nigelspeedy
Well Known Member
As part of my recent ignition timing experiments I looked at how ignition advance affects idle quality. I had some advice that reducing advance at idle would make it smoother. Well lets see.
For this test I started and taxied to the run up area and did my normal run up. For this I have the mixture full rich and prop full forward and then advance the throttle until I have 1900 RPM, then I lean the mixture until the RPM rises about 50 RPM, so 1950 RPM total, this is where I do the mag check and cycle the prop. Once I had this mixture position set I did not make any change to it for the whole test.
I used the advance shift function of the EICommander to adjust the timing. The first test point was with zero shift and this resulted in 23.8 deg advance at around 670 RPM with the throttle fully back with the engine warmed up.
I then let the engine idle for 1 minute and recorded the RPM at 16 Hz. I followed this with a run up using the throttle until I had 1950 RPM and did a mag check on each timing setting. I repeated this exercise with -1.4, -2.8, -4.2 deg shift which resulted in 21.0 deg, 19.6 deg & 18.2 deg advance respectively.
The results are shown in the graphs below. As I retarded timing from the baseline of 23.8 deg the idle quality decreased as measured by the increase in spread between the min and max RPM over 1 minute and the increased standard deviation. The RPM drop during a Mag Cx also increased as the advance was decreased.
Anyone have data to support a 'less ignition advance at idle = better idle quality' argument?
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Cheers
Nige
For this test I started and taxied to the run up area and did my normal run up. For this I have the mixture full rich and prop full forward and then advance the throttle until I have 1900 RPM, then I lean the mixture until the RPM rises about 50 RPM, so 1950 RPM total, this is where I do the mag check and cycle the prop. Once I had this mixture position set I did not make any change to it for the whole test.
I used the advance shift function of the EICommander to adjust the timing. The first test point was with zero shift and this resulted in 23.8 deg advance at around 670 RPM with the throttle fully back with the engine warmed up.
I then let the engine idle for 1 minute and recorded the RPM at 16 Hz. I followed this with a run up using the throttle until I had 1950 RPM and did a mag check on each timing setting. I repeated this exercise with -1.4, -2.8, -4.2 deg shift which resulted in 21.0 deg, 19.6 deg & 18.2 deg advance respectively.
The results are shown in the graphs below. As I retarded timing from the baseline of 23.8 deg the idle quality decreased as measured by the increase in spread between the min and max RPM over 1 minute and the increased standard deviation. The RPM drop during a Mag Cx also increased as the advance was decreased.
Anyone have data to support a 'less ignition advance at idle = better idle quality' argument?
Cheers
Nige