Spark Plug Gap For Hi-Compression

[Bob Bidonde]

The Ford Service Bulletin of August 1932, Page 44 has a chart that specifies spark plug gaps for various cylinder heads:
A-6050-A Cylinder Head (Stock 2.2:1), 75 Lbs Compression, 0.035" Gap;
A-6050-B Cylinder Head (Police Head), 110 Lbs Compression, 0.025" Gap.

Many of us run 5.5:1 & 6:1 hi-compression cylinder heads, so is it necessary / good practice to decrease the spark plug gap to 0.025"?

It appears that the physics of the situation is that a smaller spark plug gap decreases the resistance to current flow across the gap in a higher resistance environment while the ignition coil's output voltage remains the same. A case of Ohm's Law V=IR & energy conservation due to the coil's unchanged output voltage.

What is your practice & rational?

I am running a spark plug gap of 0.035" with a 6:1 head and a modern 12V coil. I also have the timing set at TDC with the spark lever halfway down the quadrant to get more retarding of the ignition timing necessitated by the hi-compression head. The engine starts smartly, the engine.s performance is excellent & the gas mileage is much improved (22MPH highway).

 

[alexiskai]

Plug gap should be as wide as possible, but no wider. In other words, the maximum width that your engine can handle without misfiring under load. The exact width will vary per setup.

 

[Jim Brierley]

I use 032" on my 7:1 head. A wide gap gives a bigger spark, better to ignite the mixture.

 

[nkaminar]

Depends on the coil. A very hot coil will jump a wide gap. Dragsters run a wide gap and a very very hot ignition setup. The plugs are thrown away after each race because the electrodes are gone. I run 0.035 with a hot coil with my 6.5 head.