I have been doing a lot of re-tuning of the mixture which may account for some of the dark deposits, but I'm considering a change of plugs soon.  Just wondered what other recommendations might be out there regarding plugs, heat range, gaps.  Lynn; I recall you weighed in on this before, but I didn't find it in the archives.

 

Thanks,

 

Al

Paul Yaw is on the dyno every day, and has forgotten more about rotaries than I have learned.

So, whatever he says to use, is the plug to use.

 

Judging the plug's  (and engines) performance by looking at it after a full power run and a clean cut is still a viable way to recover data on mixture and heat range. However, at aviation RPM, there is not much need for a real (Lethal) ignition system. Just about anything will work. We had the two distributor 12As turning over 9,000 RPM with good reliability. So, for racing we use R6725-115 NGKs with the gap at .010". This is an ice cold plug in heat range for a 12A (smaller than a 13B) making about 250 HP.

 

So, why not use that?

 

There is no need or advantage. These plugs are $25.00 each. They are hard to find. You will not be turning RPM where lighting the mixture is becoming difficult. At even 7,000 RPM the engine is hardly running at all. They will foul quickly. Running car fuel gets you a very dark clean cut plug reading as in fluffy black more like oil fouled.  Most of that will be from just one second of idle power on car gas. Avgas works just fine for plug readings.

 

A clean cut is from cruise RPM to stopped at the same throttle setting and ignition off.

 

The guys looking through the eyepiece at the track are running racing fuel with lead in it. They are looking for cement boil around the center electrode. Flat black to no color on the shell face.

 

No rounding of the electrodes. A hint of tan on the side of the porcelain facing the intake valve. Good luck on that one. Fractured porcelain and so on. 

 

From 9,000 on up, a good CD or CD with multi spark is a must, both power wise and misfire wise.

 

So, a dark plug that has idled for just a second or so will look bad because the nose temp has dropped out of the operating temp range and collects phosphors from car gas or lead compounds fro avgas. it just looks bad. It will clean up when used hard again.

 

Lynn E. Hanover