Spark plug gap?
I've seen 1mm mentioned as a size.

Some people seem to give the impression that having a small gap is better, just about nobody thinks having a big gap is good.

So is having a small gap and many amps / cm3 better? or is having a big gap good as long as you have the juice to jump it?

turbo cars use small gaps, because the high pressure can make the air so dense the spark won't jump. I also heard that the smaller gap can help with high rpm missing.

is there too small of gap?

do sparkplug heat ranges matter if your fuel doesn't deposit carbon on the plugs?

The whole point of buying those expensive MSD units is to run bigger gaps. Bigger gaps fire a wider ratios of a/f mixtures. If you don't have the spark power though you better gap it narrow and make sure the a/f ratio is not lean.

Anyways, try this, if you have an A12 in perfect tune:
1. Gap your plugs at the factory recommended 0.030 inch. Test drive, rev it up on the road
2. Re-gap at 0.035 inch. Now compare. I think you'll find the engine runs smoother with the bigger gap.
Anyways, that's what it seems like to me.

I was just wondering, I'm running something like 0.050 inch ( i think... whatever the big block american cars run with HEI ) as I basically have an HEI setup, never misses at any rpm.

I just start thinking and get to wondering.

must really light it off with that big gap and vaporous fuel.

Sometimes I start to wonder what a 25 or 50 shot of n20 would do for things... I'm serious, have it start misting at 3/4 throttle... i bet it would be fun.

Quote:
1. I do. I prefer to run the largest practical gap that will work. So too does the factory. When the series 2 Bluebird switched form points to electronic ignition [Hi Energy Ignition] the plug gap was opened up to 44 thou [1.1mm] despite the increase in compression ratio.

2. In a normal working ignition, the coil continues to build voltage untill it is able to overcome the resistance of the plug gap in the dense environment of the combustion chamber.
At idle, this resistance is lowest & the voltage requirement is lower. while at full throttle, the compression pressure is highest & so is the voltage requirement. A super duper coil is of no advantage if the plug gap is small & a regular coil is capable of providing enough voltage to fire the plug.

3. If the high RPM missfire is caused by
A. Current finding a path to earth of lesser resistance than the plug gap. [crook cap, etc]
B. Excessive plug gap.
C. Coil with insufficient capacity to build enough voltage to do the job.
Then reducing the gap, & thereby the resistance at the plug, will help in most cases.

4. I guess that there will be too small a gap at some point. As the gap closes, the voltage required to jump the gap becomes progressively less, & this spark becomes shorter & weaker. There must, I presume, be a point where it no longer does the job.
I guess you could look at it with this somewhat silly comparison.
Would you prefer to start a campfire with one feeble match, or a blowtorch?

Our combustion chamber 'campfire' needs to be given a GOOD start to ensure full & complete combustion, so a good strong ignition system that makes a good fat flame across the plug gap is just the ticket. To get these big fat flames, we need the high voltages that wider gaps produce when good ignition systems & components are used .
The leaner mixtures of smog controlled engines became hard to ignite, so manufacturers developed better ignitions to fire them & these systems are a good thing for all high performance engines.

5. Plug heat ranges always matter. The plug needs to be hot enough to burn off most carbon deposits, but not so hot that it does itself harm or starts to glow, causing preignition & detonation.

Plug makers catalogues usually have pretty pictures of used plugs with problems related to hear range in the back pages, & this should be used as a guide for proper plug selection. You can probably find this stuff online too.

If you have a high performance engine, have a think about the plug makers premium range. For NGK for example, this is their Irridium plug range. The Irridium is used for the center electrode & is able to be drawn to a finer point at the tip. It was discovered that the spark will jump a given gap, at a given compression pressure at a lower voltage if the tip of the center electrode is much smaller. They also discovered that normal operation eats it up very quickly, so exotic materials like Platinum & Irridium are used to overcome this problem. This then often results in easier starting, & better running, & if the gap is opened up a little, an even longer spark to light our combustion chamber 'campfire' with.

While these premium plugs cost more, there are many who believe this to be a small price to pay for the much longer service life & incrased spark plug performance.

I'll have to take the plugs out sometime and see what burning nothing but propane does for them. Maybe time to invest in some of the good KGK plugs finally.

I don't have any ignition issues, I just like to understand what i've done. My ignition can fire a 12:1 8cyl to 8000 rpm at least with a 50 thou gap, so it will have no problem here.

re: heat range, say i take the plug out and it's not dirty. any need to do anything? LPG doesn't leave carbon, so I would expect it to be very clean.

next addition - lpg lambda control.

LPG is an area that I have no experience with, so I am unable to help there, but you are right about how clean it burns.
I guess that you would need to compare your plugs against the pictures in the plug makers catalogue/ website to look for signs of overheating or 'other' & be guided by their recomendations.