Spark plug gap

[Mike Wilson]

Thanks for that info, Jon. I'm going to try the 6's with both .028 and .030 gaps to see how the engine responds.

Mike

[Alan Hall]

for what it is worth, Bosch says this about plug gap:

The smaller the gap, the lower the ignition voltage which is required. If the gap is too small though, the voltage reserve is high, but misfiring may occur because the spark gap is not readily accessible to the mixture or because too little of the mixture is reached by the spark. If the gap is too large, a high ignition voltage is required. This means a low voltage reserve. Although the mixture has good access to the spark gap, there is also a great danger of misfiring. The gap is usually about 0.7 to 1.1 mm. The precise optimized gap for the individual engines are specified by the manufacturer...

Sounds like bigger gaps are better until they aren't!

[Ron LaDow]

Double post

[Ron LaDow]

Sounds like bigger gaps are better until they aren't!

Sorta like compression ratios; higher is better until it's just this much too high!
I used to set the gap on the tight side as it 'wears' loose, like setting the points on the loose side, as they 'wear' toward the tight side. Now, they get replaced for age before they wear out.
Sure wish I'd have run some tests when Alan's dyno was available, but it would probably prove the factory ran hours of dyno time to find the right gap. Except for the better gas; that might have shown a bias toward one or the other.
Added by edit: I did, at one time, test the highly advertised triple ground electrode plug; engine ran like crap and didn't get close to making the recorded power before we shut it down.

[Martin Benade]

How did it manage to perform poorly? Wouldn’t it just have fired on the smallest gap leaving the other two doing nothing? Dual electrodes are not uncommon and reputedly work, aircraft use them sometimes. Still only one spark at a time I imagine.

[Mike Wilson]

Thanks for weighing in, Guys. If I recall correctly, there is a range in the the recommended gap. I'll try setting the new plugs at .030, take the car out for exercise and then read the plugs. I'm running a 6V system so maybe .028 will be fine with the hotter plugs or try .029. It's all about what the engine run the best on.

Mike

[Ron LaDow]

How did it manage to perform poorly? Wouldn’t it just have fired on the smallest gap leaving the other two doing nothing? Dual electrodes are not uncommon and reputedly work, aircraft use them sometimes. Still only one spark at a time I imagine.

Disclaimer: I have zero theoretical knowledge regarding the effects and applications of spark plugs, and (so far) just enough practical knowledge to have avoided any real harm.
Probably have notes, but I'm too lazy to do a search this evening, or even much later; it was a test like the ones on the K&N filters and other random 'improvements'. Much BS promise, no gain, or in this case a loss.
At that time, the test would have been done on my 'mule' 1720 C, base-lined at ~78HP; those plugs had the curve laying down from (memory) 3KRPM and never recovered. You can bet we both took a look at the plugs and did a confirmation run with the NGKs refitted.
They stunk. I don't know why.

[Martin Benade]

I am comfortable with Bosch or NGK- they work.

[Vic Skirmants]

Don't get silly with tiny .001" and .002" changes; ain't gonna be noticeable!

[Mike Wilson]

Thanks, Vic. Right you are...

Mike

[GregVandenbussche]

I run Bpr6hix iridium plug with factory gap, I was convinced when one day I switched back to new Bh6S and the car ran really bad, switched back to Bpr6hix right away and ran fine, checked cap and wires... but it turned out the coil was bad (but car was still running fine with the iridium plugs).

[Fred Winterburn]

In addition, with an inductive ignition such as the Kettering, a larger gap will result in a more intense spark (good) but a shorter duration than a smaller gap. The reduced duration doesn't matter because the trailing edge of the spark is too weak to be of any use anyway( in my opinion). With an inductive ignition the voltage is self regulating, so having a smaller plug gap means the voltage peak will be lower. The voltage peak includes the breakdown voltage of the gap plus the voltage overshoot that occurs which is considerable and must be factored in as far as insulating materials is concerned. 32 thou gaps are still quite conservative with the average coil and conditions in the cylinder.
With a CDI, the voltage is not self regulating so the coil will always be subjected to whatever voltage the discharge capacitor is charged to. A good CDI should have the voltage controlled to a safe maximum at the source since a CDI will give what it gives regardless of the plug gap or what is going on inside the cylinder prior to spark breakdown. The voltage overshoot with CDI is nearly constant and independent of the plug gap or conditions within the cylinder such as compression. The good thing is that a CDI will fire the equivalent spark gap with a lower voltage overshoot than an inductive ignition. But it also means that it is more important to keep the capacitor discharge voltage at a lower maximum value than with an inductive ignition.
The reason the voltage overshoot is higher with an inductive system is because of the phase angle. The current is lagging so far behind that the voltage must rise further to provide enough current to pre-ionize the plug gap. It also means that the voltage peak can be higher without punching through insulation since there is not as much current available at the voltage peak. CDI on the other hand, has current and voltage more in phase which makes a higher voltage peak more damaging, but it also means that it will fire plugs with a lower voltage overshoot (providing the CDI has its maximum voltage limited to a safe value which most don't do nowadays). The Classic Retrofit CDI + and mine are the only two I know of that are controlled to a reasonable level. Fred

for what it is worth, Bosch says this about plug gap:

The smaller the gap, the lower the ignition voltage which is required. If the gap is too small though, the voltage reserve is high, but misfiring may occur because the spark gap is not readily accessible to the mixture or because too little of the mixture is reached by the spark. If the gap is too large, a high ignition voltage is required. This means a low voltage reserve. Although the mixture has good access to the spark gap, there is also a great danger of misfiring. The gap is usually about 0.7 to 1.1 mm. The precise optimized gap for the individual engines are specified by the manufacturer...

Sounds like bigger gaps are better until they aren't!