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Dual row distributor cap firing order question

Discussion in 'Technical Q&A' started by JohnMH, Jan 8, 2024.

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  1. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #1 JohnMH, Jan 8, 2024
    Maybe a basic question but the answer escapes me.

    My distributor cap has 12 plugs for 12 plug wires on the outside, but the rotor inside has two 'arms' 180 degrees apart on 2 different levels for two different circular rows of six electrodes inside the cap. Each row gets its spark from a different coil.

    How do I figure out the firing order when installing new wires on a new cap? I am not sure the prior owner installed the old wires correctly.

    it would seem that when one electrode on the upper six electrodes is energized to deliver a spark, a moment later the lower electrode 180 degrees away on the cap gets current to deliver a spark.

    So if I am trying to install plug wires and the order is 1-7-4-10-2... etc., I do not start with the 1 then put the 7 wire next to it on the cap, but rather put the 7 across from the 1?

    Am I overthinking this?
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,509
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #2 Steve Magnusson, Jan 8, 2024
    No, that's correct. Another way to say it is if the true firing order (as shown in the OM) is:

    A-B-C-D-E-F-G-H-I-J-K-L

    then the wires are placed:

    A-H-C-J-E-L-G-B-I-D-K-F

    around the distributor cap in the direction of rotor rotation.
     
  3. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #3 JohnMH, Jan 8, 2024
    But if A is followed by B in the firing order set out in the manual of a 12 cylinder car, why is B the 8th letter? if it is directly across from A, shouldn't B be the 6th?
     
  4. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #4 JohnMH, Jan 8, 2024
    Car is an early Diablo. Firing order is 1-7-4-10-2-8-6-12-3-9-5-11.

    If I can locate cylinder 1, what should the order of the next wires be in CCW order of circumference? I believe the rotor turns CCW when viewed from the back of the car.
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,509
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #5 Steve Magnusson, Jan 8, 2024
    Last edited: Jan 8, 2024
    Because the dist cap terminal #7, directly across from cylinder #1, is in the same bank as cylinder #1 (and served by the same rotor end). If B directly follows A in the firing order (and it was a single-ended rotor) it would be in dist cap terminal #2, but, since it is a double-ended rotor, it moves across the distributor cap to dist cap terminal #8 (2+6 = 8).
    One thing you need to be careful about is that all manufacturers do not number their cylinders in the same fashion. If you can post a diagram showing the cylinder numbering, and you know the true firing order is 1-7-4-10-2-8-6-12-3-9-5-11, then I can respond to your request.
     
  6. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #6 JohnMH, Jan 8, 2024
  7. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #7 JohnMH, Jan 8, 2024
  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,509
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #8 Steve Magnusson, Jan 8, 2024
    Last edited: Jan 8, 2024
    If your true firing order is: 1-7-4-10-2-8-6-12-3-9-5-11 (which does match a possible firing order for a 60 deg V12 per the cylinder numbering convention that you posted with opposite-side connecting rods being on the same crank throw),

    Then the spark plug wires going around the two-level dist cap (with a double-ended rotor) in the direction of dist rotor rotation would be: 1-12-4-9-2-11-6-7-3-10-5-8
     
  9. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #9 JohnMH, Jan 9, 2024
    Maybe I am too thick, can this be expressed as a formula (ie order of firing + 6), or do you just need to diagram it out using the order of rotation?
     
  10. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,509
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #10 Steve Magnusson, Jan 9, 2024
    Yes, sort of, but it's more "shift all the cylinders in the bank without cyl #1 by 6 positions" (the cylinders in the bank with cyl #1 stays in the same places).
     
  11. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #11 JohnMH, Jan 9, 2024
    Ah, I think I get it. Actually more complex than I thought. So each row in the cap serves a different bank.
     
  12. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,509
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #12 Steve Magnusson, Jan 9, 2024
    Correct.
     
  13. tbakowsky

    tbakowsky F1 World Champ
    Consultant Professional Ferrari Technician

    Sep 18, 2002
    19,827
    The Cold North
    Full Name:
    Tom
    #13 tbakowsky, Jan 11, 2024
    Follow the firing order. Note distributor rotor direction of rotation. Start with cylinder #1

    You can look inside the cap and visualize what terminal will fire next based on rotor position. Install the next wire in the firing order to the corresponding terminal pole and just keep going around like that..keep adding the wires in the same direction the rotor spins..and it should all fall into place. Lambo, Ferrari, Jag all used the same cap and rotor design at one time or another.
     
  14. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,778
    Dubai / Bologna
    #14 JohnMH, Jan 11, 2024
    Thanks. The Diablo should run better with a new cap, rotor, wires and plugs. The cap and rotor are Jaguar parts, but unlike the OEM item, are without any tiny 1.5mm vent holes drilled in the cap. To prevent moisture and corrosion, I understand I need to drill a hole or two.
     

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