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Classic Engines, Modern Fuel

Comments on Topic: plug gap

Submitted by Anonymous
23-Sep-2020

Hello,

It is often stated that feeding the plugs with a high voltage coil like the Lucas sport model allows to enlarge the plug gap till 0.030".

Would it be better for combustion process ?

Laurent.

Submitted by The Author
24-Sep-2020

Laurent,

This was on of the tests we did at Manchester. Have a look at page 105 and the diagram at the top of page 106. It will answer your question.

Paul

Submitted by Anonymous
24-Sep-2020

Thanks Paul,

I had already forgotten this paragraph... Very interesting because it kills some hard rumor.

But I'm now thinking of another effect than spark energy itself. If the gap is increased, the longer spark can ignite a greater volume of mixture or at a better place. Could this have a secondary effect ?

Laurent.

Submitted by The Author
25-Sep-2020

Laurent,

We did not examine the effect of the spark gap on performance, however, my thoughts are it will make no difference. The volume of the spark is negligible compared to the volume of the cylinder.

There is a advantage of having two spark plugs (see Twin Spark engines). These create two flame fronts at different parts of the cylinder. Hence the mixture burns more quickly.

As for different place for the electrode. I have tried longer nose spark plugs in the XPAG. If you look at the cylinder head with a standard plug, the electrode does not protrude into the spark chamber. It sits in the spark plug hole. A longer nose plug moves the elctrode into the combustion chamber. In my experience it made no noticeable difference and I even had problems with the engine not running as well.

When I inspected the electrodes you could see they were being "washed" by the inflowing petrol. This could have left them "wet" or covered in an overly rich mixture at the time of the spark. Neither is ideal. I imagine that with the standard plugs and the electrode in the spark plug hole, the inflowing mixture causes turbulence around the spark plug as it passes over the hole. This is what we want.

I am now back to using the standard plugs.

Regards

Paul

Submitted by Anonymous
25-Sep-2020

Thanks Paul.

Laurent.

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