Semi-surface discharge spark plugs
Semi-surface discharge spark plugs
(OP)
I’m building a 4-cylinder, air-cooled, twin-plug, ~1.9 liter autocross engine. The cylinders are
Nikasill with JE Pistons. The pistons will be TB coated on the top and have anti-friction coatings on the
lands and skirts. The combustion chambers have been reworked to unshroud the valves
and the intake passages cleaned up. The intake valve is 42mm and the exhaust is 34mm,
both with 8mm stems. The CR target is 10:1 and rev limit will be 6500.
Unfortunately, there is some interference between the 10mm "extra" plugs and the piston
domes. I know how to accurately mark and machine the domes to solve this. However,
I have been wondering if a semi-surface discharge plug could solve the problem without
the extra machine work.
The NGK racing plug catalog- <www.NGKsparkplugs.com/docs/racing_catalog.pdf>
shows several 10mm plugs that might work, including the R0045G-x and
R0045J-x, which come in a wide variety of heat ranges. I plan to use a Crane
Hi-6 ignition system with LX-91 coil (360mA) so I think I have enough spark.
I’m thinking that the plug ‘face’ will be flush or project 0.010" into
the chamber.
Questions: How will a surface discharge plug handle the 10:1 compression ratio?
Anyone have experience with these plugs. Or an opinion about their use in my application?
Any other issues to consider? Am I going to be singing "come on baby, light my fire?"
TIA
Rink
Nikasill with JE Pistons. The pistons will be TB coated on the top and have anti-friction coatings on the
lands and skirts. The combustion chambers have been reworked to unshroud the valves
and the intake passages cleaned up. The intake valve is 42mm and the exhaust is 34mm,
both with 8mm stems. The CR target is 10:1 and rev limit will be 6500.
Unfortunately, there is some interference between the 10mm "extra" plugs and the piston
domes. I know how to accurately mark and machine the domes to solve this. However,
I have been wondering if a semi-surface discharge plug could solve the problem without
the extra machine work.
The NGK racing plug catalog- <www.NGKsparkplugs.com/docs/racing_catalog.pdf>
shows several 10mm plugs that might work, including the R0045G-x and
R0045J-x, which come in a wide variety of heat ranges. I plan to use a Crane
Hi-6 ignition system with LX-91 coil (360mA) so I think I have enough spark.
I’m thinking that the plug ‘face’ will be flush or project 0.010" into
the chamber.
Questions: How will a surface discharge plug handle the 10:1 compression ratio?
Anyone have experience with these plugs. Or an opinion about their use in my application?
Any other issues to consider? Am I going to be singing "come on baby, light my fire?"
TIA
Rink





RE: Semi-surface discharge spark plugs
-Reidh
RE: Semi-surface discharge spark plugs
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RE: Semi-surface discharge spark plugs
reidh.
Did your test conditions approximate higher compression ratios? And, tip proximity close to pistons? If not, any ideas about this?
patprimmer,
Yes, I'm aware of the Side or Wire electrodes. In fact some are shown on the NGK page that I referenced. Not as many heat-range choices, IIRC.
Thanks to both of you for your comments. Anyone else?
Rink
RE: Semi-surface discharge spark plugs
Then again my collage courses were slide rule, drafting standards and beer.
I don't know anything but the people that do.
RE: Semi-surface discharge spark plugs
With enough ignition voltage, and assuming the piston top does earth out via the rings, cut the earth straps off and try it. By using different reach plugs you can set the "gap" by grinding down the centre electrode.
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: Semi-surface discharge spark plugs
RE: Semi-surface discharge spark plugs
We did experiment with slightly higher compression ratios, and our results logically pointed to an increase in required ignition energy as CR was increased, since it is harder for the spark to jump at a higher pressure. After the official test was over, one of our mechanics for fun increased the CR to the point that the spark plug actually suffered from flashover (where the spark travels from the top of the plug under the cap down the outside to the plug to ground). He then put in a standard spark plug and was unable to replicate the phenomenon.
We did not change the set-up with regards to tip proximity.
FoMoCo: I completely agree. Maybe that is one of the good reasons that Saab never put this into production.
-Reidh
RE: Semi-surface discharge spark plugs
RE: Semi-surface discharge spark plugs
Thanks for all the comments.
I spoke to an NGK technical representative about 10mm surface discharge and fine wire plugs. It turn out there are a couple of problems that will prevent me from using these type of plugs. The most difficult one is the heat range available. NGK claims about 50deg/C tip temperature for each step of heat range. The rep. also said the tip need to be above 450dec/C to self-clean. The NGK surface discharge plug R0045J-9 is the hottest but still too cold. It’s also a 3/4” reach plug and I need 1/2”. It is possible that I could make a 1/4” spacer that would allow the use of the plug. I could then adjust that dimension to allow me to precisely place the “tip” in the chamber and change the heat range as well (making it a little hotter). But I would have to run the engine to determine that and if not suitable would have to disassemble and machine the piston domes as I was trying to avoid. Too many variables for me. I’ll just notch the pistons and run a conventional plug, NGK C7HSA, which is 2 steps or ~100deg/C hotter, and only $2 ea., and be done with it. Also, those surface discharge plugs are $20 each.
The fire wire plugs (R0373A-9) have some of the same problems but also project more into the chamber and cost $81 each. No thanks.
The suggestion to fire plugs to the pistons is silly in my opinion. Maybe okay for a 1500 RPM engine with very little ignition advance. In any case I would not consider trying this for this particular application.
Rink
RE: Semi-surface discharge spark plugs
Sorry,
Rink
RE: Semi-surface discharge spark plugs
RE: Semi-surface discharge spark plugs