So would a smaller gap plug reduce or eliminate SAG (sparkplug attention getter)?  Would a CD or MSD system eliminate it altogether?  Can either work with Tracy’s EC-2?

Bill B 

Bill writes:

Al, something else to think about. If you run lean mixtures is you may want to run the widest plug gap you can easily fire. The reason is that when doing lean operation studies engine manufacturers have found that a strong spark and wide gap is best to reliably ignite lean mixtures

Lynn writes:

Fresh plugs gapped tight.

Hum-m-m; seems to be some difference of opinion.  My B9EGVs are gapped at about .025; or a few thousands over.

I did some adjusting yesterday on the EGT spread.  It clearly varies with rpm, and I suspect on my setup that is primarily due to air flow variations.  I had close to 100F spread at low power (2000 rpm) which decreased with increased power. Using modes 4 & 5 on the three rotor I adjusted mixture so the temps are within 20F at about 15.5" MAP (below staging). It required leaning 1 and 2 to get them close to #3.  At WOT on the ground I then find that rotor 2 is 50-60 hotter than the 1 and 3 which are relatively close.  I'll do some adjusting in the air at cruise conditions.

AL G

An easy test for secondary ignition system problems it to shorten the plug gap drastically and test again.

If the miss or stumble is cured then the cause is probably voltage related. If under power when a miss develops, reduce power slightly and if the miss is cured, it is secondary ignition voltage too low for the conditions at hand. As cylinder pressure is increased, the voltage required to fire a plug goes up. So reducing the throttle slightly reduces cylinder filling, and voltage required goes back down and the engine smoothes out again.

Likewise the short plug gap reduces the voltage required to fire the plug. The Kettering system can produce about 25,000 volts with electronic switching. In a piston engine this will operate fairly big plug gaps up to .060". Then you have to deal with the carbon automotive wires that cut into available energy, and to keep the big gaps from bleeding voltage, without firing, You need no wires, or wires with actual conductors in them. So coil on plug is one answer and inductive (Monel coiled) wires is another.

So that system works even when overlean mixtures are used because the mixture is being pushed toward the spark plug. Little relative motion occurs between the plug and compressing mixture.

In the rotary, the trailing plug is shielded in a little room. The mixture is forced through a hole into the room in an orderly fashion  and like the piston engine the plug has no difficulty lighting the mixture without regard for plug gap. Not the case for the leading plug where the plug hole is exposed to the passing mixture.

The smaller the plug gap with a Kettering system the more likely the plug will fire. The Kettering system is slow time wise. So a dirty plug will provide a path around the plug gap. Just draw in a resistor around the gap, then observe the gap as the plug fires. Pretty lame. A bit of a click and a barely visible blue arc.

Without the resistor, and nice crack and a bright blue arc. A dirty plug, be it carbon or lead salts or oil soot, can bleed off so much voltage before the Kettering can reach full voltage that the alternate path remains charged and consumes all of the energy and the plug gap never arcs at all.

The CD system or the MSD system operates the 12 volt coil at 450 volts or more.  This is quite a surprise for the coil that has fairly thick windings in the primary and has little resistance. So the coil charges to near full saturation in close to zero time, and the secondary voltage fires the plug gap on the way up to fully charged, and again on field collapse. The 50,000 volts will light anything. Fouled plugs? don't care. Ground electrode melted off last year? Don't care. Heat range too cold? Don't care. It lights them all and with a whopping amount of current in each strike. The CDs and MSDs use about one amp per thousand RPM.

So for a Kettering system in a piston engine, for good mileage use .050".

In a rotary shorter gaps provide insurance for better performance and a few more units of distance should the end of the alternator belt appear from under the cowl. How long has that been there? You ask yourself, afraid now to look at the volt meter. OOPs, its been there a while. So, when the engine will not take full throttle but runs OK just off full throttle, will we be thinking wide plug gaps, or short plug gaps?

In the race car, with two MSD-6als with 9,600 rev chips and the plugs (Ice cold NGK R6725-115) gapped at .010" I don't have ignition problems of any kind. Most racers run .020" but I can not detect any performance difference between the two settings. I use a stock electronic distributor with both leading and trailing MSDs triggered by the leading pickup. No mechanical advance. Timed at 27 degrees both. Inductive MSD wires.

Lynn E. Hanover