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Dennis -> RE: Gasoline Octane??? (3/15/2006 5:15:29 PM)
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quote:
ORIGINAL: RutDogg
I have read on a couple of different Hummer forums that you should only use gasoline rated at 87 octane. After reading my owner's manual it states to use 87 or HIGHER octane. Is there a reason why you wouldn't want to use a higher octane? The owner's maunual does not state to use ONLY 87 octane, so why shouldn't I use a higher octane?
To expand on Shorty's explaination just a tad more..[8|]
The Engines are designed (compression ratio) and the PCM (engine computer) are programmed to adjust the spark firing angle for a certain range of fuel octane. The engine has what is called a predetonation sensor to signal the computer to advance the timing or retard the timing to correct for preignition (known as knocking or pinging)..But it has a limited range from retard to advance. So the fuel has to be of the correct octane for the timing to fall within this range. If you use a slower burning HIGH octane fuel, the spark control must adjust itself to it's full Advanced end of the spectrum...But it will not go far enough to compensate for the High octane fuel (usual above 90 or more)..What ends up happening is that, since the fuel burns slower and incompletely, a lot of left over unburned fuel remains and carbons up the combustion chambers. Resulting in higher compression eventually and then you are force to use the higher octane fuel by fault...So the faster burning 87 octane is optimum for proper combustion efficiency as these engines are set up for. Using any higher octane is a waste of money too...89 octane is a blend of the 87 and the 92+ at the pump's mixing valve.
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