Spark plug pressure testing (insight requestsed)

Hi all,

I recently got access to a Vixen spark plug pressure testing machine that allows you to test how well your spark plugs fire under increased pressure.

So far I've tested half a dozen ~300 hour UREM40E and a couple UREM38BY. The plugs I've tested seem to start firing irregularly and then"flame out" between 90-115psi (~0.016" gap). Is this a "good" reading?

I can't seem to find any manuals or other references that explain what's a passing grade on this kind of test.

One other note is that this machine appears to be 50+ years old. It's possible that it could be arcing over internally as the pressure increases, but if that is happening it's not obvious.

Thanks for the help!

Have you had a chance to test any "auto" plugs, perhaps using a 14mm to 18mm adapter?

rv8ch said:

Have you had a chance to test any "auto" plugs, perhaps using a 14mm to 18mm adapter?

Click to expand...

Sounds neat, but I unfortunately don't have a 14mm adapter.

digidocs said:

Sounds neat, but I unfortunately don't have a 14mm adapter.

Click to expand...

Hi David, if you really want one, they are available at places like this for about $10:

https://brillman.com/product/spark-plug-adaptor-18mm-to-14mm/

https://us.gsparkplug.com/1x-spark-plug-thread-adaptors-18mm-down-to-14mm-brass-m14-m18.html

Or if you are in the right part of the country, a farming supply company often has these.

digidocs said:

Hi all,

I recently got access to a Vixen spark plug pressure testing machine that allows you to test how well your spark plugs fire under increased pressure.

So far I've tested half a dozen ~300 hour UREM40E and a couple UREM38BY. The plugs I've tested seem to start firing irregularly and then"flame out" between 90-115psi (~0.016" gap). Is this a "good" reading?

I can't seem to find any manuals or other references that explain what's a passing grade on this kind of test.

One other note is that this machine appears to be 50+ years old. It's possible that it could be arcing over internally as the pressure increases, but if that is happening it's not obvious.

Thanks for the help!

Click to expand...

Spark plugs should fire at a working pressure of up to ~150PSI+ ( Sea Level Pressure * Compression Ratio ((8.5 to 10:1)).

If not, it's either the plugs -OR- considering the age of the machine, perhaps the machine.

Given that magnetos and their EI/CDI counterparts produce voltages in excess of what the breakdown voltage is, even under pressure - I question the value of these test devices. Measure the plug's resistance and see if it's within spec. If yes, use 'em, if no trash 'em...

Under Pressure

+1 with Brian. I’ve never owned a pressure tester but worked in a couple of shops that had them. I don’t recall ever rejecting or even suspecting a sparking plug or solving any troubleshooting problem with one. It is fun to watch the pretty blue sparks, though.

The increased pressure of the sparkplug tester simulates the engine cylinder when under compression. Under compression, the air density is increased which increases the resistance across the electrodes. The increased resistance requires the spark to have a higher voltage. if there is a fault with the spark plug, such as a crack in a ceramic insulator, the spark will go to ground through the crack instead of across the electrodes as it's supposed to. this is observed as no spark across the electrodes.

So as your pressure increases, you stop seeing spark across the electrodes. Other than a crack in the ceramic, the electrical leak can also occur along the "cigarette'" insulator or another fault with the ignition wire. even dust on the cigarette can sponsor a spark leak.

If you haven't heard this before, a cracked insulator, is the reason you should throw away a sparkplug if you drop it on the ground.

I also want to add that as an A & P, I have occasionally found sparkplugs that fail to properly spark under pressure.

I should also mention that as an A & P, I have occasionally found sparkplugs that fail to properly spark under pressure in a tester. i think they are a great tool to help rundown an ignition problem.

Follow up:

I hooked up a new ignition coil to the tester in place of the very old one. After doing so, all the spark plugs I tested worked consistently to 140 psi (the maximum for the machine).

So in this case, it was definitely the tester and not the plugs that were causing the low readings.

The main purpose of this test is to determine if the insulator is defective, cracked or has a damaged/ contaminated surface, any of which will result in arcing down the insulator to the plug housing instead of arcing across the intended gap. The increased pressure requires a higher voltage to jump the spark gap, which is better to determine if the insulator has a problem.