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Spark Plug Extenders ... Opinion

Discussion in '308/328' started by 76Steel, Dec 30, 2008.

  1. dinonz308

    dinonz308 Formula Junior
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    Just out of interest, at what point do you consider it "high" and thus open? 20k? 200k?
    Speaking as another EE.
     
  2. thorn

    thorn Formula 3
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    For myself : if the DMM doesn't read 0/OL, it's not open.
     
  3. dinonz308

    dinonz308 Formula Junior
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    True - but, in relation to spark plugs where an air gap with a given resistance needs to be bridged with a spark, once a lead goes over a certain limit that spark will no longer occur, and is akin to "open". Picture a firefighters firehose that can only shoot water 5 feet, and he's 50 foot from the fire. Technically to you the hose is not "delivering no water" but to him it is. I think this is the perception here - that at some point high resistance will prevent spark, despite not being technically open.
     
  4. yelcab

    yelcab F1 Veteran
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    The air gap in the spark plug would read infinity when measured with an ohm meter at DC. However, when 50,000 volts is applied across the airgap, there is a spark jumping across delivering a current through it. The tiny little airgap in the broken ignition wires would also be infinite resistance when measured with an Ohm meter at DC (zero frequency). But the 50,000 volts will jump across that airgap just the same. There will be spark.
     
  5. miketuason

    miketuason F1 World Champ
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    Isn’t air (gap) conductive?
     
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  6. yelcab

    yelcab F1 Veteran
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    No.
     
  7. miketuason

    miketuason F1 World Champ
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    #57 miketuason, Feb 15, 2020
    Last edited: Feb 15, 2020
    I think it is in a way.

    When the electric field is high enough, the air partially ionizes, at which point there are free electrons to carry current and the air becomes, essentially, conductive. Air does not conduct electricity in the way that metals do. ... Small voltages move the electrons and a current can flow. Also air has moisture and water is conductive.
     
  8. yelcab

    yelcab F1 Veteran
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    Technically yes, that is how lightning takes place. But, for the purpose of the measurement at DC using an ohm meter (which is what we are talking about here), the air gap has infinite resistance.

    So just because you pick up an ignition wire and measure infinite resistance across its 2 ends, does not mean that a spark will not jump across and provide ignition.
     
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  9. miketuason

    miketuason F1 World Champ
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  10. johnk...

    johnk... F1 Veteran
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    It's really a mixed bag. On one hand, resistance in the secondary limits the current across the spark gap which maintains the spark longer. But how much is too much? Carbon core wires run about 10k ohms/ft or more. As I recall, the Cavis OEM wires on a 308 QV ran about 300/ft. The Beldon spiral wound I replace the OEMs with was about 200/ft.

    Resistance and open circuits are two different thing. To start with, on a distributor car you normally have two points of open circuit in the secondary ignition path, the plug gap and the air gap between the rotor and the contact in the distributor cap. But there can be more. When I replaced my wires it was due to a misfire on one cylinder. But when I ohmed the wires, about 1/2 measured open. Still, only that one cylinder missed and that was not all the time. As I recall, it only started to miss above about 3000 RPM. After replacing the wires I probed the old ones at several points along their length. I was surprised to find that in the wire connected to the misfiring cylinder there were 3, count them, 1, 2, 3, internal breaks.

    Air gaps don't have a resistance. They have a breakdown voltage. If the voltage is high enough, a plasma is formed in the gap, which is highly conductive, and the plasma will last as long as there is sufficient energy in the coil. The current draw across the gap and the charge level of the coil determine how long the spark lasts. Additionally, the breakdown voltage depends on the level of compression and whether there is fuel in the cylinder and how well it is atomized. These and other ignition issues can be diagnosed by looking at the secondary (high voltage) ignition wave form on a scope.
     
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  11. mike996

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    FWIW, resistance in the secondary wiring is about reducing RFI. In the old days it was ONLY about eliminating interference with the radio. I'm sure there are folks here who changed the old, stock resistance plug wires to find the car's radio became useless due to static...as did the radio of a car stopped next to you at a traffic light. :)

    SInce computers began controlling engines, resistance in the secondary is more important. RFI from non-resistance wiring/plugs can cause major problems with the ECU. This is one of the reasons for individual small coils for each cylinder as opposed to one large "traditional" spark coil - less RFI emissions to affect the ECUs/other computer control systems common on modern cars.

    A more or less "standard" for plug wire resistance is 7k Ohms/foot. SO...each plug wire has a different resistance based on it's length. This is another reason for individual coils - consistency of spark output per cylinder, less chance of a misfire due to one or two wires carbon-tracking or deteriorating.
     
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  12. johnk...

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    Yes, it's about reducing RFI. But the number you quote is for carbon core wires. Spiral wound wire offer much lower resistance while being superior in RFI reduction due to the spiral winding of the conductor.
     
  13. mike996

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    Yes, absolutely right, I should have mentioned that the figure was for conventional carbon core. Of course, the resistance of the wire itself is of no consequence AS LONG AS the spark coil(s) can provide sufficient power to jump the gap in the spark plug under full load. If the spark can jump the gap with 15,000 volts, having a 40,000 volt coil does nothing at all. OTOH, the more powerful coil DOES provide some insurance that as the plug gap widens or any resistance builds up in the circuit, there will still be sufficient power to light the mixture. Also, in many performance applications, a higher output coil is coupled with a wider-than-normal spark plug gap which may be helpful lighting the mixture in higher compression/modified engines.
     
  14. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Two Time F1 World Champ
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    You mean I'm not going to get that 20 extra hp by bolting on a MSD???????
     
  15. miketuason

    miketuason F1 World Champ
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    You will but only up to 3k-4K rpm after that no more
     
  16. Rifledriver

    Rifledriver Two Time F1 World Champ

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    And Magnetti Marelli systems have been very sensitive to RFI, particularly the Microplex. Carbon wires have created a lot of problems with them.
     
  17. mike996

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    LOL!

    MSD's products are top quality. Their marketing is even better! ;)
     
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  18. Lawrence Coppari

    Lawrence Coppari Formula 3

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    I can attest to problem wires. About 6 or 7 years ago I'd be driving along just fine in my '87 328GTS. Then I'd get an instantaneous loss of power, like a hiccup. Following that car would still run fine until I stopped and noticed the idle would be at 1500 instead of the normal 800-900. I would shut the engine off, restart it and the idle would be fine for a while. Then the same thing would happen, hiccup and all. This problem was completely repeatable but the time it would take for the hiccup and the idle jump to occur grew less and less. This problem went on for probably two months. I was at wits end.

    I posted the problem here on Ferrarichat. Rifledriver said it was my spark plug wires. Mine were original so I made up a new set. Bingo, the problem vanished and never returned. We are fortunate to have knowledgeable and helpful people here.
     
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  19. BigTex

    BigTex Seven Time F1 World Champ
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    MSDs are what they say, a rapid fire of multiple spark fronts...….JRV used to refer to it as "Using a flame thrower, to light your teaspoon of gasoline."

    To proof out this discussion one of my cars had ALL the extenders deleted and plug wire run straight to the plug,
    A quick release clip, uninsulated, worked great for conductivity, the radio, no not at all, now that you mention it!!

    If I tracked a misfire to a cylinder that had a perfect looking extender, I would assume an internal fault and try a new one.
     
  20. ferrariowner

    ferrariowner Formula Junior

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    Yes, air can be a conductor. (witness a lightning strike during a thunderstorm) A number of factors determine 'air' conductivity such as humidity, pressure and other factors. Most importantly is potential.
     
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  21. redqv

    redqv Karting

    Jan 26, 2015
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    So back to the pin hole discussion. Before I put the QV away for the winter, it had an occasional miss which I thought might have been caused by old gas. But after reading this thread, I went out and pulled a few extenders off and a few do have what I think is a pin hole. But its an awfully small hole and doesn't seem to go all the way through. Could this really be the problem? Just wondering before I order new ones.
     
  22. mike996

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    If you look closely, a carbon track or a carbon tracked hole will have a hard carbon deposit that you can see - maybe with the help of a magnifying glass. So if it is a carbon track and not just a pinhole in the material, you will be able to see the brownish "carbon." It will be a hard deposit, not something you can wipe off. The pinhole on its own is not a problem UNLESS it is carbon tracked.

    FWIW, the classic shade tree "test" for secondary ignition is to have the car in a dark place with the hood up and start the engine it looking for (usually) blue random sparks . Sometimes it's quite a light show. If there are none, spritz the wiring with water from a spray bottle and repeat. Still none? System is in good shape! The problem with the Ferrari is that the plug that covers the hole in the valve cover hides the extender and top of the spark plug so you can't see what's going on there... :(
     
  23. redqv

    redqv Karting

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    Thank You! Yes I used a magnifying glass and there is one suspect extender. But I don’t think it’s the problem. The rest seem perfectly ok. So I guess I’ll pump the gas out, or maybe just wait and see the miss disappeared over the winter. Fat chance that would happen. By the way all are black on an ‘85.


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  24. Austin Newman

    Austin Newman Karting

    Dec 18, 2019
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    Well the red ones for the QV i snagged were about a 1/2 inch longer then the ones in the car. All hooked up and seems to idle great!


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