Proper bus?

[erich weaver]

I have dual pmags and fuel injection and am using Bob Nuckols Z-13/8 architecture with main bus, E-bus and battery bus. What bus is appropriate for the pmags and why? What bus is appropriate for the fuel boos pump and why?

thanks

[Mike S]

For my $.02, run the ignition off both the main and E buss, use a couple of Schotkky diodes to prevent back feed if you need to shut off one buss.

http://en.wikipedia.org/wiki/Schottky_diode

Boost pump on main.

[Bevan]

Eric, I will respond because I have the exact same setup as you and agree with much of Nuckolls theories.

I run the Pmags off the main buss. So only on ship's power once the master is turned on. This allows dedicated circuit breakers on the panel for maintenance purposes and MAYBE a one-time reset in flight if over inhospitable terrain. They don't absolutely need to powered from ship's power when operating in "ALT out" (running on backup ALT) conditions with the Ebuss in use because they are NOT drawing anything from ships power anyway at that point. I didn't want my Ebuss items powered up for engine start shut down so the Pmags are not on the Ebuss.

The boost pump is not a continuous duty item and is high draw. So this would not be on the Ebuss since it would have to be off anyway (for Ebuss ops). Since my Ebuss is off for engine start/shut down, and the fact that boost pump may be required for cold engine starts, the boost pump is on the main buss. Again, dedicated pullable breaker on the panel.

Bevan
Not flying yet but everything seems to work.

[Joe]

Originally Posted by erich weaver

I have dual pmags and fuel injection and am using Bob Nuckols Z-13/8 architecture with main bus, E-bus and battery bus. What bus is appropriate for the pmags and why?

The Main Bus. External power is only needed for starting; once they're above idle RPM they're self-generating.

Originally Posted by erich weaver

What bus is appropriate for the fuel boost pump and why?

The Main Bus. Your main bus will not fail (or have to be shut down) during the same flight that a engine driven fuel pump failure occurs so why burden the E-bus with it?

I also have a similar setup (dual P-Mags, IO engine) and agree with much of Nuckolls' teachings (and with Bevan above).

--
Joe

[N941WR]

Whatever you decide, please follow the P-mag installation manual!

Ground each P-mag to a separate engine case bolt, power them with separate pullable breakers, and provide a way to drop power to each P-mag independently (pullable breakers will do) so you can test the internal generator as a pre-flight check.

I used a three-way switch w/ pullable breakers for my P-mags.

Here is a diagram of my setup.

One other thing, regardless of which engine you have, force the P-mags to the "A" curve (jumper in) or program them for the proper timing using either the EICAD program or the EICommander.

[erich weaver]

Originally Posted by Bevan

I run the Pmags off the main buss. So only on ship's power once the master is turned on. This allows dedicated circuit breakers on the panel for maintenance purposes and MAYBE a one-time reset in flight if over inhospitable terrain. They don't absolutely need to powered from ship's power when operating in "ALT out" (running on backup ALT) conditions with the Ebuss in use because they are NOT drawing anything from ships power anyway at that point. I didn't want my Ebuss items powered up for engine start shut down so the Pmags are not on the Ebuss.

Sorry I don't follow the logic and your statement seems self contradictory with respect to when ships power is in use. Some pmags normally run on ships power and some on self power but I don't see what that has to do with which bus is used. Admittedly the chance of two different failures on one flight is remote, but what's the downside of having pmags on the e bus? they dont add a big load. Heck, there is even a Nuckolls article on his website that recommends all electronic ignitions be on the battery bus, although perhaps he didn't mean that to apply to pmags, not sure.

Originally Posted by Bevan

The boost pump is not a continuous duty item and is high draw. So this would not be on the Ebuss since it would have to be off anyway (for Ebuss ops). Since my Ebuss is off for engine start/shut down, and the fact that boost pump may be required for cold engine starts, the boost pump is on the main buss.

If you are using z-13/8, your ebus is still energized during engine start. The ebus is fed directly from the main buss until you turn on the ebus alternate feed switch, at which point it is fed from the battery bus. In looking at various electrical load spreadsheets, I observed that several people had put the boost pumps on the battery bus although I'm not sure why. Since the boost pump is only for intermittent or backup use, im thinking it really doesn't make much difference what bus it's on assuming you have open slots available on your fuse panels. If you really need it, turn it on and let the battery take up the slack.

Don't mean to come off as argumentative here. Just trying to understand the reasoning.
Thanks

[erich weaver]

Originally Posted by N941WR

Ground each P-mag to a separate engine case bolt, power them with separate pullable breakers, and provide a way to drop power to each P-mag independently (pullable breakers will do) so you can test the internal generator as a pre-flight check.

I have fuses, but my switches do allow for easy, independent testing of the self powering feature.
Thanks

[N941WR]

Originally Posted by erich weaver

I have fuses, but my switches do allow for easy, independent testing of the self powering feature.
Thanks

Only one issue and it may or may not impact you later on and I haven't seen your wiring schematic, so this may not apply.

If you wire the P-mags per their diagram, they will be powered every time you turn on the master. The ignition switch only grounds them. Thus, you have the same safety issue as a standard mag, a broken wire makes the P-mags hot and they will fire.

With pullable breakers or an independent power switch, you and turn on the Master and kill the power to the P-mags. There is nothing wrong with this, other than you have added additional complexity by including the power switch (no different than what I have done) and I'm sure there are others running the same setup.

Ignition and fuel systems are two places I urge extreme caution!

(I'm gun shy here because with my involvement with the EICommander I have helped diagnose a number of very unique P-mag installations.)

[Bevan]

Originally Posted by erich weaver

Some pmags normally run on ships power and some on self power but I don't see what that has to do with which bus is used.

Pmags are self powered above a certain RPM. Require ships power only for initial engine start. Emags, and all other electronic ignition I can think of, require an electrical power source 100% of the time.

Regarding the Bus arrangment...My 13/8 is perhaps a little non-standard in that my Ebus is powered from (A) the avionics buss, which is off (via a relay) for engine start/shutdown through, or (B) directly from the battery (always hot bus) through another relay. Either way, the Ebus is not powered for engine start so therefore not an appropriate place for the Pmags. Sorry to have confused you.

Bevan

[erich weaver]

Originally Posted by Bevan

Pmags are self powered above a certain RPM.

just a detail, but this is not universally true. For older models it's reversed, where the default is to run on ships power, utilizing self-generated power as the backup. For the purposes of proper bus selection, I suppose this might make a difference in that it changes how the bus is normally loaded, although not by all that much.