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SDS/Pmag vs dual Pmag

SabreFlyr

Well Known Member
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I'll be powering my RV-14 with the new EXP119 engine option (already on order). I had been planning on using one SDS ignition and one Pmag because I want to keep my electrical system as simple as possible. Although my Thunderbolt 390 WILL have the accessory drive where I could install a backup alternator, I don't plan on it at this time (maybe later if I choose to upgrade to full EFII).

My reasoning for wanting the SDS system may not be very well thought out. My knowledge here is sorely lacking. However, my thinking is that I could have two advance curves: one for LoP and one for RoP for better detonation margin under all conditions. However, I just watched the YouTube video, "Electronic Ignition Basics" and the presenter commented that a hybrid SDS/Pmag system would not see the benefit of the SDS system. Would that be true?

I'm also wondering if my reasoning is flawed in the first place (re: detonation margins and advance curves).

Also, how would you contrast a dual SDS (EFI or EFII) system with a dual Pmag system with an EICommander?
 
Keeping it simple would be equipping with one mag & one electronic ignition, your choice of manufacturer. Or running two mags (super simple).

The idea of the mag/EI combo is you get the redundancy of the mag (fixed timing) and most of the advantages of the EI variable timing, most of the advantages because optimally you would best benefit if both ignitions fired at the same time. The ignition that fires first is driving the engine, the mag with it’s set (24 deg setting) is along for the ride until you need it in it’s redundant role.

Best advantage if you choose dual EI is by using the same brand Lt & Rt, than they will act the same & deliver the same spark timing, best advantage for igniting the fuel charge at the same timing under the same conditions.

If you really insisted, you could use say a PMag & SDS CPI combo but you would end up spending a LONG time programming the SDS to mimic the PMag Advance curve, I see no benefits to this.
Dual SDS units would work too, you would have to plan your electrical system to accommodate this, which isn’t that hard.
Choices, choices... Good luck in your planning
 
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Best advantage if you choose dual EI is by using the same brand Lt & Rt, than they will act the same & deliver the same spark timing, best advantage for igniting the fuel charge at the same timing under the same conditions.

The reason that I'm beginning to think dual Pmag.

Dual SDS units would work too, you would have to plan your electrical system to accommodate this, which isn’t that hard.

But something that I've been struggling with and, thus, the reason that I was starting to think about incorporating at least one Pmag since it is self-generating in most inflight conditions. Most of the electrical planning difficulty, however, is because I've been too easily swayed by those who argue for their preferred method of building electrical system redundancy. Ross does seem to have a relatively simple solution in having a simple backup battery that is removed from the plane for charging. But, then I start to think about how I switch to it and, at the same time, safely isolate the engine electrical system from the rest of the aircraft. (No, I'm NOT trying to generate a discussion about how to accomplish that. :rolleyes:)
 
Trade offs

I have one bird with dual pmags and one with dual SDS ignition, so my perspective is not vendor biased !
You are trading gear backlash, bearing condition concerns, hot circuit boards, wimpy terminal strip characteristics and compromised rear engine congestion compared to rock solid crank trigger technology ( like race cars),electronics in the fuse and option to optimize advance curve to both rop and lop.
I have a Ross approved power schematic that employs a small backup battery charged continuously through a shottkey diode. It is totally isolated from any electrical problems from the main bus so you can get to a flat spot when the main buss goes TU.
Just my $.02 to consider 🤗
 
I think the best way to fire the plugs is with EI systems. The best way to do that is two separate (and redundant) EI systems. There’s no advantage in my view to use two DIFFERENT types of ignition systems for the purpose of redundancy. Personally, I chose two Pmags. I understand them, have learned to time them (easy) and they have worked very well and efficiently for me. Either one could fail partially or totally and I could continue to my destination without breaking a sweat.

BTW, when I do a Pmag check during the runup, I get a 30-50 drop on each Pmag. The drop means the engine runs better with both Pmags in play. If using two different technologies together with diffferent timing curves, the engine would not be working as well as with my two Pmags.

It’s just a guess that the difference in drop is because one Pmag fires all the tops plugs on my engine (that’s my choice), the other fires all the bottoms. So there could be a difference due to the position of the spark plug in relation to the valves. Not sure.

My 2 cents worth. YMMV

Bevan
 
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As a long time Pmag and SDS user IMHO, the Pmag/SDS system "should" work fairly well if you are willing to put in the time to test and program it. The SDS programming would be tricky though, because it would need to have a big retard to offset the radical advance curve of the Pmag at high power settings, then a big advance to offset the limited total advance available from the Pmag for LOP ops. The AV engine is particularly ill suited for the aggressive canned Pmag advance curve, but in theory, it should work with big swings from the SDS system keeping the Pmag under control.

Not the best approach though. A magneto on one side and SDS on the other makes a lot more sense.
 
Keeping it simple would be equipping with one mag & one electronic ignition, your choice of manufacturer. Or running two mags (super simple).

The idea of the mag/EI combo is you get the redundancy of the mag (fixed timing) and most of the advantages of the EI variable timing, most of the advantages because optimally you would best benefit if both ignitions fired at the same time. The ignition that fires first is driving the engine, the mag with it’s set (24 deg setting) is along for the ride until you need it in it’s redundant role.

Best advantage if you choose dual EI is by using the same brand Lt & Rt, than they will act the same & deliver the same spark timing, best advantage for igniting the fuel charge at the same timing under the same conditions.

If you really insisted, you could use say a PMag & SDS CPI combo but you would end up spending a LONG time programming the SDS to mimic the PMag Advance curve, I see no benefits to this.
Dual SDS units would work too, you would have to plan your electrical system to accommodate this, which isn’t that hard.
Choices, choices... Good luck in your planning

Not the best approach though. A magneto on one side and SDS on the other makes a lot more sense.

Completely agree with both of these post. Making something more complex to make it more redundant is going to make the normal working of the system more toublesome and less efficient. Two of the same or one simple mag with one complex EI is way to go.
 
I note with interest the concept of “plan your electrical system” for using SDS (or Lightspeed, etc.). I wonder how many people actually do that - and I wonder the ones that do how often do they check/replace the needed backup battery(s)? For that matter how long will the engine run on the backup battery? I would not accept anything less than three hours.

This is not rocket science. Most EIs will provide performance gains. If you want the least complicated approach dual pMag is the option. If you want to dive into the world of risk management (as I have done) create an electrical system to support.

For the record - I do both as I want confidence in reserve power to the panel for IFR flight. I run dual pMags as my experience with them on three airplanes has been superb.

Carl
 
Actually, there is no longer the need to "plan your electrical system" with the CPI2 - Ross has done all the planning for you. Out of the box, the system has automatic power switching, buss monitoring, and battery management. The provided battery will run the ignition for a very long time - approaching and exceeding the fuel endurance in most cases - but you are free to add a bigger battery to give you any endurance you require.

But the biggest issue here which is frequently glossed over, is the irrefutable fact that the out of the box Pmag curve is not only "sub optimal" for an Angle Valve engine like the 390, but its actually hurting performance. The need to mechanically retard the timing and/or intercept and modify the curve with a 3rd party controller is a well documented thing. The bottom line is that running a Pmag on an AV engine straight out of the box is a bad idea.

Since the OP is discussing an Angle Valve engine here (and a really expensive one at that), the idea of using the infinately flexible CPI ignition to "take the edge off" the harsh Pmag curve makes sense from a theoretical perspective, but in actual execution it is messy.
 
Regarding comments about the harsh advance curve of the Pmag WRT angle valve engines, I am fully aware of the need to get that set up right before first flight. No advance at all for the first 10 hours, at least; probably more. Then, sticking to between 20°-27° BTDC (I do need to confirm those numbers).

Point is well taken re: combining two different EI systems. Now trying to decide between SDS/mag, dual Pmag or dual SDS. In the latter case, dual alts (B&Cs with their regulator plus Russ' backup battery system). Frequent attention to the backup battery's health would be required. The difficult problems, as I see it, are, 1) how to accomplishing the switching of the EI power to the backup battery and ensuring complete isolation of engine electrical in that case, and, 2) testing engine operation on the backup battery prior to every flight.

I'm most torn between what I see as Ross' superior engineering combined with the maturity/experience of his systems and the simplicity of a dual Pmag system. Perhaps an SDS/mag system might be the best compromise but this is going to require a lot more thought.

Thanks for all the input! And, if anyone has anymore to add, I'm all ears!
 
Again, the SDS CPI2 has built in back up battery management and switching. No need to re think this or re engineer an electrical system. Buy, install, fly. Do a cap check on the battery every year (just like the main battery), and replace at regular intervals. Treat it just like you do with those mission critical backup batteries in your efis. Same concept.
 
Again, the SDS CPI2 has built in back up battery management and switching. No need to re think this or re engineer an electrical system. Buy, install, fly. Do a cap check on the battery every year (just like the main battery), and replace at regular intervals. Treat it just like you do with those mission critical backup batteries in your efis. Same concept.

Okay, I've got to look at that again. I didn't realize he'd covered all that.
 
I have one bird with dual pmags and one with dual SDS ignition, so my perspective is not vendor biased !
You are trading gear backlash, bearing condition concerns, hot circuit boards, wimpy terminal strip characteristics and compromised rear engine congestion compared to rock solid crank trigger technology ( like race cars),electronics in the fuse and option to optimize advance curve to both rop and lop.
I have a Ross approved power schematic that employs a small backup battery charged continuously through a shottkey diode. It is totally isolated from any electrical problems from the main bus so you can get to a flat spot when the main buss goes TU.
Just my $.02 to consider 🤗

Sorry, I'm a complete noob. I don't even understand your language! care to translate?
thx
 
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