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YICS & Spark Plug Indexing

Discussion in 'XJ Technical Chat' started by mdee, Aug 28, 2010.

  1. mdee

    mdee Member

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    Kidding aside regarding worthiness of plug indexing.
    It occurred to me, in my random thoughts, that the swirl of air/fuel mix into combustion chamber created by YICS has an aspect of principle similar to the purpose of plug indexing. i.e. Designed to obtain a more controlled & complete burn.
    I think YICS would virtually negate any (supposed) effect of plug indexing.
     
  2. Polock

    Polock Well-Known Member

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    here's something to ponder, when i pull a plug it always seems to have a lighter side and a darker side on the porcelain, if it's running kind of lean it might just have a dark streak on it.
    if you were to index the plugs you could move that streak, where would be be best place to have it, i got no idea, i never bothered to figure out if it's by the intake or exhaust valve but it's there and it must mean something
    the closest i came to indexing plugs was mark where the electrode was and put them in, one came pretty close, then i went for a ride
     
  3. Phyrekracker

    Phyrekracker New Member

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    I noticed that too the other day when I pulled the plugs on my bike. I had just assumed that this was because the plugs enter the top of the cylinder at an angle and the dirty side was the side facing the cylinder. The reason I would assume is that there is not enough opportunity for the air/fuel mix to get to the back side of the plug and deposit the residue or the little space is just not exposed to the combustion.
     
  4. Polock

    Polock Well-Known Member

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    here's where the dark streaks line up in the head, marked with a red dot
    this is like pondering the meaning of life :)
     
  5. mdee

    mdee Member

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    I find deposits concentrated on one side on some of my plugs. Don’t know what the deposits are. It never occurred to me to use plug indexing to help determine which direction the deposits face with plug installed.
    So I guess a part of plug indexing technique has some value in diagnostics.
    Now I gotta try this !
     
  6. mlew

    mlew Well-Known Member

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    I index the plugs on all my bikes. I seem to have a smother idle after doing it. A pack of indexing washers is cheap and I have nothing better to do.
     
  7. mdee

    mdee Member

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    mlew – I respect empirical results.
    What direction do you face the plugs? e.g. which direction is the ground electrode?
    Are you getting those results on a bike that has YICS?
     
  8. padre

    padre Member

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    Well, if you use extended tip plugs like bp7es, theres precious little space between the intake valve and the spark plug so you need to screw the plugs in so the ground arch, (thats the tang that goes from the main body of the plug and arches over the tip) is as far away from the valve as you can. If you look down in the spark plug holes next time you cange plugs you'll probably see at least one intake valve. The incomming charge has a tendency to cool the side of the plug the intake valve is on and the arch blocks flame travel so that side has the the tendency to run cooler also. (This is all weird science and no matter what you can't make it perfect.)

    So; I stack spark plug washers so the arch is more towards the center of the combustion chamber, counting on the incoming charge to cool off the the flame front side and the plugs burn more evenly.
    Extended tip plugs are risky because the tip of the plug is really close to the intake valve, my bike goes faster with them but I risk burning a valve every time I do. NGK B is the diameter of the threads, r means resistor, p projected tip, 7 heat range (theyre numbered backwards. 8 is cooler than 7) and I think es stands for length of the threads and gap.

    Now that we'vebeaten that to death does anybody have any really cool pictures to post...PLEASE!!
     
  9. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    I index my plugs too. Here's the theory I got from Hot Rod magazines.
    The exhaust valve is glowing hot, so you would want that heat to get into the spark gap.
    The incoming charge comes from the intake valve, and you don't want to "shroud" the plug with the ground strap.
    The plug gets the air/fuel charge when the piston rises, so point the open end towards the piston.

    These 3 theories would tend to contradict themselves.
    There's only a (theoretical) 1% gain anyway.
     
  10. mdee

    mdee Member

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    Here’s a question while we are on the subject of diminishing returns.
    What about the affect of lowered compression due to the increased chamber size with the spark plug raised the thickness of the indexing washers?
    Hey, maybe it compensates for those carbon deposits on the piston?
     
  11. mdee

    mdee Member

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    It does make sense to me that the intensity and angle of attack of the ignition source (spark) would play a role in the efficiency of the combustion, especially in the time scale of 4-9K+ rpm.
     
  12. padre

    padre Member

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    It's probably tit for tat, I mean it will lower the comp:ratio a little but provide a little more open area for flame travel. You'd probably gain more by blocking the YICS chamber to tell you the truth. I haven't tried that one yet but if it did much I'm sure we'd have heard about it by now.
     

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