P-mag or e-mag rpm drop on runup

I ground ran my p-mags for the first time last week. I was slightly surprised that I had a drop of over a 100rpm when I turned off each of in turn.

I would be very interested to hear from e- or p-mag users as to what is normal. (I have droped email to Bart, but no reply yet.)

It has also been explained to me by someone from the motor racing industry that with better dual ignition, but only one exhaust valve, that makes sense, but I would still like to hear the empirical experience of other users.

Thanks,

Every setup is different. What engine, compression, timing advance are you running when this "mag drop" is performed? As you know, the electronics can and do advance the timing depending on conditions.

I believe you have just determined what is "normal" for your setup under these conditions!

Hi Steve,

My dual Pmags do pretty much the same thing; usually have around a 100 RPM drop, but they're smooooth. My setup is an O-360-A1D, carbed with dual Pmags and NGK auto spark plugs. One nice thing I've noticed about this setup is that not once over 325 hours have I had the fouled plug rough runup that used to happen so often while flying rental Cherokees, 172s or Citabrias.

I've had my share of teething troubles and failures with my Pmags, but they've been sent back a few time for updates and fixes and I do believe they've found their happy place. And now, having jinxed myself by saying that, on my next flight the engine will probably fall off or something...

jbDC9 said:

Hi Steve,

My dual Pmags do pretty much the same thing; usually have around a 100 RPM drop, but they're smooooth. My setup is an O-360-A1D, carbed with dual Pmags and NGK auto spark plugs. One nice thing I've noticed about this setup is that not once over 325 hours have I had the fouled plug rough runup that used to happen so often while flying rental Cherokees, 172s or Citabrias.

I've had my share of teething troubles and failures with my Pmags, but they've been sent back a few time for updates and fixes and I do believe they've found their happy place. And now, having jinxed myself by saying that, on my next flight the engine will probably fall off or something...

Click to expand...

I usually get a 50-75 RPM drop with my dual P-mags, and they're also very smooth. IO-360 with REM37BY aviation plugs. No failures and almost no teething troubles, but multiple trips back to the factory for firmware updates.

About the same

Set up: ECI IO360- WW200RV prop, 8.5-1 pistons- Bendix Precision FI system, Auto plug harness- NGK plugs

Nothing fancy. Initially Emag on left, Pmag on right. Now 2 Pmags.

Since day one the Pmag (right side) has dropped 80-100 rpm, but smooth. Left side no drop even after switching for Pmag. I have switched out the right side Pmag and still had a drop. Did a resistance check on all plug wires with nothing obvious found

Brad provided me with new harness material and I replace all right side plug wires. Still dropped.

So, I left it alone. It has been consistent, but runs very smooth. I'm not messing with it!!!

John/Geoff/ Darwin, thank you for sharing your experience. What you say is really interesting to me, because it ranges from in my own case about a 125rpm (very smooth) drop at a 1500rpm run up on both, to zero on one of Darwin's. This has left me wondering what is going on and I offer these ideas in the hope of flushing out more knowledge. I also had a couple of other very thoughtful inputs from people who presumably did not want to contribute on the forum.

The suggestion is that with dual E or P-mags expect significant RPM drops on the ground at runup with rpm below approximately 1850(?) where the advance starts to cut in. This is because the ignition is retarded and incomplete combustion will occur in the cylinder before the exhaust valve opens. With both mags 'on' combustion will be improved since it starts at two different points at once, and therefore higher rpm will be achieved.

I had read in the past, though I cant find the reference, that in flight, the loss or turning off of an e/p mag will is barely noticable (unlike a normal mag). This makes sense, because, with the advanced ignition/higher rpm, combustion will be (more) complete before the exhaust valve opens, therefore little power lost. Further advancing the ignition will increase the combustion though with associated increase in CHT.

This leaves me wondering about your setup Darwin. What speed do you do runups? Could one ignition be in advance of the other? Could the jumper be connected one side and not the other, or if you did not use the 'blow in the tube method' they be timed differently? Sorry to ask about something that has clearly bothered you in the past that you have come to terms with.

For my part mag drop at 1500rpm is rather on the slow side, but it is a taildragger -4 and I have been advised that 1800 with chocks might pull it on its nose. This seems unlikely provided the stick is back. I have no experience in the air because I am awaiting paperwork for first flight. It will still be hard to know what happens because power loss will be disguised by a c/s prop. To date the engine has run for a total of 15 minutes to make sure the oil stays on the inside etc.

The setup is an O-320-D1A, dual P-mag. They were ganged together and set as one by the 'blow in tube' method.

Further info

I forgot to mention that I do the run up at 1700 RPM.

The current set up is manual timing on the right and blow timing on the left. Keep in mind that I have used all manual timing in the past, blow timing on both etc... It has not mattered, still get the drop.

I also have the jumper on both. I will try varying the RPM on run up and do a Mag check. I do a mag check in flight occasionally and find each one very smooth on their own with no significant difference in performance.

I do not feel it is anything worry about. I had the initial growing pains with the system as others have had but have not had any issues in nearly 200 hours now.

P-Mag check

FYI
Dual P-Mags, XP O-360, Blended airfoil constant speed, blow timing, latest version, 34 degree factory setting with no MP input by choice. Low compression cylinders. Straight 93 octane to straight 100LL and every variation in between. 210hrs and doing great.

My ignition checks are done at 1700rpm, are smooth, and drop approx 100 rpm on each.

Steve Barbour

PS Brad set a 25 timing at the factory for me so I could be cockpit selectable. The CHT will run cooler but at low MP settings, 10" and below, there is some slight roughness and hesitation. Plan on hooking up the ICAD and exploring to get the best of both worlds.

60 to 80 RPM for me

On my carureted O-290-D2 I see between a 60 and 80 RPM drop on the Dynon display when checking my P-mags at 1400 RPM.

Being a light trail dragger I don't like it when the tail comes up on run up, thus the low run up RPM.

BTW, I have a 114 P-mag for sale with all of eight hours on it (I don't need three of them, don't ask). I'll let it go for $100 off of list and Brad said he will transfer the warranty.

If interested, send me an email.