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Originally Posted by old_master
The higher the octane rating, the less volatile the fuel is. High octane fuel takes longer to burn and as a result, ignition timing must be advanced more to efficiently burn it. If an engine spark knocks, pings, detonates, what ever you want to call it, it means the fuel is done burning before the piston reaches top dead center, (TDC) on the compression stroke. The sound you hear is the piston “fighting” it’s way up to TDC. When the engine makes this sound, either the octane rating is too low, or the ignition timing is too far advanced. If an engine continues to spark knock, it will eventually burn a hole in the top of the piston. OBDII equipped vehicles, such as yours, have a knock sensor. When the sensor detects spark knock, the PCM retards ignition timing to reduce or prevent spark knock. Ignition timing is not user adjustable on OBDII vehicles, therefore using higher octane fuel will not help it, but it won’t hurt it either.
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Hey Old Master, I have seen your posts regularly in this forum and agree with many of the things you have written so I have respect for your opinions. I would like to comment on a couple of items in this particular post though. Volatility is the measure of how easily liquid fuel changes to vapor fuel (i.e. vapor pressure), it has nothing to do with octane, av-gas is high octane and high volitility so at high altitudes (low temp, low pressure) gas will still vaporize. Premium gas sold in Pheonix is high octane (well sort of) and lower volitility especially in the summer so that the gas will not boil in the gas tanks parked on black top in the sun. Refineries very the volitility from region to region and season to season. In the summer you want a little lower volotility to prevent excess vaporization (and overloading evaporitive emission charcoal canisters) and vapor lock on cars with mechanical pumps mounting on the engine in the old days. In the winter you want a little high volitility to improve cold starting (if you have ever driven from Pheonix to Flagstaff and seen a 50-60 degree temp change w/Pheonix gas in your tank you'll know what I mean, not as big a problem now as it was with carburatored cars).
Octane is a rating of the anti-knock index of a fuel, fuels with otherwise identical characteristics with different octanes will combust (i.e. burn) in the same period of time in identical engine characteristic. The anti-knock index is the ability of the fuel to prevent knocking (detonation) which is the fuels tendency to combust due to heat and pressure as opposed to due to a spark and the resulting flame front. During normal combustion the spark plug ignites the fuel and the flame front traverses the combustion chamber gradually, think of a grass fire in a field. The edge of the grass starts on fire and the flame burn across the field roughly in a line, same thing in a combustion chamber. During detonation as the flame front move across the combustion chamber the remaining unburned fuel mixture is heated and compressed, if the heat and pressure exceed the fuels ability to prevent detonation then all of the mixture burns at once rapidly raising the heat and pressure in the combustion chamber (think of the field with the grass fire, the whole remaining field of grass instantanously lights at once). This rapid rise in pressure actually causes the engine block to vibrate, this is the sound that can be heard during detonation (and why knock sensors are usually screwed into the engine block). Detonation can occure prior to the piston reaching TDC or while the piston is dwelling at TDC during the time known as the constant volume combustion period (the few degrees before, during and after TDC when the crank is moving but the piston is not). Detonation can happen after TDC but this is not as common. While causes for detonation include low octane fuel and excess timing advance other factors can cause detonation including excess combustion chamber deposits and lean fuel mixtures (which increase combustion temperatures).
Vehicles with knock sensors may or may not benefit from using higher grade (octane) fuels. If your engine does knock on 87 octane and the PCM retards your timing you will most likely not know it, the effect of the retarded timing is poorer performance and lower fuel economy. By putting higher octane fuel in you may improve performance and economy, HOWEVER unless your vehicle is specifically designed for premium fuel (because of higher compression ratios) you will never notice the change in performance and economy since most vehicles are designed to run on 87 octane gas. In the event that your PCM is retarding the timing it is so slight that even a professional driver wouldn't notice the difference. In an article I read recently on Yahoo finance concerning saving money on fuel it said that 80-90 percent of the people putting mid and high grade fuels in there cars were not getting any improvement in performance or economy. In another article I read in Yahoo Finance a couple of years ago it was stated that retail gasoline suppliers were cheapening up there lower grade fuel by skimping on additives like detergents and anti-injector fouling agents, although they haven't removed them from higher grades it still isn't worth buying the higher grade gas just to get the better detergents and additives. The article recommend every 5-10 tanks to put a bottle of fuel system additive in, at a few dollars at the autoparts store you are way ahead of putting $2-$3 of higher priced gas in every tank.
Don't know if anyone is actually going to read this far down, but I gotta ask this - Has anybody noticed in the Shell gas ads about preventing the build up of gunk in your engine that when they show the Shell gas valve that they are showing you an INTAKE valve and when they show you the cheap gas gunky valve that it is an EXHAUST valve?
Sorry for the length, hope this info is helpful in deciding which gas is right for you.
Sparky.