Hi Chris,

Anywhere within the recommended range will provide adequate ignition potential.  Nevertheless, I suggest initial adjustment at the minimum-gap recommended, which will increase over time due to wear to the mid-gap and eventually to the max-gap.  That procedure minimizes maintenance since no adjustment is required until reaching the max-gap stage, except for cleaning lead and carbon accumulation and monitoring the gap.

According to magneto data published by manufacturers such as Vertex Magnetos, which I use in my 426 Chrysler Hemi powered water-jet ski boat, the spark plug gap can be set between .018 - .045 inch depending upon the engine involved, fuel used and the application operating environment.  The smaller gap improves starting, the larger gap favors performance.  Flying at high altitudes involves additional tradeoffs up to and including pressurizing the magneto.  Obviously two characteristics that are universal with magnetos;  the higher the RPM the stronger the current, and no battery is required for ignition.  Moreover, magneto discharge is instantaneous.  A 12 or 24 volt battery energized coil inherently needs time to build charge between each spark ignition event.  Electronic and conventional battery-coil ignition systems inherently produce less spark energy as RPM increases.  Aircraft magnetos typically feature an impulse coupling to increase current and reduce timing advance at starter cranking speeds until the engine starts.  Therefore, the magneto lower output at cranking speed has been mitigated.  I should also mention that most of our aircraft piston engines do not operate at extreme high RPM, as do racing boats and automobiles.  Therefore, the superior current capability of magnetos at high RPM may not be as important in aviation as in automotive applications.

The 426 hemi in my boat has 10.25:1 compression ratio, and I set the platinum plugs at .020 gap.  I manually time the magneto dynamically to maximize top speed.  Yet it starts quickly and reliably, and is very powerful easily igniting the fuel and air supplied from two 750 CFM 4-V carburetors.

For my Legacy, I plan to use one Bendix magneto plus one Slick electronic ignition combining the advantages of both types.  In addition, I am using a dual independent bus architecture, dual battery, dual alternator electrical system with the Vertical Power V-200 electronic circuit breaker system.

Fair skies and tailwinds,

Rod Pharis

N96RT