Mag check at altitude-EGT increase?

[Pmerems]

I decided to perform a mag check at altitude yesterday. I have an IO-360 with a CS prop. I was crusing at 8.5K a little ROP when I tested the mags.

The mags were fine but what I noticed was the EGT's were increasing. This was true with either the right or the left mag and I repleated this twice to confirm the reading. When I did switch mags I only let it run with one mag off for about 5 seconds. But the increase in EGT's was noticable.

I am trying to understand why the EGT's would increase with a less than efficient combustion.

Any ideas?

[kkmarshall]

EGT's will rise as less efficient combustion has more burning of fuel still going on when the exhaust valve opens.

Keith

[scard]

Nothing special here. You'll see it at runup as well.

[Pmerems]

My first thought was that less efficient combustion would lead to a richer mixture. Since I was already ROP I would have thought the EGT's would drop a bit.

[zilik]

Originally Posted by Pmerems

I
I am trying to understand why the EGT's would increase with a less than efficient combustion.

Any ideas?

You answered your own question. With one mag off the combustion is less efficient allowing the still burning mixture out the exhaust. This is perfectly normal.

[Aggie78]

Found here:

https://www.cirruspilots.org/cfs-fil...10_2D00_09.pdf

Great article on mag checks by Mike Busch.

Enjoy that mag check even more fully now!

[bhester]

That is one of the best indication when you lose a mag in flight, rapid rise in EGT.

[Pmerems]

Great info. Thanks for locating it.

Originally Posted by Aggie78

Found here:

https://www.cirruspilots.org/cfs-fil...10_2D00_09.pdf

Great article on mag checks by Mike Busch.

Enjoy that mag check even more fully now!

[lr172]

Ignition timing, in combination with other factors, determines where peak cylinder pressure occurs relative to crank rotation and EGT will change as this point moves. Optimum PCP positions will leave more heat in the cylinder (heat converts to work moving the piston) and less optimum will lose more of that heat to the exhaust (heat is wasted).

Timing in a twin plug engine is based on the combination of two spark events. When you shut down one mag, you are retarding your effective timing and therefore the position of peak cyl press. This causes your EGT to shift as a result.

Larry