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Benjamin Gradler
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29 June 2009 02:02 I was reading a story about how right after WWII the Norton factory team could tell the octane of the fuel they were using by reading their spark plugs. This let them add or remove shims under the cylinder to optimize their compression ratio for that fuel and possibly get a little bit more of an edge on the competition for that race. This got me wondering if there was a correct compression ratio for the late short-stroke 500 for different octane fuels. I think the 1960 and later bikes had 11:1 compression, and Ken has alcohol pistons for sale that give a 14:1 ratio. So what are the compression ratios for the different Manx engines for different years and the octane fuel they require? And just as important is the spark lead to run with those ratios? Some of this information may not be in manuals. For instance I may want to lower the compression of my late Manx so I can use less expensive or more readily available fuel. If the best I have is 94 octane, it would be nice if some tuner somewhere already worked out the best compression ratio and ignition spark lead to use with that fuel and would share the information! Of course you can do some adjusting with spark lead and cylinder shims, but not in all cases, and it surely is not as good a job as using a different piston. Also, with too many shims added under the cylinder you lose the squish and quench effect in the combustion chamber and this can end up making an engine require the same octane fuel after all! Thanks for your thoughts....
Benjamin Gradler
(read 20733 times)
01 July 2009 03:07 Tuning for speed by P.E. Irving has some of this information on octane, compression and spark timing as well as on Norton Manx combustion chamber squish.

On 80 octane fuel he states that the long-stroke Manx needs 7.5:1 compression and 37 1/2 degrees of spark lead, while the short-stroke needs 9.5:1 and 34 degrees for the 500, and 9.7:1 and 39 degrees for the 350.

The ability of the short-stroke to use a higher compression-ratio with the same octane fuel might have something to do with its more modern combustion chamber with a squish area between the piston and head, which Irving says must be maintained at between 45 and 55 thousandths of an inch. There is other information in Irvings book, but as my copy is from 1959, a lot of it is not any good because the petrol fuels he talks about are no longer available. He does give information on running alcohol, but that is not legal in vintage racing in my part of the globe!

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