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On 9 Jun 2005 at 22:40, Daniel Baum wrote: > I had wondered whether I needed a higher-octane petrol but I would be quite > surprised if this is the case. I use 96 octane. Since they stopped selling > leaded petrol over here a couple of years ago, the 96 octane has had a > potassium additive added to it (I think this is what they call LRP in > England). Other options are 95, 98 and 99-octane unleaded. Surely it > shouldn't require 99 octane? There are 2 different ways of measuring octane: the Research Octane Number (RON) and the Motor Octane Number (MON.) The difference is in the kind of test that they run to make the determination. Pump gas in the US is sold by the average of these to scales. I THINK the difference in the results is about 4 points, or about 2 points between the higher number and the average. [BTW, aviation gas is measured on a completely different scale, so 100 octane avgas is no good in cars, in spite of what some will tell you.] I think the stickers on our late type 3s refer to the european rating, which is the higher number. If Israel uses the same scale as used in europe, then your 96 should be equivalent to our ~94. I would expect that to be high enough, but there are several reasons why your your engine's needs might have changed over the years. If your engine has been rebuilt, then it's possible that they flycut the heads, either to increase the compression or to clean up the sealing surface. This will increase the compression ratio, which I suspect started out the same as our '69s at 7.7:1. If there is a significant amount of carbon buildup in the combustion chamber, either in the head or on the piston crown, this will also decrease the chamber volume, increasing the CR. Either way, it's worth trying some better gas just as a test. -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~