[Brantel]

I posted this in my prop swap thread but wanted to put it here as well to get a more focused audience...

One weird thing that has changed since switching props...

With the FP during mag checks, running on the Mag only would drop about ~50 rpm. Running on the P-Mag only ~10 rpm.

Now with the CS, running on the Mag only drops about ~60 rpm. The P-Mag only drops about ~50 rpm.

Nothing changed on the ignition, no ignition components were removed, no timing adjustments were made.

Why does the P-Mag now drop ~40 rpm more? I noticed that the higher the RPM during the Mag check, the less the P-Mag dropped.

I know there is more inertia involved now and less load on the engine at typical mag check RPM of 1700 RPM but would this make the difference?

I checked my timing via the LED on the back, checked all the wires and plugs last night just to be sure I did not knock something loose with all of my work swapping props and adding a governor. Everything appeared fine.

 

[SmilingJack]

I see about the same 50 rpm drop with my pMags.

I'm thinking its the weight being much more and the inertia only help during the first rotation of the prop and then the engine feels the weight.

 

[David Z]

Load on Engine

I'm assuming you are doing the mag check at the same rpm.
A constant speed prop in full fine pitch is less coarse than a fixed pitch prop. That means that during the run-up with the c/s prop, the engine is under less load (lower manifold pressure). I know very little about electronic ignitions and their programming, but I bet that's how it reacts to lower manifold pressure during run-up.

 

[Brantel]

I'm assuming you are doing the mag check at the same rpm.
A constant speed prop in full fine pitch is less coarse than a fixed pitch prop. That means that during the run-up with the c/s prop, the engine is under less load (lower manifold pressure). I know very little about electronic ignitions and their programming, but I bet that's how it reacts to lower manifold pressure during run-up.

I am thinking you are correct... I think the Pmag is operating with more advance with the CS at the same 1700rpm runup vs the FP.

I looked at my log files and the FP averaged around 20" during runup and the CS averages around 16"....same RPM

 

[penguin]

I think it is about the map used by the P-mag. It may be that the additional 4" of manifold on the fixed pitch puts the P-mag into a greater advance portion of the map, so minimizing the impact of the mag. At 16" there may be little advance over the mag's 25*, so the impact of turning off the mag or the P-mag is similar? Just my thoughts, no data or knowledge to back up!

As an aside I know there is a knee in the P-mag's advance curve at 1800 rpm, so I use 1900 for my mag checks.

Pete