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No, I have the ‘European’ with ‘manifold’ vacuum advance…
The reason for the variable advance is that combustion inside the cylinder takes time. As the mixture ignites at the spark plug, a flame front spreads at a constant speed of some 30 m/s.
There is an ideal position of the piston position at combustion completion for max power. As revs rise piston; speeds increase - and earlier ignition is required to complete combustion before the piston reaches the ideal position…
Compression pressures is a function of manifold pressure; the higher the pressure (the lower the vacuum) - the higher the compression pressures. As the mixture is compressed it heats up, adding to the heat and pressure generated by the burning mixture.
When the temperature reaches the ignition point of petrol, and there is still unburnt fuel - remaining fuel ignites explosively; the engine knocks. To avoid this the spark must be retarded, delaying ignition - particularly when high manifold pressure is detected.
Using ‘manifold’ advance this is achieved; high manifold pressure means no vacuum advance - the timing falls back to centrifugal advance. The ‘European’ philosophy, for historic reasons, aimed to extract maximum power from a given engine volume - maximum advance while avoiding knocking.
Other advance methods described has other priorities, but does not aim for maximum performance. ‘Ported’ vacuum is taken from a pickup port at the carb - the air speed past the port decides the vacuum; the higher the air speed, the lower the vacuum. It can be used for both ‘advance’ and ‘retard’ - but in all cases; the lacing of the port is material…
Complicating matters is the analogue, mechanical distributors used. They are imprecise and cannot give optimal timing throughout load and rev range - so safety margins are required to avoid knocking. Modern ignition systems are digital - and are also equipped with antiknock sensors…
The main point is that the advance influences engine performance and indeed emissions. But altering timing cannot alter the engine itself; you cannot force the engine against its will…
Frank
xj6 85 Sov Europe (UK/NZ)
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