Ignition Timing with Premium Fuels
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DUE to the high anti-knock qualities of the premium grades of motor spirit, it is possible, when using them, to over-advance the ignition setting beyond that necessary for optimum performance without producing audible warning. Maximum combustion pressures can thus be produced before the piston reaches top dead centre, which would dangerously overstress the engine and result in a loss of efficiency.
It is for this reason that some precise method of setting the ignition is recommended when motor spirit of this quality is used. It has long been the practice to set the distributor in its approximate position with reference to the position of the piston, the final precise setting being by a trialand-error method on the road. Such a setting would be one that, under full-load and full-throttle conditions with the engine at its correct operating temperature, a "trace pink" could just be heard.
This method, however, is not entirely satisfactory when running on a premium fuel. The first step is to obtain the correct setting from the makers of the vehicle. Usually, this is given in the number of degrees of advance permitted over the standard setting, or in degrees or inches before top dead centre.
Of the various methods of obtaining an accurate ignition setting, one of the best is by use of a lamp in the ignition circuit. To arrange this, a 6-v. or 12-v. bulb, whichever is applicable, should be wired in series between the coil and the distributor, the normal wire, of course, being disconnected.
With the distributor in the approximately correct position for firing on, say, No. 1 cylinder, the ignition switch "on," and the modified marking (which you will have made on the flywheel corresponding with the timing pointer), the distributor should be slowly rotated until the light goes out. This will indicate the precise point at which the contacts open, and the distributor should be locked in this position.
In general, an overhead-valve engine will permit a greater degree of advance when using premium fuel than will a side-valve unit. Most of the recommendations by manufacturers lie between 2 degrees and 5 degrees