andrew phillips

andrew phillips

Well Known Member
I have one electronic ign and 1 slick mag. Did the mag timing after annual and removal of mag for gasket replacement. Not fun the first time but after 10 or so tries I got it right...I think.

Question, I have never run on the mag only while in flight. It controls the lower plugs. Today I thought I would give it a try. I have the timing set at about 24 1/2 BTDC. 25 is spec. When I switched to the mag the rpm held strong but the EGTs when up quite high. I didn't let them go to see where they stopped. Is this normal? Do I have advance or retard the timing?
 
I have two mags, anyway I think it?s normal, I get the same thing when I do in-flight mag checks the EGT climbs fast so I get it back to both in short order, just long enough to see that both mags are running good.
 
Andrew, I am no expert but I am pretty sure it works like this.

When you run on two mags ignition starts in two different places in the cylinder. When you run on just one the flame front has further to go to complete the burn, and before it does the exhaust valve opens. As a result you now have flame not hot exhaust going down the pipe. (I know its hard to imagine this all going on at 40 times a second!) The flame is hotter than the exhaust so you see the EGT shoot up.

I have two P-mags and it was interesting to see when I advanced them both by 5 deg. the EGT dropped - the combustion was more complete. Conversley, the CHT rose very slightly because the work was being done in the cylinder, not the exhaust pipe.

So the bottom line, you are seeing what you should expect.
 
That's right, the dual plugs are there because Avgas burns so slow and the piston is so big you need to start on either side and meet in the middle to complete the combustion event in time. With a single plug, you have effectively doubled the distance that the flame front has to travel... Just doesn't have enough time to burn completely, and it becomes an "external" combustion engine.

The "safety" aspect of dual magneto ignition is simply a byproduct.
 
Steve Sampson said:
(I know its hard to imagine this all going on at 40 times a second!)
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That's because at 2700 rpm, it is going on ONLY 22.5 times per second.....:p

So then if you had to run just one mag, would it be best to reduce power to say 55% to reduce the egt? At 2100 rpm it would happen 17.5 times per second and might be enough time to burn more of the charge before the exhaust valve opens................
 
andrew phillips said:
I have never run on the mag only while in flight. It controls the lower plugs.
Click to expand...

Did you set this up on purpose? Every aircraft I have worked on each mag will control the top plugs on one side and the bottom plugs on the other side of the engine. The other mag does the opposite. I don't know if this would make a difference but that is how certified aircraft run them.
 
MNAv8or said:
Did you set this up on purpose? Every aircraft I have worked on each mag will control the top plugs on one side and the bottom plugs on the other side of the engine. The other mag does the opposite. I don't know if this would make a difference but that is how certified aircraft run them.
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He is using electronic ignition on the top plugs. That is standard to use the mag on the bottom plugs............
 
gasman said:
So then if you had to run just one mag, would it be best to reduce power to say 55% to reduce the egt? At 2100 rpm it would happen 17.5 times per second and might be enough time to burn more of the charge before the exhaust valve opens................
Click to expand...

No need to reduce power setting on 1 mag...the reduction in effective timing is already reducing the output power. The increase in EGT due to single mag operation won't hurt anything, at least in non-turbocharged applications where TIT is not an issue. Note that in the case of operating on a single mag, even though you'll see an increase in EGT over dual mag operation, the CHT will actually be lower due to the reduction of internal cylinder pressures...and this is what we are really interested in managing anyway.

Skylor
RV-8QB
Under Construction (Finishing Stuff)
 
Why not run the EI on the bottom plugs where the hotter spark and plugs will have less or no likelihood of fouling? Which is typical of the bottom plugs.

Glenn Wilkinson
 
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