Spark Plug Testing

k_dodd@swbell.net

k_dodd@swbell.net

Member
When testing Spark Plugs, in my case Champion W16Y, what should the Ohms be? I've got 2 plugs reading fairly high (4.4), and 2 plugs reading fairly low (about 1.7).
 
I'm not sure if the manufacturers of spark plug preasure testers make bushings to fit the 7/8" threaded ends for those made that way but they should since some aircraft plugs are that size. The ones with rubber grommets should have a large size grommet that will fit the spark plugs. Some folks referred to them as bomb testers but I don't know the origin of that. They are made to be pressured up to an approximation of cylinder pressures in operation. I pressure them up to 120 psi and run the high tension ******** about 60 seconds while watching the spark. If they keep arcing smoothly for 60 seconds then they are good to go. If the arcing is poor or non existent then they are junk.
 
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Not sure you can test a plug via ohms. But hey, I am not the brightest guy out there and don't know everything. If you can find someone with a shop sized SUN engine analyzer it may have a tester for plugs. I have an analyzer where I can watch the plug spark so I can see if they are good or not.
 
This is my spark plug cleaner and tester.
 

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Meauring Ohms from the top terminal (threaded) portion of the spark plug to the bottom of the center electrode where the open spark would jump to the side electrode. I recently bought new (identical) plugs. They measure 0.2 Ohms b/c there is no resistor in these plugs.
 
The type that Ivan has in his post #4 is the same basic unit that has been used for decades. They have a cleaning feature that uses a replaceable blast media then the other port is for pressure and has a little hand wheel to increase and decrease pressure. These units are now very expensive. Even the ATS unit is over $600.

Most folks just replace the plugs now days so mostly only aircraft shops have plug testers due to the very high price of aircraft spark plugs. Automobile plugs are not made the same way so the testers don't view an auto plug directly but the flashes can be seen well enough to know if they are working.
 
The center electrode should have close to zero ohms resistance. If you are measuring any other amount it is probably because of carbon or oxidation at the tip. The spark voltage is about 20,000 volts so the small amount of resistance you are measuring will have no effect.
 
nkaminar said:
The center electrode should have close to zero ohms resistance. If you are measuring any other amount it is probably because of carbon or oxidation at the tip. The spark voltage is about 20,000 volts so the small amount of resistance you are measuring will have no effect.
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This is exactly what I'm learning. So, when I test a spark plug (without a resistor, like on a Model A), if there IS resistance, it's indicating a problem with that plug. That's where I was confused. Thank you for helping clear this up.
 
Only in the unlikely case where the center electrode in broken internally will you get a large amount of resistance (over 100 ohms). Anything less than that is not going to effect your spark plug performance. After all, the spark jumps a 0.035 inch gap with high pressure in the cylinder. I don't know what that resistance is but it is a lot more than 100 ohms.


I looked up the resistivity of air and the gap represents about 30,000,000,000 ohms, if I did my math correctly. The resistivity is 1.5 to 6 X 10^13 ohms/meter. So even 1,000 ohms is not going to do anything.
 
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