I fear replacing one magneto by another would be a bit too much for a 337
You absolutely cannot just remove a dual mag and insert a single mag. It would be a Major Alteration (a Form 337) and that needs one of these
If their STC doesn’t cover this then you can’t do it. Well, not legally 
I also believe that the STC does not only have to cover the engine but the airframe as well.
In EASA-land, probably. In FAA-land, I don’t think so because the Skytec starters (e.g.) are covered just for the engine.
Their words “You can keep the dual mag install with one side capped off” don’t relate to anything I recognise.
What I wanted to say is that I don’t think the FAA would signoff on a 337 without an STC. They are pretty anal about the ignition system. This is too big a change to do as a field approval in my view. Therefore, the manufacturer better get an STC that includes the whole procedure. This will probably be rather difficult as I would not expect the FAA to approve a mag change without extensive flight testing for each and every engine variant.
Peter I guess EHT is just as much of a concern in this install – they say you have to use specific RF-insulated spark plug wires to protect the controller… not to mention that to have less radiating EHT is bound to be good for your avionics as well!
another Q is their product reliability. Found this in pilotsofamerica.com
I experienced 3 failures in 11 years and a little over 800 hours. Two of the failures were the pick-up for the hall effect sensor and one was a failure of the circuit board in the system’s brainbox. The circuit board failure resulted in me retiring the system and moving to a different ignition.
What did you replace it with?
A pMag, which is another electronic ignition.
Yes – I found that thread.
The feedback is rather mixed and not a lot of data there. I think too few have been sold. Maybe the conclusion one can draw is that the performance gain might be negligible.
It is true that if you want the best MPG, you fly at say 2200 RPM (rather than 2300 or 2400) and that lower RPM requires a different (reduced) ignition advance. However, the existing mag just happens to largely take care of that, because if you fly LOP the mixture burns more slowly and that needs a more advanced spark, but that’s exactly what you do get because the ignition timing is fixed!
This is why flying LOP at a low RPM gives you better MPG than flying at peak-EGT at a higher RPM, despite simple physics saying it is impossible (setting aside the mechanical losses) because once at stochiometric (about 25F LOP) or beyond, there is no more power to be extracted from the fuel.
Unfortunately my faith in the competence of electronics designers in piston GA is low. I have opened up a lot of the “peripheral” instruments like the EDM, Shadin, etc, and the internals are the kind of junk you might have built in a spare garage in the late 1970s. Electronics that go onto a vibrating lump like an IO540 need to be rock solid and either totally encapsulated or built using special techniques.
It is true that if you want the best MPG, you fly at say 2200 RPM (rather than 2300 or 2400) and that lower RPM requires a different (reduced) ignition advance. However, the existing mag just happens to largely take care of that, because if you fly LOP the mixture burns more slowly and that needs a more advanced spark, but that’s exactly what you do get because the ignition timing is fixed!
Yes, if you can make enough power at lower rpm. The main efficiency benefit of mapped ignition timing for an aircraft is actually when you are intentionally running the engine inefficiently at high rpm in order to maximize aerodynamic efficiency, i.e. at high altitude with a non-turbo engine. In that situation the higher aerodynamic efficiency at altitude may dominate over the worse engine efficiency: you may get good MPG because the aircraft has high TAS concurrent with a lower IAS that has lower parasitic and induced drag. However the available manifold pressure is low meaning high, inefficient rpm to make sufficient power for the desired TAS. One solution to increase high altitude engine efficiency is turbo normalizing to bring the rpm down, with attendant complexity. Another simpler solution is huge ignition advance when at high rpm and low MP, far more than fixed timing would normally allow – as much as 50 degrees BTDC, which as a fixed ignition timing would cause detonation at higher MP. That gets a bit more energy out of the fuel, regardless of high rpm being otherwise inefficient in terms of friction losses.
Its no more complicated than old fashioned vacuum advance on a car engine, but because aircraft engines don’t run highly throttled in service very much, it isn’t particularly useful for an aircraft engine until its at high altitude.
BTW and FWIW the guy posting with the same user name as me on other site(s) is not always me!
A straight or opposed six shouldn’t be a vibrating lump.
If it’s built properly :D
Yes, if you can make enough power at lower rpm
Spot on. At 2200 I can’t fly below about FL100 (well not with a wide open throttle, which is another efficiency improvement, because the engine goes rough, and also it’s not allowed) and I can’t fly above about FL140 (not enough power). But on nice days, when trying to optimise MPG, it’s the way to go.
Re high RPM, I wonder if the wrong ignition timing is the reason for high CHTs?
the guy posting with the same user name as me on other site(s) is not always me!
Good to know that
A straight or opposed six shouldn’t be a vibrating lump.
I had mine dynamically balanced by the best shop in the USA, but isn’t there always a second order imbalance?
You may also have a view on this data.
I think there is a strong component from the exhaust gas pulse, and the airframe itself suffers badly from the prop aiflow which is extremely dirty and turbulent. The vibration spectrum contains practically nothing at the crankshaft fundamental.
I thought third order is more important on six potters. I drive a straight five which is much more difficult to balance – yet it is smooth and has a higher power to weight ratio than a comparable output aero.
With enough R&D it could have been done.
This AML stuff is without much diligence. If your airframe is missing on Garmin’s AML, it is because they forgot to put it on or didn’t know it existed. I guess in this case they tried to come up with a comprehensive list of all aircraft equipped with certain engines.
I would bet if you submitted an AML that includes “Peter’s fantasy aircraft”, it would get an FAA (and now also EASA) stamp…
My aircraft is missing though (basically same engine as the TB20), apparently they knew it was equipped with a dual mag.
