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High CHTs and magneto timing

Hornet2008

Well Known Member
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.
 
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.
 
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.
 
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.
 
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
 
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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.
 
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.
 
So how close in timing do the two mags need to be? Mine are currently 1-1/2 degree different. Is that a problem? The engine is running fine.
 
So how close in timing do the two mags need to be? Mine are currently 1-1/2 degree different. Is that a problem? The engine is running fine.

no problem. I have one mag at 25* and an EI that ranges from 23 to 35* of advance, depending upon MAP. What you are looking to optimize or set is the total effective timing, which comes from the combination of the two sparks. You have two flame fronts and what we care about is where peak combustion pressure occurs relative to TDC.

Larry
 
Timing

Clearly the way the OP posted his question the mags timing is RETARDED, IE both mags are firing quite late. Late enough so the engine is likely low on power.
I have an 0 320 with 10-1 pistons. I was faced with breaking it in in very high ambient temperatures. One of the most respected engine builders in the US recommended mag timing of 22 degrees for the break in. This worked extremely well. It did not cause any significant loss of power.
Further there are some angle valve 360's that require the timing at 22 degrees.
Timing of 27 degrees will likely cause a higher CHT but is unlikely to cause any damage, especially on a stock engine.
PLLEASE do not bring EGT into this discussion. It is totally meaningless in this discussion.
The last check after resetting timing is to turn the prop backwards a few degrees past the timing mark and then slowly tap the prop forward until the timing light comes on.
 
no problem. I have one mag at 25* and an EI that ranges from 23 to 35* of advance, depending upon MAP. What you are looking to optimize or set is the total effective timing, which comes from the combination of the two sparks. You have two flame fronts and what we care about is where peak combustion pressure occurs relative to TDC.
Thanks, Larry. Tomorrow I'm going to try and get them both to 23 degrees, but won't obsess over it.
 
One of the most respected engine builders in the US recommended mag timing of 22 degrees for the break in. This worked extremely well. It did not cause any significant loss of power.

Great you bring this up. I had been asking myself this because we also have a new stock engine that is running very hot.

WHY don't we use this as a standard to break in period as we know that they will hotter than normal especially in the RV cowl? what is the drawback? Wouldn't this give us a bit of a margin to get through the initial break in?
 
Great you bring this up. I had been asking myself this because we also have a new stock engine that is running very hot.

WHY don't we use this as a standard to break in period as we know that they will hotter than normal especially in the RV cowl? what is the drawback? Wouldn't this give us a bit of a margin to get through the initial break in?

There is no drawback for the break-in application. You will see lower CHT and increased EGT. Do not be concerned about the EGT increase. It is just an artifact of slightly higher pressure at valve opening:

http://www.vansairforce.com/community/showpost.php?p=1201862&postcount=29
 
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