About spark plug gap

[Oldgearz]

Just got my first Model A and am enjoying working on it. I ordered new spark plugs (W-18) and set the gap at .035". This got me wondering about spark plug gap. I am very experienced (shade tree style) at working on my five Ford 8N tractors (1947-1951), and have changed many plugs in them They have the Ford 4 cylinder engine and their plug gap is .025-.028, quite smaller than the A's .035. For those of you who understand Ford 4 cylinder engines better than me, why would there be such a large difference in plug gap for the A. All the W-18s came with a gap of about .025. Why the difference in gap needs? The tractors are also 6V positive ground.

[Patrick L.]

Its all about available voltage. The old tractors should be able to handle the wider gap. And I think Ford was still trying to figure things out back then.

A wider gap allows for better/smoother low speed operation, but, is more susceptible to high speed missfire.

[Bob Bidonde]

The spark plug gap is set on the basis of:
> The amount of hi-voltage available to jump the gaps in the distributor & spark plugs;

> Volumetric efficiency of the engine's intake system;
> Compression ratio;
> Combustion flame development in the cylinders;

> Density of the fuel in the cylinders;
> Power the engine makes.

A rule of thumb is that the more power an engine makes, the smaller the smaller the total gap needs to be (rotor to distributor + spark [plug gap).

[The Master Cylinder]

Originally Posted by Bob Bidonde

The spark plug gap is set on the basis of:
> The amount of hi-voltage available to jump the gaps in the distributor & spark plugs;

> Volumetric efficiency of the engine's intake system;
> Compression ratio;
> Combustion flame development in the cylinders;

> Density of the fuel in the cylinders;
> Power the engine makes.

A rule of thumb is that the more power an engine makes, the smaller the smaller the total gap needs to be (rotor to distributor + spark [plug gap).

In laymen's terms, The Model A engine is an inefficient design that needs a big spark to light the fire.

[redmodelt]

The gap maybe larger then you think. Add in the .025 at the distributor and makes it about the same as my S10 4 cylinder, around .060.

[Oldgearz]

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Bob's answer is the tech talk I need to understand. I knew somebody out there would have the info. Thanks to you all for helping the rest of us nubies learn.

[40 Deluxe]

Originally Posted by redmodelt

The gap maybe larger then you think. Add in the .025 at the distributor and makes it about the same as my S10 4 cylinder, around .060.

Well, not really. It's the larger gap that determines the voltage needed to fire the plug. The larger the gap (and the higher the compression pressure), the more voltage required. So the spark jumps that .025" rotor gap like it wasn't even there. Thus all the coil sees is the .035" plug gap.