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RPM drop expected with Electronic Ignition

flightlogic

Well Known Member
Patron
I currently run a P mag on the left and Lightspeed on the right. Carb. equipped O-320. During run up, I get 200 rpm drop with the P mag. I get almost no change when on the Lightspeed. I have spare P mag that is getting upgraded to 114 from 113 at E Mag. My thought is to install it and having matching systems. Over time, the Lightspeed runs OK, but is "fiddly". I have to periodically pull the cowl and clean the pins on the D sub at the back of the engine. That and the fact that I could clean up all the extra components. Coils, main box on the floor of the RV etc.
But, the mag drop or lack of..... seems to indicate it makes a hotter spark than the P mag. Should I keep it?
Any observations and opinions welcome. Cheers.
 
Two different kinds of ignition, one CDI, one inductive. And the Lightspeed CDI may be multispark, IIRC. Both electronic, but very different.

I currently run a P mag on the left and Lightspeed on the right. Carb. equipped O-320. During run up, I get 200 rpm drop with the P mag. I get almost no change when on the Lightspeed. I have spare P mag that is getting upgraded to 114 from 113 at E Mag. My thought is to install it and having matching systems. Over time, the Lightspeed runs OK, but is "fiddly". I have to periodically pull the cowl and clean the pins on the D sub at the back of the engine. That and the fact that I could clean up all the extra components. Coils, main box on the floor of the RV etc.
But, the mag drop or lack of..... seems to indicate it makes a hotter spark than the P mag. Should I keep it?
Any observations and opinions welcome. Cheers.
 
Thanks Dan. Have read both manuals... but at separate times. That fundamental difference is one that escaped me. I don't tend to run lean of peak and have accepted the economy of fuel burn on the rich side. I suppose it comes down to complexity of hardware. In general, I prefer simple. I will probably list the Lightspeed for sale when I get the second P mag back from conversion. Appreciate your time.
 
The RPM may or may not drop when doing an ignition test with dissimilar ignitions, like you have.

It all depends on which one is firing first at your test RPM.

Let's say the LS fires at 27*BTC and the P-mag is firing at 25*BTC. If you ground the P-mag, the RPM probably won't change because the LS is igniting the fuel air mixture before the P-mag and all the P-mag is burning is smoke.

However, if you ground the LS, you will see an RPM difference because now the fuel air mixture is being ignited at a different prop angle than the LS.

All that test is doing is confirming you have two working ignitions.
 
Over time, the Lightspeed runs OK, but is "fiddly". I have to periodically pull the cowl and clean the pins on the D sub at the back of the engine.

I have no position regarding which ignition is best.

However it sounds like your Lightspeed may be installed incorrectly? There should not be an extra D-sub connector installed at the firewall. From the manual:

"Drill a 7/8?-1? hole through the firewall to feed the 15-pin connector to the Plasma CDI location. Reseal and fireproof the penetration of the firewall using established practices. Do not modify the sensor wires, they are assembled and tested professionally at LSE!."

My Plasma II has never missed a beat in nearly 1200 hours of operation.
 
good thoughts

Bill, burning smoke. I like the way you put that! Had not thought of those realities.
Alan, that cable has a D sub at each end. On the Hall effect sensor... mine needs periodic spraying with contact cleaner. That means pulling the top cowl... or stretching my arm through the oil fill door. I can do it.... but it isn't fun.
 
I don't know the LS timing curve but it is possible that the LS and P-mag swap "lead" a few times in flight.

Not a big deal but the reality of using different types of EI ignitions.

If you had one fixed time mag, than the EI would probably lead all the time, except when at idle with a P-mag. With low RPM and low MAP, the P-mag moves the timing up to 19.6°. That is why people are able to get exceptionally low idle speeds with P-mag's (and some other electronic ignitions). My idle RPM with a two bladed Catto prop is in the high 500"s to low 600's, depending on my engine temperature.
 
I had 1 light speed and one slick Mag. I never saw a drop with the light speed. This year I installed two new P-mags and i get 90 rpm with one of them and 135 with the other! it is a very smooth drop not like a fouled plug. I am using BR8ES spark plugs set at .033. Not sure why theses darn P mags drop so much. I have been less than impressed with the P-mags so far!!!
 
On the Hall effect sensor... mine needs periodic spraying with contact cleaner. That means pulling the top cowl... or stretching my arm through the oil fill door. I can do it.... but it isn't fun.

Oops, didn't realize you had the hall-effect sensor. These do seem to cause more than their share of problems (a friend had one fail completely). I've got a crank sensor, which has the advantage of no moving parts and no connectors or electronics forward of the firewall. Its been far more reliable than the mag it was originally paired with.
 
I had 1 light speed and one slick Mag. I never saw a drop with the light speed. This year I installed two new P-mags and i get 90 rpm with one of them and 135 with the other! it is a very smooth drop not like a fouled plug. I am using BR8ES spark plugs set at .033. Not sure why theses darn P mags drop so much. I have been less than impressed with the P-mags so far!!!

Tim, see my explanation above regarding the lack of a mag drop on your Light Speed ignition.

As for the mag drop on your P-mags, I am not sure but it could be plug wire issue. We need to dig into that, maybe tomorrow, if the weather allows us to go flying.
 
I have dual P-mags on my IO-360 and I can change the amount of RPM drop by aggressive leaning. It seems to drop less the closer the EGTs are to the high 1200s. Dan fro Reno
 
Dan, good observation. I will try some comparative lean settings. I live at 5000 feet and so taxi out quite lean. Maybe my habits are having an effect. Headed for Burbank this week. Will try some full rich, sea level tests as well.
 
Just as another point of reference, I have an O-360 with two P-Mags that is timed at 2 deg ATC instead of TC and using A curve. I get a 20 rpm drop per P-Mag at 1700 rpm, and I also watch for an EGT rise when doing the mag check.
 
When quoting rpm drop, it may be useful to state if each mag is firing two top plugs and two bottom aka with a traditional mag, or one P-mag firing the tops and the other firing the bottoms as I have.

This arrangement is the cause of more of a drop on one p-mag over the other (I believe) but the engine runs smoother when on a single ignition this way (in theory, and mine is very smooth) as if I was to need to run for any distance on a single mag to get to down to an airport.

Bevan
 
Christer, are you full rich at the time of runup? And what is your home field elevation?
Thanks to all Van's contributors who have offered their experiences!
 
But, the mag drop or lack of..... seems to indicate it makes a hotter spark than the P mag. Should I keep it?
Any observations and opinions welcome. Cheers.

The mag drop difference between the various systems (assuming plugs and wires are operating correctly), is more a function of the different advance curves between the two systems, not the ?hotter spark?. I believe the Pmag incorporates rpm in the curve, LSE does not and you will see it fully advanced at low manifold pressure like at idle or partial throttle...like when doing a mag check.
There is nothing wrong or weak with a magneto spark, the big difference people see when comparing on a mag check is the timing Delta.
Tim Andres
 
learning

I watched the whole webinar recently on the topic of timing. When I look at my 1950's O-320 I always thought it could not be any simpler. The actual physics of firing off that compressed mixture is fascinating. But, that is pleasing to me. I like learning something new everyday. This thread has not let me down.
 
Identical Installation

I have an ignition setup idential to the OP: PMag on Left, Lightspeed Plasma III on the right (Hall Effect) on an IO-360 A1B6 Angle Valve. The LSE fires the top plugs, the PMAG fires the lower plugs. I get 20-40 rpm drop during mag check on each ignition at 1800 rpm at sea level, full rich. I would suggest there is nothing inherent about having an installation with these 2 ignitions which would cause one ignition to behave so much worse during a run-up ignition check.
 
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Just as another point of reference, I have an O-360 with two P-Mags that is timed at 2 deg ATC instead of TC and using A curve. I get a 20 rpm drop per P-Mag at 1700 rpm, and I also watch for an EGT rise when doing the mag check.
Not needed for starting, if you have version 40 or higher.
In Tim's case, we shifted the curve with the free EICAD program from Emag a -1.4°.
Give me a call, let's talk about your timing.

When quoting rpm drop, it may be useful to state if each mag is firing two top plugs and two bottom aka with a traditional mag, or one P-mag firing the tops and the other firing the bottoms as I have.

This arrangement is the cause of more of a drop on one p-mag over the other (I believe) but the engine runs smoother when on a single ignition this way (in theory, and mine is very smooth) as if I was to need to run for any distance on a single mag to get to down to an airport.

Bevan
And add the RPM used for the mag check.
 
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