Are dual spark plugs still needed?

Discussion in 'Firewall Forward / Props / Fuel system' started by Hot Wings, Aug 22, 2019.

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How necessare is dual ignition or dual spark plugs.

  1. 100% required. We all know that magnetos fail.

    3 vote(s)
    10.3%
  2. They are a very good idea because they help the engine be more effecient.

    10 vote(s)
    34.5%
  3. Dual ignition is required but not dual plugs. A back up ignition system is just as good.

    5 vote(s)
    17.2%
  4. Dual plugs are no longer needed with distributed COP style ignition systems.

    8 vote(s)
    27.6%
  5. Dual plugs are still needed with COP systems unless there is no single point failure mode.

    3 vote(s)
    10.3%
  6. Dual plugs lets me use both a magneto and an electronic ignition.

    7 vote(s)
    24.1%
Multiple votes are allowed.
  1. Aug 23, 2019 #21

    don january

    don january

    don january

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    I find dual plugs and ignition is like insurance on the car. You hope you never need it put is a great stress reliever and can help out at times.
     
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  2. Aug 23, 2019 #22

    blane.c

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    That's how I feel about a couple extra engines.
     
  3. Aug 23, 2019 #23

    pictsidhe

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    Do you have a head thread?
     
  4. Aug 23, 2019 #24

    Hot Wings

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    If that is directed at me - no. Way too early. All I have are some ideas and electronic versions of napkin sketches. Example : 1/2 VW head with since abandoned stock rocker arms.
    Vw Doodle.JPG
     
  5. Aug 24, 2019 #25

    BeemerNut

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    After reading all other replies to this thread I must agree with you TFF as the best (bestly ha ha) explained reply.
    Flame propagation comes to mind as mentioned by mag drop, this telling you something good about dual plug ignition vs single plug? It shows the known facts. Presently looking at the 5.750" diameter piston out of a 4360 Pratt,my dust collector next to my computer. PS anyone ID the piston ashtray used on Cournal Blake's table on M.A.S.H.? Thinking 2800 Pratt, I worked on them back in 74 in an engine shop. Had to spout off......~~=o&o>.......
     
  6. Aug 24, 2019 #26

    Pops

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    Bob Hoover had a drawing of his idea of a custom made VW head on his blog but I haven't been able to find it. He had the exhaust coming out the bottom like the small Continentals instead of the ends of the heads as on the stock VW head.
     
  7. Aug 24, 2019 #27

    Hot Wings

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    I remember a sketch from the days on RAH when we were discussing this. I think he sent it to me via e-mail. Getting the exhaust out the bottom, like a continental or TP IV VW is impossible given the valve train arrangement. This is one reason when Bennett was talking to Hoover about the possibility of a 'for aircraft' crankcase I was pushing them to move the camshaft so we could do the down exhaust - and get some better valve train geometry.
    The sketch I remember has the exhaust lowered but still out the ends. It does free up a lot more room around the exhaust guide boss for air flow.
    The CAD above needs to have a lot of credit given to Hoover.
     
  8. Aug 24, 2019 #28

    pfarber

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    You can play the redundant game all day and still have an engine issue. What good is dual X,Y,Z when a mechanical problem like a snapped crank or blown cylinder seizes the motor? Clog the fuel line with debris and you're just as SOL.

    Are mags outdated? Sure are. They have what, 500 hour inspection periods?

    I'll take a modern EFI/ECU over a magneto any day.
     
  9. Aug 24, 2019 #29

    Pops

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    The down exhaust was not directly below the exhaust valve but down at the end of the head. The exhaust would have to make a 90 deg bend. The way I remember, I wake up in a new world everyday. :)
     
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  10. Aug 24, 2019 #30

    Vigilant1

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    The way the head is set up on the Motorav engine makes the intake and exhaust plumbing fairly neat, and having both plugs up on top is convenient. I'm sure it's got VW roots. 2868cc, 98.5mm bore, 94mm stroke. upload_2019-8-23_23-14-33.png
     
  11. Aug 24, 2019 #31

    ncmc47

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    Dual mags were introduced to prevent detonation, not so much for redundancy, but they do provide spark if one quits.
     
  12. Aug 24, 2019 #32

    Hot Wings

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    It is at least VW inspired, but this thread is drifting into head design rather than dual ignition. Maybe a new thread is needed?
     
  13. Aug 24, 2019 #33

    Vigilant1

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    My apologies -- I did some of that.

    To keep the market appeal as broad as possible this seems like the best bet, as long as dual plugs can be done without a huge sacrifice in other areas. I wonder (I don't know) if the specifics of flame propagation are critical in an engine turning just 3600 max RPM and a total piston-top area of 70 sq cm (11 sq in).
    We've got a small sample, but with 30 votes in, 11 (37%) want dual plugs. I'm sure opinions might change when we go from a general question on a survey to a specific case of one airplane project and actually talking with dollars.
    Other: It will be best if there's enough meat around the holes to allow for steel inserts (e.g. Timesert-style) for the spark plug.
     
  14. Aug 24, 2019 #34

    Hot Wings

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    No apology needed! I too participated in the thread drift. :oops:
    << >>
    Hadn't considered the repair aspect. Thanks for the idea!

    Flame propagation, IMHO based on my read of history, was the original idea behind dual plugs. I don't know either if it's really important for such a small diameter cylinder at direct drive speeds. I'm sure the optimal timing would be different.

    I think the number of cylinders involved is also relevant if using COP or a discrete ignition system. If I were to market, or use, a 2 cylinder dual plugs would be the only option. This coming from the guy that got the single ignition option through the ASTM process. o_O I'd probably be comfortable with a 6 cylinder and single plugs with a modern ignition system.
     
  15. Aug 25, 2019 #35

    Dana

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    The two cylinder (half VW) Mosler engines had custom heads made by Scat. The exhaust was still on the end (front in this case) but the intake was moved to the back of the engine instead of the top, allowing a much shorter intake manifold.

    I had a Datsun pickup truck some years back, that had two spark plugs per cylinder, and they weren't the same. I presume it was necessary to meet some emissions or economy target, but it was part of the factory design. Always ran great until it threw a rod. :eek: I wonder, reliability questions aside, does anybody see any significant performance difference with dual ignition on a VW based engine?
     
  16. Aug 26, 2019 #36

    AdrianS

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    I would be wary of COP in a tightly cowled engine due to heat concerns.

    I've no opinion on dual ignitions for aircraft, but in the historic forest rallying world, we always fit dual coils (and fuel pumps).
     
  17. Aug 28, 2019 #37

    Hot Wings

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    Got the Nissan COP unit in the mail today and put it into CAD. Way too large for a VW head. Have a BMW COP unit and the rocker arms on the way. Once they are in CAD I may start a VW head thread.
     
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