If I'm using a spark tester with an adjustable gap set to the correct gap for my car's ignition system voltage, what should the spark look like?
I ask because I've seen conflicting opinions on this matter. For example, this Brigg's and Stratton site says:
A bright blue spark is best. A yellow/orange spark signifies weak ignition. Not true. Spark color determines virtually nothing. The hottest spark is ultraviolet which we can't see. Blue spark is cold in comparison to ultra-violet. Orange and yellow come from particles of sodium in the air ionizing in the high energy of the spark gap.
However, they are talking about small air cooled engines, and are using a specific Briggs and Stratton spark tester with a fixed gap for small air cooled engines, the 19368 model.
Here's another person saying spark color doesn't matter:
I might note though that the very fast, very high voltage sparks in todays systems usually are not blue. Your mileage may vary.
On an automobile ignition system the spark should be bright blue. That's because the compression ratio is higher than that on an air cooled small engine. An automobile engine operates under a transient condition where the RMP's change so fuel demand changes. Where with most air cooled small engines they operate under a static condition or a steady state.
Compression ratio of an engine and the amount of fuel being delivered can have an effect on how well the spark plug fires. This is called quenching the spark. A low compression engine running at a steady state with a constant fuel supply won't quench even a spark plug with a yellow spark. But on an automotive engine with a higher compresssion ratio and under transient conditions the yellow spark can get quenched thereby causing a misfire.
Here's another guy saying basically the same thing:
The recommended 7/16" gap has always worked for me on outboard motors. I use mine in line between the spark plug installed normally in the engine and the plug wire. This means that the spark must jump the 7/16" gap and the spark plug gap in the motor while running. You can then also increase engine rpm to verify spark at higher speeds. My experience has shown a strong blue spark with a noticeable popping noise in a healthy ignition. The gap is determined by the amount of voltage generated by the ignition, spark plug gap, and the compression ratio. Most manuals will give an air gap recommendation for a spark test. An air/gas mixture at high pressure creates a much denser gap for the spark to jump then in out in the open.
I don't have the background to tell who's right and was wondering if someone more knowledgeable could give me some verification here.
As a side note 7/16th is about 1.1cm, and the breakdown voltage for air at sea level is about 30kv/cm. Also of note is this answer on the physics stack exchange which says that the blue color of air sparks comes from ionization of nitrogen atoms.
One other point of interest is that the power of light is inversely proportional to it's wavelength, meaning blue light is about 40% more powerful than orange light based on the ratio of their wavelengths.
In fact, someone else has basically asked a very similar question over on the physics stack exchange:
Has anyone had personal experience with seeing a yellow/orange spark, doing a repair and then seeing a nice strong blue spark with a disappearance of the performance problem?