Since I had nothing better to do last night, I decided to change the plugs in my hillclimb car in prep for an event.
Old NGK BP7ES plugs came out. Checked their resistance out of curiosity. Very low, under an Ohm with continuity test beep. These are non resistor plugs remember.
New box of same plug. Tested and got very high resistance on the 1st, 2nd 3rd.... I have around 20 of these and they all had resistance values of 3-4 Ohms lowest, into the mega Ohmns for the highest. None of the 20 plugs gave a continuity beep on the multimeter.
Any comments?
Resistance of non resistor spark plugs
Moderator: Team
Re: Resistance of non resistor spark plugs
30-35 years ago this was brought to my attention by a pal who is a deep thinker, always looking for the last little detail to make things better on his race car. We measured a bunch of NGK and Autolite racing plugs and found the same thing you have. New plugs measure some small resistance or open, as if there is no connection at all.
Asked the local NGK rep about it and he called upstairs, I don't know, to Japan maybe, and they replied something to the effect, "Install them and after they are run check them again, you will find they are like the ones you just removed, zero resistance."
Well, they were right. Apparently, a bit of oxide will grow between the terminal end and the electrode, where they butt together to join inside the ceramic insulator, and the heat cycles and electricity will simply remove it.
Your ignition system is capable of high enough voltage to jump a half-inch or an inch or more and still fire the plug gap, so the tiny bit of oxide inside the plug has zero effect and is gone almost immediately on the first fire up. Put the new plugs in, run it some and pull one and measure it.
In the late 50s and early 60s Champion made some plugs with an internal gap the spark had to jump to get to the electrode in the firing end. They were effective in firing fouled plugs but vented the ozone through the hollow plug wire terminal and naturally that caused deterioration of the connector inside the rubber plug boot because the corrosive ozone was trapped inside the boot.
viewtopic.php?t=45565
Asked the local NGK rep about it and he called upstairs, I don't know, to Japan maybe, and they replied something to the effect, "Install them and after they are run check them again, you will find they are like the ones you just removed, zero resistance."
Well, they were right. Apparently, a bit of oxide will grow between the terminal end and the electrode, where they butt together to join inside the ceramic insulator, and the heat cycles and electricity will simply remove it.
Your ignition system is capable of high enough voltage to jump a half-inch or an inch or more and still fire the plug gap, so the tiny bit of oxide inside the plug has zero effect and is gone almost immediately on the first fire up. Put the new plugs in, run it some and pull one and measure it.
In the late 50s and early 60s Champion made some plugs with an internal gap the spark had to jump to get to the electrode in the firing end. They were effective in firing fouled plugs but vented the ozone through the hollow plug wire terminal and naturally that caused deterioration of the connector inside the rubber plug boot because the corrosive ozone was trapped inside the boot.
viewtopic.php?t=45565
Re: Resistance of non resistor spark plugs
Ok. I decided to heat the tip and unscrew the contact point from the porcelain. I noticed lots of a loctite type sealer extending down into the copper electrode. Cleaned it up and screwed it back on. 0.03 Ohms now and problem solved. Live and learn. It may be as you say, when they fire and heat up for the first time the glue melts and it's back to 0 resistance.