Ignition Timing affecting RPM

[Robster]

I have just set my ignition timing on my Xflow to 14 BTDC as recommended, but have noticed that as I turned the dizzy back to retard the ignition, the revs rose very noticably, to the point that by the time it is retarded to 25...it is running twice the speed as the recommended setting. I have heard about this before but am confused as to why it happens. Surely if the engine revs higher at the same throttle opening with more retarded ignition, it is running more efficiently? I have set the ignition to the 14 degrees as I know that this is correct (the car is due at RK rolling road soon anyway) but if anyone has any theories as to the effect of ignition timing on rpm and why we choose the settings that we do, I would love to hear!

 

Thanks

 

Robster

 

 

[oilyhands]

By turning it you are *advancing* the ignition not retarding.. the timing at idle can usefully be increased to well over 20 degrees, but this is normally done by the vacuum advance retard mechanism. The static setting is correct at 14 degrees. The reason this happens is that the fuel/air mix at idle is very slow to burn since the cylinder is only part full , therefore the flame front hits the piston when most of its energy has been expended, by advancing the timing the flame front hits the piston earlier and gives it a bigger kick, hence the higher idle.

 

The problem with setting the 'static' timing to a higher figure than recommended is that the moment the throttle is opened the cylinder charge density increases and the fuel/air mix burns much faster and hits the piston on the way up towards TDC causing pinking. Therefore the timing at any one point (idle included) has to be set so that the engine will not pink when at full throttle.

 

The vacuum advance retard operates to add to the advance when manifold vacuum is high (closed throttle) and therefore cylinder filling low, the moment you step on the gas, the manifold vacuum drops and therefore so does the extra advance.. magic..

 

There is a fuller explanation here.

 

Without the vacuum advance retard the engine will be a lot less efficient at part throttle and therefore less economical.

 

Oily

 

 

Edited by - oilyhands on 17 Jan 2002 17:22:27