Spark plug overgap-- effect on ignition system?

My 1949 Plymouth uses cables that have about .01 ohm/foot (copper wire). Big whoop. Ohms/foot are not a measure of performance- the system is designed to operate with a specified cable impedance and lowering it doesn't necessarily make the spark "hotter".

| Ohms/foot are not a measure of performance Prove it Shops here compare cables on ordinary ignition coils fed by 240v AC rms 50hz current, the biggest sparks come fr cables with the lowest resistance. Magnecor claims its 200 turns of wire on per inch of carbon core maximise spark size, but its site has no video to prove this.

| the system is designed to operate with a specified cable impedance You're confusing with tv signal cables' requirement : tv tuners use resonant circuits, cables must have a 75ohm impedance, so tuners can amplify the incoming signals' voltages. No resonant circuit exists between a distributor cap & spark plug.

The "biggest sparks" don't necessarily yield the best performance. What you see in free air at atmospheric pressure has practically zero relation to what goes on under compression squeeze in the combustion chamber, but those "bigger sparks" you like so much are very effective at getting people to spend money on "performance" ignition wires. There's a bigger/brighter spark between the electrodes of nonresistor spark plugs than between the electrodes of resistor spark plugs. There is no performance difference between the two types of plug, all other factors equal.

Of course it does.

The best _consumer_ performance....:)

Hi, Big spark and hot spark are two different things. Tony

Here is a hot spark

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs

You've missed it.

It occurs to me that if you have sufficient voltage capacity in the secondary ignition system, having the gap wider can force a large and hotter spark, everything else being equal. That's one reason for electronic HEI type ignition - lot's of voltage, big spark gap, very hot spark, and you can ignite unfavorable mixtures ie leaner, more efficient, higher compression.

A coil produces voltage up to the point required to create the spark, then it quits. Balancing the gap to the coil to the mixture is the key.

Next time I am on the dyno with a motor, I will take a monster coil and try to see if putting in a big gap creates a difference in ideal ignition advance or peak power. Any bets on what will happen? I have no idea - I've always been a proponent of the theory that if you got a spark and combustion, that was enough. I have one engine where I need to ideally run about 48 degrees advance, and playing with a big coil to get less timing would be a good thing.

Brian

Brian

Brian wrote in rec.autos.tech

The first thing you have to remember is that the gap acts like a resistor. Thus heat is generated according to the amount of voltage or current jumping the gap. And using a bigger coil, and and bigger gap is what the manufacturers are doing. Of course, there is a trade off there. The bigger coils mean a bigger voltage, but for a given current supplied then the current drops proportionately. And a bigger gap will help to limit the current.

One thing that is not stated is that a coil produces voltage on the output side by the voltage changing on the input side. A steady 12 volts will not produce anything on the output. (that is why you need more than a transformer to get 110v off the car). And the faster the change in voltage the higher the output. And this where the Capacative discharge ignitions come in. A high value capacitor coupled with some very powerful transistors trigger a very fast voltage into the coil. Giving you a higher output voltage, one that is capable of firing a very wide gap plug. The old Kawasaki Mach III (500 cc 2 stroke triple) had such a system. And it had the strangest spark plugs you have ever seen. They were nothing more than a spark plug with the center electrode cut even with the outer ring, and no riser off the ground. In other words, no gap to set. Just put new plugs in when needed, and that was not very often. That ignition system was great, no problems with fouled plugs, no problems with misfires.

Go ahead, but I think you will find that one of the High Energy type of ignition systems will be what you need, it will include, I am sure, a bigger coil.

Standard marine-type surface-gap plugs, nothing fancy there.I've still got a box of Champion L78Vs lying around my office. I used them as the triggers in a spark-gap closing switch system I built a few years ago- they work great in that application. The down side to that type of plug in an engine is that the arc produced doesn't penetrate very deep into the combustion chamber, and isn't exposed to much of the mixture. In fact, it forms right on the surface of the insulating ring between the center electrode and the annular ground electrode and relies entirely on the IxB force created by the current in the spark interacting with its own magnetic field in order to lift the spark off the surface of the ceramic and into the open part of the combustion chamber, and by the time that happens the spark is dissipating. They're pretty crappy in terms of emissions and misfire rate for that reason, BUT they don't foul easily, which is why they're so commonly used in 2-stroke engines like big outboards.

I have tried this with a SBC on a dyno. The timing will advance as you narrow the gap. Why? Less voltage required to jump a smaller gap. one unexpected problem with this is on newer engines with knock sensors. If you have worn plugs in the engine and only replace one due to a failure, the knock sensor will keep retarding the timing because it thinks the new plug firing earlier is ping. One good reason to always replace them all at the same time.

Steve wrote in rec.autos.tech

That sounds vaguely familiar. Its' been thirty years since I owned that bike, so my memory is likely to have some holes. With a nice ignition system the plugs would work fine in a 2 stroke, especially one that was revving like my bike did. Strange side effect of that setup. Sometimes while coasting for a while (such as coming off a freeway onto a down hill offramp) you would get a very loud back fire. The cylinders would still get some fuel, even with the throttle closed, and sometimes it would build up in the exhaust header, where it would ignite, possibly from the ignition system. Scare the crap out of you. But it was normal behavior.

Sure, those, but also JC Whitney Fire Injectors (of "Run Your Car Without Spark Plugs!" fame...)