When I adjusted the timing by the apex seal in the spark plug hole method, I was getting spark plugs with the appearance of overheating and significant erosion of the center electrode along with SAG after 15 hours or less of operation on stock spark plugs and mostly unleaded fuel. 

This lead me to verify the ignition timing and I found that I had been operating at 35 degrees BTDC at 5200 RPM and 23” MAP with the default controller timing setting and Tracy’s recommended CAS position.  After setting the timing using the minimum chamber volume method,  the spark plug life has increased along with the time it takes SAG to develop.  I did not notice any difference in power or fuel consumption as a result of the change.  I may have been fortunate that at my location, the density altitude is nearly always in excess of 7000 ft and MAP is limited to 23.5” since the engine is normally aspirated. 

These are just my observations and I am not contending that they are of any consequence.

Steve Boese   

First let me point out the big error in the procedure I wrote:

-The center of the combustion chamber is 60^o on the rotor from the tip seal; or 120^o (should read 180) on the E-shaft.

- Count the total number of teeth on the ring gear, and divide by 3 (should read 2). Now rotate the engine by that number of teeth (either direction) and rotor 1 will be very close to TDC.

I wouldn’t argue the precision of the method; but for the housings you show, the offset appears to be much less than 10 degrees. The e-shaft TDC offset would then be 1 ½ times that as you split the difference between the plugs and multiply by 3:1 ratio. And being a few degrees off on the rotary is not of much consequence.

Ultimately the timing at which we operate is determined by the timing curve built in to the engine controller.  I’m pretty sure Tracy has modified that some over the years.  And then there in-flight tuning of the timing at cruise condition that you may like to look at.

When I got my 20B with a 12A front housing from Atkins, the relationship of the pointer to the marks on the pulley made no sense. I used my simple method to determine a TDC mark, and worked from there.  On the dyno we put a pressure transducer into a trailing plug hole and adjusted timing a bit while operating (I don’t recall conditions – I think 22” MAP) to get the pressure trace to look right. We chose a point 2 steps retarded on the EC2, - 3.75^o.

Subsequently I did some variation at cruise conditions to see effect on performance, and again ended up at two steps of retarding, although I found performance change almost undetectable over a 3-4 step range (~7^o) to either side. So that setting is pretty consistent with Lynn’s data that the method leads to a TDC that was a bit BTDC; but is of no consequence.

That’s my story, and I’ sticking to itJ.

Al