Everything you didn\'t want to know about octane - a novelette
(Was: 93 Octane in a stock engine)
Well I'm kind of afraid to post this since I'll get scorned from the anti-math and education crowd here /wwwthreads_images/icons/tongue.gif - but - if you recall that little yellow sticker telling you the "Minimum Octane Rating" on the pump you will remember it has (hang on tight)a /wwwthreads_images/icons/shocked.gifmath formula./wwwthreads_images/icons/shocked.gif
Octane is simply the "general" term used for the "Antiknock Index" or AKI Method. AKI is determined based on an average of the "Research Octane Number" or RON and the "Motor Octane Number" or MON. The (hang on tight again)/wwwthreads_images/icons/shocked.gifmathematical formula /wwwthreads_images/icons/shocked.gifis (RON+MON)/2 or "the amount of the Research Octane Number plus the Motor Octane Number added together, then divided by two" but is usually abbreviated "(R+M)/2 Method" on the little yellow sticker.
This "octane" method is tested using only a single cylinder octane test engine. The MON is the measurement of the fuel's ability to resist knocking under severe operating conditions. MON will affect "top end" or highest speed RPMs, as well as acceleration and performance under load. The RON on the other hand, is the measurement of the same fuel's ability resist knock under less severe operating conditions. Therefore RON will effect the low end and mid range RPM knock, and engine "dieseling." Under most circumstances RON will be 8-10 points higher than the MON. For example: If the octane is 90, then one possible formula would put the RON at 85 and the MON at 95. But remember, this "Method" uses a single cylinder engine.
The octane that is required to make your engine run "knock free" is called the Octane Number Requirement or ONR. The exact ONR for an engine is affected not only by design factors but environmental and load conditions also. Engineers today have to balance performance with economy standards and environmental standards in the design of each engine. In doing so, they must factor in variables such as chamber design, timing, combustion ratios, operating temperatures, and so on. This ONR will then have an impact on fuel economy. As was stated many times before, compression ratio will have the most impact on the ONR and the engine efficiency. GENERALLY speaking, the higher the compression ratio is, the higher the ONR will be, so the engine will have a higher combustion efficiency. But if you change your rich and lean air/fuel ratios, retard your timing, and increase your engine's compression ratio through additives such as "Kreen," all of these will work to reduce your engine's ONR.
But it isn't that easy - there are other factors that affect the ONR. One you have already discussed - temperature. Others are barometric pressure, relative humidity, dew point, elevation, engine deposits, fuel purity, oil viscosity, and on and on and on. Increases in barometric pressure or temperature, increase the ONR. Increases in humidity and elevation, reduce the ONR.
And we have just scratched the surface - this is remedial fuels, not even fuels 101. We could add in such factor as flash point, front end volatility, pre-ignition tendency, final boiling point, . . . .
As you can see, there are hundreds of combinations just in the fuel that will affect the engine's performance. That's why NASCAR engines are practically built for an individual race. This stuff is an exact science now - not just what ol' Bubba felt might run good today.
So what does all this mean to us? You will have to find out what octane runs best under what circumstances for your vehicle and use it! Don't use higher, you have already met your requirements! Certanly don't use lower, that knocking is causing severe wear in your engine. Why would you buy all the best components for your rig, then starve it anyway? If you don't feed your engine properly, it won't preform for you, and it will break down - just like your body. Don't over feed it, or it will end up with fat and nasty deposits from the fuel that isn't totally consumed - just like your body.
My temperamental MPFI Cherokee was ONR'ed from the factory at 87. If I burn 87, I might as well push it! I HAVE to run 89 or better. If it is a LONG trip (300mi+) I have to run 91 or 93. I ran 86 ONCE to save money when I first bought it, and paid well over a hundred big ones to get it running right again - but the $3.00 I saved at the pump sure helped offset the cost. My 304 in the CJ? 1 part moon shine to 1 part kerosene and that old brute would probably still run just fine! /wwwthreads_images/icons/crazy.gif Just kidding.
This is just a portion of the intoductory knowledge you have to attain to qualify for an aircraft refueler/defueler license in the Marine Corps - and you all thought MARINE stood for "Muscles Are Required, Intelligence Non-Essential." /wwwthreads_images/icons/mad.gif