Oc­tane ain’t the an­swer

Old Bike Australasia - - LETTERS TO THE EDITOR -

An­other Velo Rally (the 2015 V.O.C.A. Good Com­pan­ions in W.A.), an­other holed pis­ton. Th­ese are not melted through, they are blasted through by pre-ig­ni­tion. At ev­ery rally there is at least one. I have been told that a stan­dard Ve­lo­cette will “ping” (det­o­nate) on high oc­tane petrol. What they are hear­ing is not det­o­na­tion but far worse; it is pre-ig­ni­tion caused by run­ning too high an oc­tane fuel that is still burn­ing long af­ter the ex­haust valve opens, and so over­heats the ex­haust valve. Det­o­na­tion and pre-ig­ni­tion are not the same thing al­though they can be con­nected. The calorific value of all petrol is about the same but the higher the oc­tane, the slower it burns, and so it has to run in a high com­pres­sion en­gine to speed up the burn. The burn should be com­pleted when the ex­haust valve opens. The com­pres­sion ra­tio gov­erns the speed of the burn; the higher – the faster.

The eas­i­est time to light the fuel/air mix­ture is at the start of the com­pres­sion stroke where it can be lit by an over­heated ex­haust valve. When this hap­pens the flywheel in­sists that the com­pres­sion stroke be com­pleted and the re­sul­tant ex­plo­sion has nowhere to go but into the crank­case via the pis­ton crown. The ideal oc­tane fuel to use is one that will just det­o­nate un­der gross abuse; too much throt­tle at too low revs. Once I ac­ci­den­tally filled my Moto Guzzi Le Mans with 91 oc­tane and on lit­tle throt­tle I rode it home, but it went so well that I care­fully used it up un­til at the end I used too much throt­tle for a gap in the traf­fic when it in­di­cated its dis­tress. When I re­moved a cylin­der head I found no dam­age at all. It has a com­pres­sion ra­tio of 10.0:1. An­drew Dun­can Wil­ton, NSW

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