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Why are spark plug gaps important?
That spark can jump half an inch!
by Jud Hildebrant

It seems odd, doesn't it? Auto manufacturers specify
spark plug gaps of .035, .045, .055, but if you've ever
tested for spark by bringing the distributor end of the
coil wire near the engine, you've seen the spark jump
MUCH further than that.

A visible spark is a stream of free electrons coursing
through air. Air is typically considered to be an
insulator (a poor conductor of electricity). When
electrons flow through air, they do not connect with
the various molecules in that air, instead they bounce
around and bang against those molecules, essentially
having to muscle their way through. In a car engine,
when a piston is approaching Top Dead Center on the
compression stroke, the pressure in that cylinder
rises sharply, often to 100 PSI or more. Then the
spark occurs, igniting the air/fuel mixture around it,
and KERPOW, we are generating power. That spark
had to occur while the surrounding atmosphere
(fuel/air mixture) was tightly compressed. A
compressed gas, simply stated, is gas molecules
jammed closely together. And the spark energy had
to muscle its' way through them!

The specified gap for spark plugs, then, is a
calculation based on an engines' peak pressure
in the cylinders verses the amount of electrical
energy available to jump that gap under pressure.

ENGINE BUILDERS: Here's a tip for you. If you
are dealing with much-modified engines, which
include a high-powered ignition system, the original
manufacturers spark gap specification probably
doesn't mean much to you, so how should you gap
the plugs?

This method will provide you with a very good
approximation. Have someone crank the engine,
while you hold the end of the coil wire close to a
grounded spot. Using a wooden ruler, (and wearing
gloves), measure exactly how far you can draw a
visible spark. Then divide that number by the first
figure in the engines' compression ratio. For example,
if you can get a visible spark .600" long, and the
compression ratio is 9:1, then .600 / 9 = .064 .

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