Originally Posted by nailem
I'm not an expert on octane but doesn't the compression ratio of the engine play into the equation? Some motors are designed to run on higher octane. My wife's VW said to run 89 we would run 87 and it ran fine but definitely ran better on 89 and got better MPG. Do the Hemi's suggest to run 89
Everyone needs to understand that octane is nothing more than the relative measure of a fuel's propensity to combust under compression (rather than when your spark plug fires). A diesel actually fires under compression - hence the term "dieseling", or pre-detonation.
So higher octane is no "better" than lower octane, other than the fact that it will withstand higher compression ratios w/out combusting. Many engines of the day will adjust their "timing" based on the octane of the fuel. If lower octane is uses, the ECU "retards" the timing - firing the cylinder before
it fires under compression.
I have several "toys" with high performance engines that require 91+ octane. Without it, pre-detonation occurs (which can cause damage) - if detected by the ECU, it backs off the timing resulting in lower performance of the engine.
What does this mean? If the engine is designed for it, running higher octane results in maximum performance. This may or may not translate into better fuel economy. However, if an engine is not designed for higher octane, you are wasting your money using it (and the oil companies love you for it).
BTW - nearly all gas contains the exact same detergents... so it's not "cleaner" either. I do put it in my small engines (mower, snowblower, etc.) because 91+ usually contains little (if any) ethanol, so it stores better.