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Retrofitting a Fixed/Variable Timing Switch to 6-Cyl P-Mags

Dad's RV-10

Dad's RV-10

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DanH said:
...if your 540 is a parallel valve, hot, high DA days can make the fixed timing option worthwhile for CHT control.

Paul, one switch... ... Just make sure to use a double pole
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Ironflight said:
Never closed this out…yup, went with a single DP switch….
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I'm contemplating retrofitting the fixed/variable switch into my RV-10 (at least temporarily). DanH and Ironflight indicate the use of a DP switch.

But the P-Mag manual specifies use of a SPDT switch:

"(*) Mode Switch (optional): Ignitions can work with a cockpit switch to select either Fixed Mode or Variable Mode operation. The switch circuit adds a 1K bias signal to the existing kill wire (pin E) to enable Fixed Mode operation. The switch E-MAG offers is a SPDT (one switch controls two ignitions) with 1K resistors preinstalled."

What am I missing?
 
Carl Froehlich said:
Easy to add as the needed wires are already in place. Wire to the existing pMag control switch. No new wires going to the pMag.

Carl
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Yes, I see that that diagram shows the switch is simply wired in parallel to the P-Lead wires.

Buy what I'm not clear about is why the quoted posts say to use a DP switch while the manual says to use a SP switch?
 
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Dad's RV-10 said:
Yes, I see that that diagram shows the switch is simply wired in parallel to the P-Lead wires.

Buy what I'm not clear about is why the quoted posts say to uses DP switch while the manual says to use a SP switch?
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It just keeps the two ignitions electrically independent should a switch fail. Of course, the switch can mechanically fail so that you don’t get EITHER ignition to switch, but its just not worth going down that rabbit hole in my opinion- the complications to truly make them independent while making sure that you don’t have one in one mode and the other in another will freeze you into inactivity. Remember that - like any single system - ignition designers are giving you an ignition, not an entire aircraft with inter-related systems. System integration is up to you.

In all reasonable probability, it probably doesn’t make any difference which way you do it… but that is an opinion not grounded in analysis.
 
Ironflight said:
It just keeps the two ignitions electrically independent should a switch fail. Of course, the switch can mechanically fail so that you don’t get EITHER ignition to switch, but its just not worth going down that rabbit hole in my opinion- the complications to truly make them independent while making sure that you don’t have one in one mode and the other in another will freeze you into inactivity. Remember that - like any single system - ignition designers are giving you an ignition, not an entire aircraft with inter-related systems. System integration is up to you.

In all reasonable probability, it probably doesn’t make any difference which way you do it… but that is an opinion not grounded in analysis.
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Thanks for elaborating, Paul. Makes sense.
 
What Paul said. Independent circuits with one handle.
It came to mind because the same function MUST be on separate circuits with the paired Magajolt controllers I use.
 
DanH said:
What Paul said. Independent circuits with one handle.
It came to mind because the same function MUST be on separate circuits with the paired Magajolt controllers I use.
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I think the question is why does the P-Mag manual specify the use of a SP switch? Keeping the two ignitions independent seems to be needed but with a SP switch probably not possible?
 
I need to order some 1K ohm resistors for the Fixed/Variable switch.

Some are 1/2 W. Some are 1/4 W. Some are 1 W.

Not sure which to use?
 
Dad's RV-10 said:
What am I missing?
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Does this set up allow for an in-cockpit display of your timing advance? (Not a P-mag fan and therefor not very familiar with it) If not, that may be what you are missing. It is crazy IMO to mess with variable timing on an aircraft engine w/o seeing -in real time- the relationship between timing, CHTs and Oil temp.
 
dmattmul said:
I think the question is why does the P-Mag manual specify the use of a SP switch? Keeping the two ignitions independent seems to be needed but with a SP switch probably not possible?
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The two igntions are independent.

The nomenclature is SPDT, as in single position (on/off) double throw (separate contacts for each ignition). So when the switch is on (FIXED position), both ignitions have a separate 1K bias resistor in parallel with that ignition control wire. When the switch is off (VARIABLE postion), the 1K bias resistor are out of the circuit.

Note the caution in the install instructions about pre FW V55:

Mode Switch (optional) – Fixed Mode or Variable Mode can be selected by a single cockpit
Mode Switch that directs both left and right (Series 200) ignitions. Fixed Mode is enabled when
the Mode Switch applies a 1K bias resistance to the kill (p-lead) circuit(s). The Mode Switch
can serve one or two (Series 200) ignitions. The Mode Switch is required for all boosted
engines. For non-boosted engines the Mode Switch can be eliminated, in which case the
ignition will operate in Variable Mode only.
a) Fixed Mode - Mode Switch “FIX” will fire plugs at the MIN firing position. RPM must be
above cranking speeds – i.e. you cannot start in Fixed Mode. [In FW V55 and after, Fixed
Mode starting is enabled]
b) Variable Mode - Mode Switch “VAR” allows RPM and MAP inputs to select a plug firing
position between the MIN and MAX boundaries.

A couple of years ago I asked Brad about all this. I got the impression that that the Fixed/Variable option was an issue on the path to get the Series 200 certified for GA.

Carl
 
Carl Froehlich said:
The two igntions are independent.

The nomenclature is SPDT, as in single position (on/off) double throw (separate contacts for each ignition). So when the switch is on (FIXED position), both ignitions have a separate 1K bias resistor in parallel with that ignition control wire. When the switch is off (VARIABLE postion), the 1K bias resistor are out of the circuit.


Carl
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Thanks, I was thinking SP meant single pole (not single position) and the switch did not have individual paths and a DPST was needed but I'm a Chem E. ;)
 
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