Why a different plug gap?

[Steve Jelf]

Ford recommended ¹⁄₃₂" (.03125"). Some other sources say .025". How come? Does it matter?

[John Codman]

The way I learned it is that you want the widest gap that the spark will reliably jump.

[Been Here Before]

I have always used narrow plug gaps. And this is why....

"Typical spark plug gap for the aviation plug is .016-in to .021-in., as opposed to as much as .050-in. for automotive type plugs. Why the big difference? The tighter gap of the aviation spark plug requires less voltage to arc across the gap, reducing the chance of misfire, and lower voltage means less electrode wear due to material transfer loss during the arc event. If operating stress is reduced, reliability is improved. "

https://www.aviationpros.com/home/artic ... spark-plug

[Henry K. Lee]

Way to many variables..., ie, compression ratio, contact material of the points, condenser rating, coil type and construction with output voltage. Stick with the recommendations by the vehicle specs, they did all the homework for you.

For what it is worth,

Hank

[Wayne Sheldon]

If the gap isn't wide enough, OR in some circumstances the spark not HOT enough (a case with many types of magnetos)? The ignition will be slow and not burn cleanly away rom the plug gap in turn resulting in fouling of the spark plug. So wider gaps help to keep the plugs cleaner and firing reliably.
However, the widest gap that will reliably fire is not always a desirable goal! The wider the gap, the more stress it puts on all other electrical components in the ignition circuits. Wider gaps cause the points to burn away more quickly, causes condensers to short out and fail, and can burn through the insulation inside the ignition coil! These damages may not show up for some time. However, even a few minutes of running with too much spark resistance might irreparably damage any or all those other components.
Many factors go into an optimum spark gap. Fuel quality, ambient and engine temperatures, ambient humidity, altitude, all make a slightly wider or narrower gap perform better in both long term and short term reliability.

With hundred year old coil windings, I tend to prefer a slightly reduced gap (about .0028) because I have no way of knowing how much the old coil windings may have been overly stressed in the past. I find I have very little trouble with plug fouling at that gapping. The slightly smaller gap probably works better on modern gasolines than it would have on the heavier stuff from seventy to eighty years ago?

Just as a point to ponder. A Bosch DU4 magneto is supposed to have the plugs gapped at .0018 to .0020. A rather small gap. The reason is that at very low rpm, like crank starting, the magneto output is too low to fire the wider gapped plug reliably (makes hand cranking really difficult!). However. as the rpm increases, the spark gets hotter and hotter! So much so, that even at normal idling speeds, the plugs will rarely foul on that narrow gap. At higher rpm, the spark against a wider gap, hotter still, can damage the magneto armature windings! (I did that once!)

[TXGOAT2]

As discussed above. the wider the gap, the higher the voltage required to fire the plug, all else being equal. The condition of the electrodes also affects firing voltage. Worn, eroded, rounded electrodes will require more voltage to fire, all else being equal, than "sharp", as-new electrodes. The required higher voltage to fire a wider gap is more likely to leak from a plug wire, coil secondary, or any other path it can find before reaching and jumping the plug gap. The advantages of a wider gap include a better chance of firing a weak fuel mixture, especially at light throttle. A stock Model T may not have enough available voltage to fire a plug with a wide set gap at higher compression pressures. (Wide open throttle) Modern engines typically develop 2 to 3 times the compression pressure of a Model T. Modern cars have higher voltage available to fire plugs and better insulation to contain the high voltage. They often run very lean mixtures to reduce emissions and conserve fuel. A wide plug gap and sufficient voltage to fire it reliabley are needed. For a T, I'd think it would be advisable to follow Ford's recommendation as regards plug gap, if you have a stock ignition system, stock engine, and period spark plugs. An engine run at high altitude might benefit from the maximum recommended gap. Running a plug gap of .025 to .028 might give better results at low altitudes, and might get better performance from an aging ignition system, or under very humid conditions. Using high ethanol blend gasoline might require a wider gap for best performance, and higher system voltage to support it. A narrower gap might make crank starting easier with a weak magneto.

[JohnH]

Weren't Champion X's original equipment for the Model T? If so, why does Champion say .025" and Ford says 1/32"? Seems like a contradiction here.
I just put my X's straight into the car without reducing the gap - they were about .035" and it has always run with good power and top speed.

[TXGOAT2]

.025 vs .030, does it matter? Probably not, under most conditions. A car with a less than excellent ignition system or a weak magneto will probably do better overall with a tighter gap. I think that .028 is a good setting for general use. The gap will get wider as the miles accumulate, and the condition of the electrodes will slowly deteriorate. Cars with oversize cylinders and shaved heads might do better at around .025, especially at low altitudes. Both the plug gap and the electrode condition are important. I don't think there is a magic number, any more than there is a perfect setting for the carburetor adjustment.

[TXGOAT2]

Why the difference in recommended gaps?
Speculation: Ford wanted best overall engine performance. Champion wanted best overall plug performance. A new Champion plug set at .025 could be expected to give better magneto starts and cold weather starts on older cars and could be expected to run more miles on any car without needing cleaned or re-gapped. The Ford recommendation would probably be just fine for a car in top shape in general service that received regular recommended maintenence. The Champion recommendation would probably be more realistic for a used car on its second or third set of plugs that might not get service until an overt problem developed, if then. Ford sold parts and service for the entire ignition system. Champion only sold spark plugs.