Spark Plug Gap

[MadBill]

...
The posted required firing voltages are based on pure air, not fuel vapor or
relative humidity factors.

Seems like every engine should have an on-board recording scope.

In talking to operators of bi-fuel (propane/CNG-gasoline) fleets, they reported that worn spark plugs would misfire thousands of miles earlier on the gaseous fuel...

[dfarr67]

Any issue with taking a double iridium pregap 0.060 and gapping to 0,035?

[David Redszus]

Any issue with taking a double iridium pregap 0.060 and gapping to 0,035?

Per Bosch, spark plug gaps should not deviate by more than .004" from the plug
manufacturers specifications. The gap size is indicated by the third suffix of
the Bosch plug number.

Iridium is not the best choice for a high performance spark plug. It does not have
good electrical conductivity but does have a high melting plug.
It is designed for long life to meet emission requirements.

[dfarr67]

Any issue with taking a double iridium pregap 0.060 and gapping to 0,035?

Per Bosch, spark plug gaps should not deviate by more than .004" from the plug
manufacturers specifications. The gap size is indicated by the third suffix of
the Bosch plug number.

Iridium is not the best choice for a high performance spark plug. It does not have
good electrical conductivity but does have a high melting plug.
It is designed for long life to meet emission requirements.

plug suggestion- EFI?

https://www.airflowresearch.com/190cc-s ... nder-head/
AFR suggested plug Autolite 3924

[Geoff2]

How would the difference in conductivity [ resistance in ohms ] of a small 0.100" slug of Iridium, compared to the same length of nickel/steel, affect the spark energy with all the other resistance in the circuit [ leads, coil secondary etc ]?

[Circlotron]

Iridium is not the best choice for a high performance spark plug. It does not have
good electrical conductivity

Iridium has a resistivity about 28 times that of copper, so if you have an electrode length of 0.5 + 0.5mm of iridium then that will have as much effect as 28mm of copper. That is nothing compared to maybe an actual mile of hair thin copper wire that is the coil secondary.

[RDY4WAR]

I had that experience with iridium plugs in a turbo 4 cylinder. It would start easier and quicker, and idle smoother, on the iridium plugs, but would occasionally miss at WOT which my tuner (at the time) claimed was likely due to spark blowout and something he sees commonly with fine tip platinum and iridium plugs in boosted engines. I switched to a cheap set of copper (nickel alloy) plugs and had no issues. Both sets were at .025" gap. I did try the iridiums again at a tighter .020" gap and they did okay, but I just stuck with simple conventional plugs from then on. Also, the misfire issue only started when I cranked the boost up. If the boost pressure was less than ~12 psi, it was fine.

I went through a few plugs in that engine at lower boost and the worst one I tried was E3. Misfires galore.

[dfarr67]

I have Ruthenium HX FR6AHX-S 94279 in now, Iridiums were slightly smoother.

I have a set of NGK BKR5E I could try at 0.035?

[David Redszus]

The use of high melting point electrode material allows the use of a very fine wire
center electrode that will last longer than lower melting point materials.

The benefits of a smaller center electrode can easily over come the increased resistance.

Over the years, it has been my experience that when an ignition system output is adequate,
almost any spark plug material can be used. It is only when faced with weak ignitions that
small plug details make any difference. And even then it is only at the limit of performance.

[Truckedup]

I have been experimenting with NGK Iridium plugs in my modified vintage British street bikes with magneto and modern electronic ignition...Non resistor just like the convention NGK plugs. I notice no difference other than the ground strap has more color change than a copper plug of the same heat number...
Plug gaps... a iridium non resistor plug can make an engine with a weak magneto start quicker, sometimes...
Vintage bike with a weak magneto may need a .012 plug gap so it will start...

[Geoff2]

The fine wire centre electrode plugs require less arc over voltage than a conventional plug [ all else being equal ], & that is one of their benefits. Less misfires as gap erosion increases the plug gap, & better for emissions.

[RDY4WAR]

I think there's a lot of factors that affect how the plug will perform. My engine had tight quench, and I think turbulence in the chamber may have been extinguishing or impeding expanding plasma of the spark with the fine wire plugs. At least that's my limited hypothesis based on my limited understanding from limited research on the subject.

[Schurkey]

Early 90's Chev 350- small cap HEI called for 0.035in gap (I think). Using any of the newer Iridium's and such- is 0.040in a reasonable gap?

If you're going to run an Iridium plug, run it at the plug manufacturer's gap. I wouldn't change the .040 to .035. I don't see an extra .005 causing problems. But what do I know?

Any issue with taking a double iridium pregap 0.060 and gapping to 0,035?

Iridium plugs should be pre-gapped. Like all plugs, the side electrode is engineered to support the expected gap--longer side electrodes for larger gaps. Smashing it shut is not wise.

OTOH, opening the gap isn't good for iridium plugs either--I think they're concerned that you'll break-off the little chunk of Iridium on the side electrode, or bend the fine-wire center electrode.

One indication of counterfeit iridium plugs is that the dirtbag manufacturers don't get consistent plug gaps. I discovered this the hard way.

[Geoff2]

Yes,
very easy to damage the small tip on Plat/Irid plugs. I run 0.060" [ sometimes more ] gaps on these plugs; they come with a smaller gap. I made up a gapping tool using bull nose pliers. The end of the pliers hooks under the tip of the grd strap to pull it back to open the gap.