Chris,

 

The answer does change with an EI. 

 

Assuming a naturally aspirated aircraft engine using 100 LL.  Mags only produce a limited amount of energy (in joules) most of the time.  The gap is dependent on the energy, compression ratio and transmission efficiency (distributor points, health of ignition leads, etc.).  Iridium plugs (fine wire) are a consideration because they don't lose electrode material that causes a change in the gap.  Platinum is next best but, forgetaboutit.

 

Even more interesting is the resistance in the plug.  Just ask Lorn.  Champion plugs have a screw in the plug where the spring on the "cigarette" makes contact.  That screw holds down a spring loaded slug of carbon used to ideally provide a few thousand ohms of resistance.  There is a problem when these carbon slugs degrade by increasing the resistance that leads to misfire, a weak spark and potential damage to the mags.

 

Check out Tempest plugs ( http://www.aircraftspruce.com/catalog/eppages/temptestplugs.php ) as they have the resistance buried in the ceramic (Gee, just like the moped Iridium plugs I use with my EI).

 

Gap too small = not a good fire starter, can lead to fouling.....

Gap too large = misfire if the energy isn't great enough to span the gap.

 

Scott Krueger

 

I have a question for all the engine experts on the list. 

What are the pros and cons to wider vs. smaller gap in our spark plugs.  The given range by Champion etc. must balance several parameters.  I am trying to get a feel for why one would choose to be on one end of the range over the other.  Also. what might be expected outside the specified range, too small or too large.

I am assuming magnetos here.  I imagine the answer might change with EI.

thanks,

Chris Zavatson