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


Staff member
Nov 13, 2006
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 .

Everybody: Here's a food-for-thought quiz;
What is the electrical resistance across a spark gap in ordinary air, while the spark is occuring?

I'll give you the answer next week.

Bye for now,
Jud Hildebrant