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12-28-2021, 09:53 PM
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Join Date: Aug 2020
Location: Los Alamos, NM
Posts: 31
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Help understand in-flight mag check result
Hello all, I recently had both of my slick mags taken off and re-installed for the 500-hour inspection. I was suggested to be more vigilant about the mag checks at least in the first 10 hours. During my first flight after the mag installation, I did both the ground check and in-flight LOP check, and I don't really understand the results. For reference I have an O-360-A1A engine, with slick mags on both sides (left mag has impulse coupling).
The ground check was pretty good. The test was done at 1700 RPM, and both mags showed 50-60 RPM drop. During the in-flight test, although the engine seems to run fairly smooth with either the left or the right mag, the power reduction and the EGT rise are much more pronounced when the right mag is turned off. By power reduction, I mean that I could feel very strong deceleration when the right mag was turned off. Below are the EGT numbers I wrote down during the test.
Please take a look and let me know if it's something worth worrying about, and/or if there are some further tests I should be doing. Thanks!
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12-28-2021, 10:41 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 8,174
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IMHO there is nothing terribly wrong here, although the numbers suggest the right mag is firing slightly later than the left one. I’d check the timing one more time.
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12-28-2021, 10:55 PM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 16,318
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Quote:
Originally Posted by liuk
the power reduction and the EGT rise are much more pronounced when the right mag is turned off.
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Methinks it is the left that is a bit retarded.
Late firing means the mixture is still burning as it is going out the exhaust, and thus higher egt readings.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
Last edited by Mike S : 12-28-2021 at 11:00 PM.
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12-28-2021, 11:11 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 8,174
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Opps, I agree with Mike, if you really mean ‘right off’. I mis-read the column label as the usual ‘right only’.
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12-28-2021, 11:16 PM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 16,318
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Quote:
Originally Posted by BobTurner
Opps, I agree with Mike,
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Thanks Bob, your check will be in the mail 
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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12-29-2021, 05:07 AM
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Join Date: Nov 2017
Location: Eatonton Georgia
Posts: 694
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Ideal
I see nothing wrong with your numbers posted, your LOP numbers.
I sorta doubt any fine tuning will make them more alike, and there is really nothing noticeable you'll gain if they were. Fly on !
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Butch
RV6A Purchased N72TX
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12-29-2021, 07:08 AM
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Join Date: Oct 2005
Location: 08A
Posts: 10,714
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Quote:
Originally Posted by liuk
The ground check was pretty good. The test was done at 1700 RPM, and both mags showed 50-60 RPM drop. During the in-flight test, although the engine seems to run fairly smooth with either the left or the right mag, the power reduction and the EGT rise are much more pronounced when the right mag is turned off. By power reduction, I mean that I could feel very strong deceleration when the right mag was turned off. Below are the EGT numbers I wrote down during the test.
Please take a look and let me know if it's something worth worrying about, and/or if there are some further tests I should be doing. Thanks!
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As others have noted, the left mag may be a bit retarded. The overall higher EGT rise for #3 (greater increase when running on either mag) may be due to mixture distribution issues typical for a carbed engine, or possibly an intake gasket leak.
Can't expect the same results at 1700 RPM and some higher inflight RPM. Mixture distribution is likely different.
Quote:
Originally Posted by Mike S
Late firing means the mixture is still burning as it is going out the exhaust, and thus higher egt readings.
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Late firing means cylinder pressure is higher at exhaust valve opening. Pressure and temperature are proportional when temperature is expressed in degrees Kelvin.
Although there are ongoing reactions, 100% mass fraction burned is reached well before exhaust valve opening. A good reference like Haywood's Internal Combustion Engine Fundamentals has typical burning duration at 40 to 60 crank degrees, starting at ignition BTDC. The exhaust valve doesn't open until roughly 135 ATDC.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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12-29-2021, 09:40 AM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,842
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In addition to what everyone said, it would be good to know how are your sparkplugs connected? Does one mag fire all the top and the other fire all the bottom? If that is the case, that might explain the difference.
__________________
Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - SOLD
N258SM RV14A - IO390EXP119 - Out of Paint shop & flying
Dues paid
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12-29-2021, 11:35 AM
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Join Date: Aug 2020
Location: Los Alamos, NM
Posts: 31
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Quote:
Originally Posted by Bavafa
In addition to what everyone said, it would be good to know how are your sparkplugs connected? Does one mag fire all the top and the other fire all the bottom? If that is the case, that might explain the difference.
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Yes. The left mag fires all the top plugs and right mag fires all the bottom ones.
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12-29-2021, 11:40 AM
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Join Date: Aug 2020
Location: Los Alamos, NM
Posts: 31
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Thanks!
Thank you all very much for the fantastic explanation!
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