Spark Plug Gap

Bob Bidonde

Bob Bidonde

Well-known member
Generally, the higher a compression ratio is, the more current it takes to make a spark at a spark plug. Thus if the compression ratio is increased, the ignition coil needs to produce a higher current.
So if a hi-compression cylinder head is run on a Model "A" or "B" engine, should the spark plug gap be reduced to accommodate the existing ignition coil's performance?

If so,what plug gap is appropriate for a 5.5:1 cylinder head?
What plug gaps is appropriate for a 6:1 cylinder head?
 
Actually it is the voltage that would need to increase to jump the higher chamber pressure
In reality moving from stock to 6:1 the difference is very small

I run 14:1 in the Bville car on 116 octane and gaps up to 035
Not necessarily comparable but you get the drift

J
 
You could reduce the gap to 0.030, but John is correct that a higher voltage is the way to go. Get a hot coil. Drag race engine run a large gap and very high voltage. The spark plugs are replaced after each run because there is nothing left of the electrodes.
 
Good evening Bob,


Your assumption is correct.



Instead of a smaller electrode gap, which reduces the spark distance and therefore unfortunately also the amount of gas molecules that cause initial inflammation, it is better to use spark plugs with a pointed center electrode (chk: 'Iridium').

This type of spark plugs requires a lower ignition voltage and delivers a hotter spark.
 
I don't have it at hand right now, but somewhere in the '32-'37 service bulletins Ford does recommend decreasing the plug gap to .025 for higher compression heads. Having said that, I agree with the others that you should run the largest gap you can that doesn't cause misfires. If you can run at .035 or .040 without ill effect, do so.
 
Good afternoon all...I have a 1931 Tudor with a 5.9 head and a strong engine by John Cook in Chandler, Arizona. For years I have run W16Y plugs and lately they have started to miss on excelleration. I put in an old set of Autolite 3076 plugs that may have come with the car 18 years ago and she runs back to normal...leads me to ask: What Plugs are most of us running these days? The W16Y plugs ran a bit foul and needed to be cleaned fairly often...Prehaps not the correct plug...just my sense of things. Just want to see what you guys are running. Thanks, Ernie in Arizona
 
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