Firing sequence of 24 cylinders H engines, like Napier Sabre

Discussion in 'Engines' started by Elmas, Feb 8, 2015.

  1. Elmas

    Elmas Active Member

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    #1 Elmas, Feb 8, 2015
    Last edited: Feb 8, 2015
  2. kmccutcheon

    kmccutcheon New Member

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  3. Elmas

    Elmas Active Member

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    Thanks!
     
  4. AMCKen

    AMCKen Member

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    The 2 links here seem to have different numbering systems for the cylinders. 'Flight' says: Upper and lower banks of 12 cylinders are numbered on the left 1 to 6 from rear to front and, on the right, 7 to 12 from front to rear."

    Enginehistory has 7 opposite 1, not 6.

    ??

    Looking at just one bank, the order is the same as most in-line 6 cylinder car engines: 153624.
     
  5. camman

    camman New Member

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    #5 camman, Jul 23, 2015
    Last edited: Jul 23, 2015
    If you look at the AEHS page closer, you will see that the dataplate cylinder numbering and firing order of the sabre are still there and that the firing order has been normalized for ease of comparison by using the cylinder numbering of a German v-12. It's kind of like how a Ford small-block has a different firing order than a Chevrolet small-block but if you normalize the cylinder numbering, you can see that they fire identically.
     
  6. GregP

    GregP Well-Known Member

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    #6 GregP, Jul 25, 2015
    Last edited: Jul 27, 2015
    Dan Whitney's corrected firing order is just WRONG. If you learn it and use you can't communicate with anyone who works on the actual engines.

    Use the data plate, that is what it is FOR. Do NOT attempt to set a firing order without the BOOK on the engine. All factory engine books explain it perfectly. Some engine are looked at from the prop-attach end and some from the rear, opposite the prop attach end, and the book tells you, right up front in the first 20 - 30 pages. It's hard to miss.

    Anyone smart enough to be curious about the firing order should also be smart enough to look at the book and ignore that post above, which is mostly not correct if you are working on the engine using the book. If you aren't an engine guy, then you probably aren't even curious about firing order.

    But if you ARE, then by all means, get it RIGHT. That being said, I like the simplification ... but you can't use it to set the iming by the book.
     
  7. GrauGeist

    GrauGeist Well-Known Member

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    Just like any engine (be it aircraft or automotive), never assume.

    *most* inline sixes use a 1-5-3-6-2-4 firing order, BUT there were changes, like in the 1970/1971 Chevy 250 (4.1L), it was: 1-4-2-6-3-5 and then it changed back to 1-5-3-6-2-4 between 1972 and 1974. However, from 1975 onwards, it reverted back to the '71/'72 order.

    So the rule of thumb is to always check the service manual that is model and year specific to the engine you're working on...
     
  8. GregP

    GregP Well-Known Member

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    That firing order for the sixes depends on which way the crank throw go, doesn't it Shortround? The firing order for the Allison V-1710 changes between a left and right hand engine because the crank turns around 180° for a left hand engine versus a right hand engine. There are only about 6 - 8 parts difference between them plus a starter that turns the other way.
     
  9. kmccutcheon

    kmccutcheon New Member

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    GregP-

    The original data from which the Napier Sabre firing order display was derived appears below
    A57462-013-InterUnitAssyCrankRods.jpg


    Could you please explain exactly how the Napier Sabre firing order depicted in the V-12 Firing Order Display is "just WRONG??"

    The V-12 Firing Order Display is intended to allow engine firing order COMPARISONS. This requires normalizing the cylinder numbering; it does not "correct" the dataplate firing order, but simply provides a different way of looking at the same data. It was never intended to replace the technical data that is used for valve and magneto timing.


    Kim McCutcheon
     
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  10. GregP

    GregP Well-Known Member

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    #10 GregP, Jul 27, 2015
    Last edited: Jul 27, 2015
    Sure.

    Dan's explanation covers a lot of V-12's and they are not all seen from the same point when setting the firing order by the book. If he gets it right for the Sabre, bully for Dan. It means the number one cylinder was seen by the designers the same as Dan saw it. Nothing wrong with it if Dan's firing order agrees with the engine book.

    But some designers look at the number one cylinder from the front of the engine and some from the back. If Dan's firing order disagrees with the book, the cylinders may be technically correct from a logical standpoint, but using the actual engine book will result in an engine that won't run or, if or does fire, may even damage the engine with reverse torque some gears weren't designed for.

    If you want to look at the firing order from above the engine and from the same vantage point Dan's explanation is fine.

    If you set timing on an actual engine that is to run by the data plate, then don't do it by memory or by ear or by hook or crook, do it by the engine book. If you are an engine designer, it makes little difference since you KNOW. Most people who overhaul WWII engines and actually set cam and valve timing are engine overhaulers, not powerplant engineers. They use the BOOK written by the engine designers ... and they get it right using both the data plate and the book.

    You won't see Mike Nixon, Joe Yancey, Jack Rousch, or anybody else setting timing on a WWII aero engine that is to fly in an actual airplane without an engine book. If you do it by memory on an engine on a stand, so what? If you get it wrong, nobody dies and any subsequent damage or whatever is on YOU, and you're still on the ground.

    If it is a flyer, the engine is worth some big money if it is a Merlin, Allison, Radial, or whatever ... any damage caused by other-than-factory timing settings is covered by your wallet. If that's OK with you, hey ... go for it. Your first mistake might cost you tens of thousands of dollars and if that's OK with you, then all is good.

    If you KNOW which way the crank turns and which way each cam turns and which way the distributors turn, and how many teeth are on each gear and the size of the gears, adjusting timing isn't too difficult. If you aren't completely sure, use the factory engine book.

    All the guys overhauling flying engines use the book and are required to do so by law. If you doubt that, just invite a local FAA inspector over to watch you jackleg the timing on an engine that is to fly without having and using a book and see what happens to your A&P license. That should be enough for anybody. You are supposed to have the book open to the proper page when you are doing work on an engine that will be bought off as flyable.

    I'm not an A&P like some in here, but I know going by the book is required for aircraft unless you are in the experimental or experimental exhibition category. If so, knock yourself out with Dan's method. All the guys at Reno use a BOOK.

    It ain't easy to get it right to start with and deviating can make it easy to require disassembly and resetting basic cam and ignition timing, but your time is yours, so go for it if the engine is also yours. If you are building an engine for a boat or a tractor or a car / truck / snowmobile then, hey, whatever you want to do is OK. If it runs, you're good to go. If it doesn't you can figure it out.

    An automove V-8 has one distributor with 8 wires. A Merlin or an Allison has two distributors, each with 12 wires, and the intake and exhaust mags fire at different points. The automotive V-8 and the aero V-12 aren't necessarily each as easy to time right as the other one.
     
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