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High altitude starting/operating with PMAG

JDA_BTR

Well Known Member
I've got the IO-390 in my RV-14, with dual PMAGs. In Louisiana, I set them up with 0 start delay, -7 retard (to account for the different BTDC setting for this engine), and 24deg max retard. Timed at TDC as it should be with this -7 retard offset (because the PMAG already accounts for 20 degrees of it based on IO320/360 operations).

So now I operate on an airport with a base elevation of 6500 feet. I sometimes go to 4500 but mostly higher not lower. Has anyone any experience in changing the settings to accomodate higher airport operations?

I think once in the air it is same/same because the airplane doesn't care about the height above ground. Another way of thinking about my question is what setting makes the best start and takeoff scenario at 6500'. or 10000' (Leadville).
 
I think the question you are asking is, how does altitude affect the speed of the flame front in the cylinder, I don't know the answer! The other variable is mixture setting - assuming you are starting way rich but taking off leaned to some extent (to what extent?).

If the flame front speed is decreased due to a weaker mixture (and I don't know if that is true) you may need less retard to achieve best performance. Really needs input from someone who knows more about combustion in a cylinder than I do!
 
More questions about high alt operation, the Pmag thinks it's running at reduced power due to low manifold pressure so it will be advanced, not sure what effect this will have on "full power" TO.
 
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generally leaner mixtures require more advance and lower MAPs require more advance, though less so than leaner mixtures. There should be no need to add advance when starting at higher altitudes compared to lower altitudes. The extra advance is for optimizing output and won't meaningfully affect the ability to start. Most EI system have a sepearate advance setting (often configurable) for starting (under 300-400 RPMs) and won't follow the normal advance tables until above the starting RPM threshold. Have no idea if the Pmag follows this convention though.
 
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