Spark Plug Gap...

[Steptoe]

I look. If there's a gap, i fit it.

These are very basic engines and just need a spark, it doesn't care or take any notice and will run just the same no matter how much you paid for the spark plug.

And if you worry about the precise gaps in your old low tech engine you have no idea how a twin/multi electrode spark plug works.

 

[uncle dick]

If it's a basic engine which just needs a spark then why did they go to the trouble of fitting twin spark on the later ones? The truth is, the 1100 / 1150 boxer engine is fairly primitive and has always struggled to cope with running on an incredibly lean fuelling regime.

A more 'modern' engine will have improved swirl and mixture distribution by a variety of methods (VTEC etc) which enables it to run on a leaner mixture. With our fine examples of German engineering our only hope is to have the best spark we can provide.

If you don't believe me then try setting your mixture to 0.5% CO and see how far you get. Then do it with different plugs and/or gaps and see what difference it makes. You'd very likely find that opening the gap up to around 1.5mm or more would enable the weakest mixture setting but would complain of misfiring under load when out on the road. This is due to the improved cylinder filling which then requires a greater voltage to jump the gap. All very well until it finds an easier place to jump to.

Some very interesting work done by David Vizard in the 80's on all this stuff. Well worth a read if you're so inclined.

 

[STUBBS]

thumb nail

 

[Steptoe]

If it's a basic engine which just needs a spark then why did they go to the trouble of fitting twin spark on the later ones? .

EU emmisions test.
Same old swirl and mixture pattern. But now with an extra electrode stuck in the chamber in the only space available to fit it.

And multi electrodes do away with widening gaps, as no matter how many electrodes a spark plug has it only sparks on one of them, the nearest one. Hence once a gap gets larger the spark then transfers to the smaller gapped electrode.
It pings and pangs about the electrodes keeping wear to a minimum while the gap stays as close to the manufactureres specification until it's time to change the plugs at the manufacturers recommended time.

It's why modern spark plugs have such a long life compared to old plugs. Mostly for longevity of a certain guaranteed performance level, not outright performance.

 

[uncle dick]

EU emmisions test.
Same old swirl and mixture pattern. But now with an extra electrode stuck in the chamber in the only space available to fit it.

Precisely.

The 'simple' spark wasn't enough to burn all the fuel kicking around in the combustion chamber. Unburnt fuel = emissions.

 

[Steptoe]

Precisely.

The 'simple' spark wasn't enough to burn all the fuel kicking around in the combustion chamber. Unburnt fuel = emissions.

A bigger factor was the fuel mapping at the same engine speed as the emmisions test.
Which caused the surging, and which still wasn't cured by the extra spark plug being crammed into an already over crowded head .