Under sea level, full throttle and rpm conditions, the Lightspeed fires at about the same ignition timing as the fixed mag ignition. As the aircraft climbs or as MP is manually reduced, the Lightspeed will advance its ignition timing and the mag will stay fixed. The Lightspeed system changes the timing based on manifold pressure and rpm. Most of the change is based on manifold pressure and only a few degrees of possible change is based on rpm. The lower the manifold pressure, in cruise, the more spark advance you get up to the max advance set into the Lightspeed, when doing the initial setup. The Lightspeed system fires at TDC for start then as the manifold pressure increases, the less advance it supplies. For engines with standard compression the spark fires, in the power range, anywhere from 25 BTDC to 42 BTDC. With the higher compression setup, it fires anywhere from 20BTDC to 37BTDC. The issue is to limit the max advance from 42 BTDC on lower compression engines to 37BTDC on higher compression engines. In most cruise power cases, the Lightspeed is firing before the mag. At sea level full power, the mag may fire before the Lightspeed depending on the way the engine was set up, but in most cases where the engines isn't getting full sea level manifold pressure, the Lightspeed will optimize the advance for the spark to occur before the mag. If you timed the mag to 20BTDC with the Lightspeed set to high compression spec, you could mostly assure the Lightspeed would fire first under most conditionsbuty at seasl level conditions you would have timing of 20BTDC. If you time the mag to 25BTDC for 9:1 pistons, you may get the mag to fire before the Lightspeed at close to sea level full power manifold pressure conditions. But at full throttle and RPM settings where the manifold pressure is less then what we would have at sea level, the Lightspeed will start to advance the timing towards before when the mag fires. We feel the compression increase the 9:1 pistons supplies, does not warrant retarding the sea level full power ignition timing below 25BTDC. So any time the Lightspeed wants to fire after 25 BTDC, the mag is firing first and the Lightspeed is cleaning things up, firing a few degrees later. This will optimize power output at close to sea level full power, as the engine will have 25BTDC timing for close to sea level conditions. But this will limit the max advance that the Lightspeed will attain, to 37 BTDC instead of 42BTDC. So we get 25BTDC timing for full sea level power but we limit max advance to 37BTDC.
That is the way we set up our engines. Others may do it differently and have a little less power at sea level conditions due to the retarded timing. It doesn't really matter that much one way or the other.... more of a preference thing. If you had significant compression increase, you might want to time the mag more retarded but at 9:1, I don?t think it is necessary.
Good Luck,
Mahlon
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