P-Mag, Lightspeed and LASAR use a timing map or algorithm based on RPM and MAP. LASAR optionally allowed one to limit the advance based on overly high CHT for inadequately cooled engines (C172). 

 

LASAR determined that its timing data was proprietary and its map is unknown.  After 18 months of trying different LASAR brains (O320, IO320 and IO360), the system was rejected because at best power cruise the fixed magneto timing produced better results than whatever Unison determined the electronic map to be.  Perhaps it was my 9:1 CR.

 

For a 320/360 Lightspeed suggests setting the base timing at 20 DBTDC instead of the standard 25 if the CR is 8.7 or greater.   A rheostat optionally may be externally added to adjust the timing in flight.  P-Mag similarly has a setting that can retard the base timing from 25 to 20.

 

Note that Mark's old single Lightspeed is not the more modern Lightspeed Plasma III system where the spark duration was extended to ensure ignition at extreme F/A ratios.

 

Evaluation of spark strength is frequently described in joules with Slick magnetos far down the list - thus the small plug gapping and wee spark.

 

Remember that P-mags don't produce juice until around 700 RPM and there is no windup spring as with magnetos.  Working batteries are important with P-mags at start and with Lightspeed all the time.

 

Gary -- MAP (deck pressure) is an important component that can only be measured by tapping into the induction system.  Ambient pressure is unreliable unless taken from a static port. Cooling plenum pressure varies at the same altitude depending on attitude, cowl flap and speed.  Cockpit pressure is less as speed increases and changes depending on venting.  Induction systems enhanced by ram air need to inform the electronic ignition of the engine parameters as measured by MAP.  Any throttle reduction from WOT changes MAP to a value lower than "ambient."

 

Scott Krueger

NA IO320, 9:1 CR, ram air, ram air equalized injectors, Moped Iridium plugs, Dual Plasma III and base timing at 20.

 

Timing (DBTDC):

Idle and taxi at 700 MSL, about 34

Take off power at 1000 MSL, about 22

WOT 2500 RPM best power cruise between 5000 and 9000 MSL, about 25-27

High altitude and LOP operations produce greater advance.

 

 

Thanks Gary,

 

> I now have the Lightspeed set at 22 (can't retard it any more than that) and the mag at 20 and so far, so good.

 

Looks like Lightspeed is a mapped system (advance based deterministically on MP, rpm...)?  GAMI's (as yet unreleased) PRISM appears to be very feedback intensive by measuring peak cyl pressures (psi and relation to TDC).   A nice halfway point on the complexity spectrum is probably what LASAR did by measuring CHT and limiting advance when CHTs exceeded a pre-set limit.

 

Does Lightspeed have different mappings for different compression ratios?   One cool thing they do is to optionally display the spark advance.  Wonder if there would be a way for the pilot to set max-allowable advance?

 

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