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  #1  
Old 03-19-2017, 04:30 PM
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N941WR N941WR is offline
 
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Default How does the P-mag respond with a MAP line leak?

On another thread, someone asked how the P-mag will respond with a leak in the MAP line.

This morning I unplugged the MAP line from one P-mag and plugged the system so the right P-mag was operating off of a closed system while the right one remained open to the atmosphere.

All tests were performed with a Fixed Pitch prop. The higher RPM generated with a Constant Speed prop on takeoff or lower RPM in cruise may cause the P-mags to react differently.

What I found was that on startup and initial climb out both P-mags tracked each other exactly.

21, 22.4, 23.8, etc. degrees at low RPM's and MAP.

During the Run up, both the left and right P-mag's stayed at the low end of the setting. (25.2* BTC in my configuration.)

During the takeoff roll and initial climb (My airport elevation is 1064 MSL) both ignitions stayed at 25.2 up to about 4,000'. At that point, the right P-mag started to advance and the open left P-mag stayed right at 25.2 degrees.

The left P-mag's internal MAP sensor started to recognize the reduced atmosphere pressure and started letting the disconnected P-mag start to advance.

As I continued my climb, the open P-mag eventually hit the Max advance limit but it did so after the P-mag that was connected to my MAP system.

What I deducted from this was that should you have a failure in your MAP system, the P-mag will not allow your timing to advance to the point where it will harm your ignition. However, every installation is different and I could very well be mistaken.

(BTW, the EICommander was going crazy because it recognized the timing divergence in my configuration. Normally this wouldn't happen because a leak in the MAP system would impact both P-mag's.)
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Last edited by N941WR : 03-19-2017 at 07:29 PM.
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  #2  
Old 03-19-2017, 06:42 PM
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catmandu catmandu is offline
 
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Thanks for experimenting, Bill. Good info to know.

Quote:
Originally Posted by N941WR View Post
(BTW, the EICommander was going crazy because it recognized the timing divergence in my configuration. Normally this wouldn't happen because a leak in the MAP system would impact both P-mag's.)
For whatever reason, a very animated visual of a troll-like thing under an RV cowl popped into my brain when I read that, made me chuckle.
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  #3  
Old 03-20-2017, 06:54 AM
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I have had a broken vacuum tube on my system and I did not notice a performance difference until I had the cowl off the next time. YMMV
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  #4  
Old 03-20-2017, 08:10 AM
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At WOT the timing advance should be almost identical (difference would be due to ram air affect on the true MAP versus warm more-still air on the broken line). When you start to pull the throttle, the disconnected Pmag will see a higher MAP than the engine is getting, keeping the timing retarded based on that MAP.
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  #5  
Old 03-20-2017, 11:21 AM
guccidude1 guccidude1 is offline
 
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Default How does the P-mag respond with a MAP line leak?

I was told by Brad, at P-Mag, that if there is no manifold pressure sensed, the P-Mag reverts to 25 degrees max advance, accodrding to my bestest rememory. Dan from Reno

Last edited by guccidude1 : 03-21-2017 at 08:16 AM.
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  #6  
Old 03-20-2017, 11:44 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by guccidude1 View Post
I was told by Greg, at P-Mag, that if there is no manifold pressure sensed, the P-Mag reverts to 25 degrees max advance, accodrding to my bestest rememory. Dan from Reno
Yes and no. It reverts to your minimum setting down low. Eventually, as you climb, the P-mag senses the drop in pressure and will let it advance. Eventually, it will hit the Max Advance limit for your configuration.

If you have the jumper in, which I do not, your advance will go to the lower limit. The jumper in, A curve (I prefer "configuration") starts 26 degrees and some change and will advance to 33+ degrees as you climb. Nothing I saw will jeopardize your engine, which is good news.

Because I have an EICommander installed, my ignitions always run on the configuration in the B memory location. I have my P-mags configured to mimic the A configuration less 1.4° on both the starting timing and the max advance.

The no-jumper B-curve should not be used in a standard (or higher) compression parallel valve engine. If you have an angle valve engine that requires 20° timing, you need to customize the B Configuration to match your engine by entering a negative offset and a reduced Max Advance.
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Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html

Last edited by N941WR : 03-20-2017 at 11:51 AM.
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  #7  
Old 03-20-2017, 11:45 AM
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Jesse Jesse is offline
 
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With a broken line it's sensing manifold pressure, just not the engine manifold pressure. It's sensing ambient pressure which is around 30 at sea level, about 20 at 10,000', etc.
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352-427-0285
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RV Hotel (hangar, room, car) at X35 in north FL

Commercial ASEL/S A&P/IA EAA Technical Counselor

Dynamic Prop Balancing, Prebuy Inspections, Condition Inspections, Repairs and Mods, Injector Tuning, Airframe, Engine, Panel Upgrades, Ferrying, etc.
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  #8  
Old 03-20-2017, 12:25 PM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Jesse View Post
With a broken line it's sensing manifold pressure, just not the engine manifold pressure. It's sensing ambient pressure which is around 30 at sea level, about 20 at 10,000', etc.
Exactly, and that is why it advances as you climb, despite what Emag says.

The good news is, it will not advance beyond the same limits used when there is no leak.
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Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html

Last edited by N941WR : 03-22-2017 at 07:36 AM.
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