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On 1 May 2003 at 7:32, Daniel Baum wrote: > > > Hopefully i will not break my car again and will promise to my engine > > > to only use premium fuel > > Out of curiosity, what type of petrol do you get over in the US? Is it > really something like 82 octane, Ours ranges from about 87 to 92 octane depending on which grade we choose. The rating is based on the average of the MON and the RON (MON + RON)/2 where MON is motor octane number and RON is research octane number. I think VW used whichever of these tends to be higher, so that our octanes tend to be equivalent to those VW used with about 3 points higher. (So our 87 is ~90 on the scale that VW used.) > to the point, what difference does octane rating make to engine performance and > the way it is tuned? Higher octane allows an engine to operate with higher compression ratios. If you use too low an octane you will get knock which is hard on an engine. Once you have the octane high enough that knock does not occur there is no additional benefit from higher octane. In some cases using higher octane may allow you to run with more spark advance without knocking. This will generate more power, but the higher octane was not the real source of the power, it just allowed it to happen. I don't think there is any significant difference in the actual chemical energy content of the various octane levels of gas, but adding alcohol does reduce the energy content. -- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------------------------- List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org