Octane ain’t the answer

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