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Anti-Knock Quality (Octane Rating)
- The octane rating defines the gasoline's anti-knock quality (resistance to pre-ignition). The higher the octane rating, the greater the resistance to engine knock. Two differing procedures are in use for determining the octan rating. These are the Research Method and the Motor Method.
- The number determined in testing using the Research Method is the Reseach Octane Method, or RON. It serves as the essential index for acceleration knock.
- The Motor Octane Number, or MON, is derived from testing according to the Motor Method. The MON basically provides an indication of the tendency to knock at high speeds.
- The MON method differs from the RON method by using preheated mixtures, higher engine speeds and variable ignition timing, in the process placing more stringent thermal demands on the fuel under examination. MON figures are lower than those of RON.
- Octane Numbers up to 100 indicate the volumetric content in percent iso-octane contained in a mixture with heptane at the point where the mixture's knock resistance in a test engine is identical to that of the fuel being tested. Iso-octane, which is extremely knock resistant, is aassigned the octane number 100 (100 RON and MON), while heptane, with low-resistance to pre-ignition, is assigned the the number of 0. Bascially researchers take the two methods and compare laboratory values (Rersearch Method) to real world values (Motor Method) for the fuel being tested, hence at the Gas pump you see the RM/2 symbol.
- Normal untreated straight-run gasoline displays little resistance to knock. Various refinery components must be added to obtain a fuel with an octane rating which is adequate for modern engines. The highest possible octane level must also be maintained throughout the entire boiling range. Cyclic hydrocarbons and branched chains provide greater knock resistance than straight-chain molecules. Additives based on oxygenous components (methanol, ethanol, MTBE) have a positive effect on the octane number, but can lead to difficulties in other areas. Basically alcohols raise the volatility level and can damage fuel system components if not manufactured or built to handle these components.
- If I recall me and Scott C were talking about this on a thread about a year ago (I'll try and locate it) and some interesting things were revealed. For example if Car A has zero knock present it will not benefit the owner to run a higher octane level as Car A is ideally getting it's full potential from the current fuel. However as temperatures do fluctuate and we do live in an imperfect world who's to say the knock value will always be zero. So the key is finding the threshold or the "sweet spot" at which the octane level and zero knock occur. As you can decipher it would be much easier for a enthusiast who uses his vehicle purely for 1/4 mile runs to know where this sweet spot is rather than someone who uses their car on a daily driving basis with all kinds of different loads and stresses. Some who are infact tuning guru's here (Dyno Brian, 12.99someday98) unlike myself can manipulate fuel and timing curves to find this "sweet spot" and fullyt take advantage of it. For the rest of us (myself included), we are left to utilizing our Scan-tools and local Gasoline providers. But hey, there's always room to grow and time to learn. :8
 
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