Spark Plug tech - how to increase the gap?

[BLSTIC]

So once upon a time I side-gapped the (copper-core equivalent) plugs on my 2NZ Toyota Echo while searching for a load-dependent misfire that turned out to be fuel pressure. Surprisingly it did *something*, in that it was able to handle marginally higher loads (and produce equally marginally more torque at lower loads) before misfire, implying that I had indeed improved spark ignition. Eventually I tracked down the problem, and the plugs stayed in there until it started misfiring again. It turns out that they wore out to a 1.5mm gap before misfiring.

Now I *thought* 1.5mm was guaranteed bad idea on anything, but I have 18 year old coils firing a 1.5mm gap with 10.5:1 compression, and they only just began to misfire. Meaning that at 1.495mm they were just dandy. So what gives? And how can I abuse this effect?

I've looked into electric theory and it states that curved surfaces tend to build charge and pointy surfaces tend to allow it to arc. The sparking region of a typical plug is the end of the electrode and the side of the ground strap, a rectangle on one side and a circle on the other. Both flat areas equidistant apart. You'll note that when a plug is new it has sharp edges, and when it is worn it has rounded edges. You'll also note that pointy iridium plugs run larger gaps.

So if I were to take side-gapping to the extreme, by rounding off the opposite side of the electrode, grinding the ground-strap-facing side to a point, sidegapping the ground strap and performing the same no-edges-that-arent-facing-the-spark ritual, how much gap could I run without misfire? And how much would that actually help?

[Baltothewolf]

Coil strength plays a huuuge part on gap. On my Miata, that I supercharged last week, I had to close the gap from 0.040 to 0.030 and it's still not enough to prevent spark blow-out (misfires above 4800). I'll probably end up at 022-027 before the misfires go away. With toyota coil over plug (COP) It's common to be able to run stock gap or even bigger.

[oldtamiyaphile]

Originally Posted by BLSTIC

You'll also note that pointy iridium plugs run larger gaps.

Seems like the logical thing to do is just swap to iridium's. Cost 4x as much but last 4x longer so any MPG or drivability gains are just icing on the cake.

[s_t]

I don't know much about spark plugs, but yes, sharp points increase electrical stress in insulating/dielectric material (compressed air/fuel mix in this case). A sharp point on either electrode will increase electric stress and encourage breakdown at lower voltage (ie. spark). A sharp point on both even more so.

[DSMHondaGuy]

A wider gap, can create a hotter spark according to my father, he's an old GM tech. It may also effect the dwell time of the coil's before they fire, which would likely result in increased voltage and the hotter spark. But it can also result in a weaker spark. Depends on the system in question and the condition it is in.

I bet in most cases you would gain more torque by going with the OEM spec gap and advancing the ignition timing 1*


ETA: in 4cyl engines where the plugs are easy to change, personally I recommend copper plugs, (now keep in mind my background is performance engine related not mpg). They're cheap and seem to provide the most stable ignition under high load and high intake velocities, as well as excellent idle performance. They're also cheap, the downside though is they do need to be replaced MUCH more often. But at $2-3 a piece, I generally don't care or mind, I tend to replace them in my Talon for example every other oil change so, 6,000 miles. But it's a turbocharged engine that is flowing close to 40lb/min of air and much harder on the spark plugs than any mild mannered NA 4cyl.

[BLSTIC]

They also seem to not break when there's detonation present. My cousin killed all four iridium (or was it platinum?) plugs in his EJ20G by being cheap and running regular. I mean the engine still fired, but all the electrodes were missing and it had a massive gap

[Hersbird]

I don't know the specific answers to your questions but I added a Jacobs electronics ignition and coil to an old 360 Dodge that was running rough. At first there was little change but as soon as I opened up the plug gap per instructions it smoothed right out and had a noticeable increase in power across the board. I never even checked economy on that, a 95 4x4 Ramcharger with oversized tires, but it had to have helped considering how much better it ran after. Jacobs got bought by Mallory, and now Mallory is bought by Accell, they all probably use similar technology.