EGT and dead cut mag check

[pilot-lite]

Does anyone have an explanation as to why the EGT rises when you cut one of the mags on a piston engine?

Explaining the loss of power, resulting in a drop in rpm is easy, but my instructor has me baffled as to the reason for the EGT increase.

Any answer, especially one so devilishly complex he'll be left speechless, will be gratefully received!

pilot-lite

[HugMonster]

I may be misleading you, and this is from an awful long time ago, so don't take it as gospel...

But my understanding is that the mixture in the cylinders is burnt, but does not all ignite at the most desirable time (a bit before TDC) and is still burning as it's expelled by the piston rising again.

Thus you have burning gases instead of merely exhaust slung out the exhaust manifold, thus resulting in both decrease of power available and an increase in temperature at the EGT probe.

[Check-in]

Could it be that less rpm for the same throttle opening, and therefore the same amount of fuel, equals less efficient scavenging of the hot exhaust gases?

[Danish Pilot]

HugMonster: You are very right. The flamewall of the fuel travels at about 50 meter/sek, and the speed increases a bit when the rpm goes up, and slow down a bit when rpm goes down. When switching off one ignitionplug, the flamewall now starts at the other ignitionplug, and since each cylinder have two ignition plugs, they can not be centered. Therefore the flamewall have a longer distance to travel, which means that the time the flame is pressent is longer, therefore leaving shorter time for the exhaust to cool before it reaches the gauge on the manifold = rise in egt. (There are other factors)