: Meli,
: A 1/4 inch gap?! What are you talking about? A spark plug gap is only about .040" at the most.

Pay Attention!!! Well...
    Steve at Mainely magnetos says they don't let a mag out of their shop unless the spark can jump a 7mm gap. Here's what Steve says:

    "7 millimeters = 0.275590551 inches to be exact. This is important because I talk to so many people that say their mag sparks but won't run the engine. They just lay the plug on the block and see it jump the plug gap and think that is good. That's because, yes, it sparks in the outside air but won't jump the gap under compression. It takes about 6 times as much voltage to jump the same gap when under compression."

: Okay, so, WHY??

Pay Attention!!! That's just what we asked Steve and he sent us this quote:..

    "Spark discharge is a breakdown in the air insulator between the electrode and ground. Yes - air is an insulator, a pretty good one at that. As voltage builds at the electrode, atoms in the air within the air gap gain electrons. This process is called ionization. Ionization turns the air gap into a low resistance path between conductors. When a spark plug fires under compression, the quality of the spark will be very different from when it fires under ambient pressure. Barometric (air) pressure at sea level is approximately 30 lbs. per square inch. If cylinder compression is 75 psi (measured with a compression gauge), this is actually 75 psi greater than ambient pressure because the compression gauge is zeroed at ambient pressure. It is much more difficult for a spark to jump the gap under higher pressure, because higher pressure makes the air a better insulator. A plug may throw a strong spark when held against the side of a cylinder for testing, but when installed in the cylinder and subjected to an additional 75 lbs. of pressure it may not."

: I STILL don't get it. It's just AIR in the gap in BOTH cases.

Pay Attention!!! Yes, but there is MORE of it! MORE molocules of insulation. Look at it this way, if you put a piece of paper in the spark plug gap, the spark would jump right through it. But, if you filled the gap with ten sheets of paper, the spark would not make it. More paper, MORE insulation. More air, MORE insulation.


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