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Oorspronkelijk gepost door: Paul_Met_Puch
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... gegeven door Niek (http://www.brommerforum.nl/print/reply/1/1359368))...

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Beside the type of fuel being burnt, there are other factors which influence flame
speed. Very rich and very lean mixtures both burn slowly, hence more spark advance
will be needed. A mixture close to full power lean burns the hottest and requires the
least advance.

An increase in the compression ratio increases the density and temperature of the
compressed fuel/air charge. This increases the rate of combustion. Likewise, an
improvement in the volumetric efficiency of the engine will have a similar effect.
Therefore a change in porting, a new expansion chamber, or a carburettor which
allows the engine to breathe better, may require that the spark lead is reduced.
As the engine speed (toerental) increases, fuel atomisation improves, which means that the
fuel will be broken up into smaller particles. Small fuel particles have a proportionately
larger surface area, therefore they burn more rapidly, which means less advance minder voorontsteking is necessary. ...

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