High CHTs and magneto timing

[Hornet2008]

Hi All. I have an IO 320D1A with slick magnetos. Put a buzz box on the engine and found the right mag is timed at 2 teeth after 25 degrees and the left mag about 8 degrees after 25degrees on the flywheel. I have been experiencing elevated CHTs. My question is could this be causing high CHTs. The engine only has 10 hours but is using very little oil, about quarter inch in 2.5 hours. Temps run away in climb, especially #2(usually 20 degrees hotter). I plan to do the duct mod to the right inlet ramp while the lower cowl is off. Any help with the relationship of mag timing and elevated CHTs, if any, would be greatly appreciated. Thanks in advance.

[N941WR]

Just a fee degrees to much advance in your timing van have a significant impact on your CHT's.

If you are saying your timing is set at 27 and 33, then you will have very high CHT's and you should retime them before your next engine run.

If they are set with less advance (23 & 17 degrees), then you will have low CHT's.

[Hornet2008]

Many thanks Bill

[Jesse]

I had high CHT?s on my -6 back in the day and found that one mag was set at about 30-31 degrees. Set to 25 and the CHT?s dropped 25-30 degrees.

[krw5927]

Originally Posted by Hornet2008

Put a buzz box on the engine and found the right mag is timed at 2 teeth after 25 degrees and the left mag about 8 degrees after 25degrees on the flywheel.

From your description, it sounds like your timing may already be retarded, not advanced. The way I interpreted your note:

While rotating the propeller in the normal direction of travel for flight, you passed the 25-degree mark and then two teeth later the right mag buzzed.

Is that correct?

If so, the spark is happening closer to TDC than 25 degrees. This is retarded timing. Changing timing so that the buzz box makes noise before reaching the 25 degree mark in the normal direction of travel for flight would make the problem worse for you.

[lr172]

The further you advance timing toward the "optimum level, the more CHTs rise. It is an inverse relationship with EGT. A more advanced timing keeps more heat in the cylinder, which converts to force on the piston and less of the heat goes out the exhaust port.

Your primary concern with advanced timing should be detonation potential during high load scenarios. More advance = greater detonation potential.

Larry

[Hornet2008]

Could I just time them both for 25 degrees TDC? What I understand from the group is to advance timing, greater than 25 degrees will produce higher CHTs, and to retard it lower CHTs less than 25 degrees.

[JonJay]

Originally Posted by Hornet2008

Could I just time them both for 25 degrees TDC? What I understand from the group is to advance timing, greater than 25 degrees will produce higher CHTs, and to retard it lower CHTs less than 25 degrees.

Time them per your engines data plate for standard mags. It is most likely 25d BTDC.

Also, you might want to check that your timing marks are indeed correct. Many of us have had ring gears (fly wheels) that where not accurately marked. I have been "told" that a degree or two doesn't make any difference, but I have experienced it myself and it does.

[lr172]

Originally Posted by Hornet2008

Could I just time them both for 25 degrees TDC?

Yes, always best to follow the operating manual.

Larry

[Hornet2008]

Many thanks to the collective brains trust. Much appreciated