fuels; in modern engines, the ECU controls everything.
Now, there is a contention on the part of oil companies that their high octane fuels contain detergent packages that will keep your engine cleaner. This may be the case, and not being a petrochemical engineer, I can't say. However, if that's the only reason you're buying the expensive stuff, save your money and throw a bottle of good fuel injector system cleaner in before an oil change once or twice a year (or better yet, drive the car hard and get those combustion chamber temps up high enough to burn off the carbon) and call it done.
posted by 140.186.252...
, Fri, 18 Feb 2005 15:54:09, Fri, 18 Feb 2005 18:46:26, Sat, 19 Feb 2005 12:07:59, Mon, 21 Feb 2005 15:40:13, Sat, 19 Feb 2005 12:12:59
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