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RPM drop mags vs EI
- Thread starter Dorfie
- Start date
MiserBird
Well Known Member
I see about a 50 RPM drop on both the PMag and the Sure*Fly.
D Weisgerber
Well Known Member
At least on the P Mag the plug gap is bigger and it has a hotter spark. This burns fuel more efficiently.
Lately I?ve been flying a certified aircraft that comes from the factory with one magneto and one LSE Plasma ignition. The POH specifies a maximum RPM drop during the ignition check of 100 RPM when running on the EI (magneto off) but allows up to a 300 RPM drop when running on the magneto (EI off). Pretty clear indication that they know which one is doing most of the work!
a mag drop test is designed to test two things. First, presence of a failed mag/ignition system or a failed plug and second for improper timing. Assuming no failed plugs, the amount of EGT increase or RPM drop is based upon the amount of timing that is removed during the test. ignition timing is a composite of the timing from the two discrete firing events. If both ignitions have the same timing, the drop should be the same on both. If they are different, you will see differences in drop on each. THe amount of drop is directly related to the change in total timing (which is a composite when on both ignitions) on both ignitions versus just one.
As you get closer to optimal timing, things change a bit, as too high of a composite timing keeps the RPM less than peak and dropping one ignition will result in a smaller drop, as the reduced timing first increases RPM, approaching optimum, then reduces as it goes below optimum.
Larry
As you get closer to optimal timing, things change a bit, as too high of a composite timing keeps the RPM less than peak and dropping one ignition will result in a smaller drop, as the reduced timing first increases RPM, approaching optimum, then reduces as it goes below optimum.
Larry
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So if you have two of the same engines, one fitted with dual standard magnetos and the other with dual EI, both engines with timing fixed at 25 degrees BTDC, will the EI still have much less RPM drop than the engine with magnetos?
If that is true, then is the difference in RPM drop solely due to the "heat" or "energy" of the spark? I am keen to understand how the spark energy affects the rate of combustion. (It would seem that once the mixture is lit the rate of burn should be the same). Is there less increase in EGT switching from one to the other EI compared to switching between the magnetos?
How do fine wire aviation spark plugs compare to massive electrode aviation spark plugs if both are lit by a magneto?
I am asking out of ignorance hoping for better understanding.
Thanks.
Johan
If that is true, then is the difference in RPM drop solely due to the "heat" or "energy" of the spark? I am keen to understand how the spark energy affects the rate of combustion. (It would seem that once the mixture is lit the rate of burn should be the same). Is there less increase in EGT switching from one to the other EI compared to switching between the magnetos?
How do fine wire aviation spark plugs compare to massive electrode aviation spark plugs if both are lit by a magneto?
I am asking out of ignorance hoping for better understanding.
Thanks.
Johan
alpinelakespilot2000
Well Known Member
I only see a 30-50 rpm decrease on each of my slick mags and this has been the case for all 500+ hours I have on them, so I?m not sure that it?s always true that electronic ignitions will have lower drops. Perhaps it?s more a function of how clean the plugs are which may or may not always be a function of the type of ignition?
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RV10inOz
Well Known Member
Lets clear this question up first.
IF the spark timing is the same, then as lr72 has said it should be the same. But you are making an assumption that the actual spark timing is fixed and the same. I only know of one aero engine dyno that you can watch this on, and when yo do you come to appreciate the variability that exists.
With an EI that triggers electronically the plug will fire pretty darn consistently every combustion even (10-20/sec) compared to a magneto which wanders around a bit.
Also there is lag in a magneto. So if you statically time bth mags at say 25 degrees, the EI fires at close to 25 degrees every time but you Slick Mag will fire the plug somewhere around 23 degrees.
Lycoming Chieftain engines with the dual mags set at 20 degrees statically fire nowhere near the same as the seperate slick mags (earlier) and they wondered why they had much higher CHT's. This was discovered on the GAMI dyno a decade or two back.....nobdoy knew.
So what is a degree or two between friends? A fair bit apparently
Hope thats helpful.
I have a bendix mag and Electoair ignition. When I turn off the mag, I haven't had any RPM drop since I've owned he plane. Mag recently went out in flight so I had it rebuilt. Rpm drop when I turn off the electroair, is between 30-50. Saw higher on the mag before but was within limits. RPM drop on the electroair is 30-40 Max. So potentially there is a very small chance the mag could perform better than the EI, but I haven't seen it. A bud has the same set up and he has the same results.
