Spark plugs. Why you should check yours, today!

The lowly spark plug is one of those aircraft parts we all take for granted. It’s not exciting like a new glass panel, and it doesn’t have the ramp appeal of  a new four-blade prop. But when a spark plug fails, you’ve got some real trouble that focuses your attention on what matters most – a smooth running engine.

Do you have to lean aggressively to keep your spark plugs from fouling during taxi?   Do you have persistent trouble with your pre-flight magneto check? Do you experience engine roughness or vibration during flight? Maybe the problem isn’t your leaning technique, or the magnetos. Maybe it’s your spark plugs!

Over the last few years, sophisticated engine monitoring has given us a wealth of information on the health of our engines. The data allows us to diagnose problems that used to go unresolved for far too long. Using this data, we are now able to pinpoint problems with spark plugs. These plugs seem to work fine in the shop’s bench tester, but give trouble when installed in the engine. At Platinum, we can analyze your engine data and determine if your engine can benefit from new spark plugs.

Spark plugs contain an internal resistor to absorb excess energy, which protects the electrodes from premature erosion. This resistor is subject to extreme heat from the cylinder head, and the stress of 25,000 volts of electricity passing through. If the resistance increases, the spark energy at the electrodes decreases. Resistors dissipate energy in the form of heat. This results in intermittent roughness, elevated cylinder head temperatures, and completely fouled spark plugs in some cases. So the ideal resistor must remain stable throughout the service life of the spark plug.

Let’s examine the two different styles of resistor.  Champion uses a carbon-pile design. This resistor element is found inside the spark plug body, held under pressure by a spring, which is in turn secured by the screw.

Champion RHB32S fine-wire spark plug, with resistor removed

As you can see in this photo of a two month old Champion RHB32S fine-wire spark plug with only 110 hours in service, there is corrosion on the conductive surfaces.  When it was new two months ago, the resistance was normal, about 1500 ohms. But today, this spark plug’s resistance is over 12,000 ohms!  That means some of the spark energy isn’t passing through to the electrodes. This spark plug was causing elevated CHT’s in the affected cylinder. Installing new spark plugs resolved the CHT problem, and stopped the intermittent vibration problems at low rpm.

left-to-right: resistor, spring, retaining screw, note the corrosion on each part

The second plug is also a Champion RHB32S fine-wire spark plug. It is one year old, with 350 hours in service. It has over 35,000 ohms of resistance.

Champion RHB32S, one year old, 350 hours total, 35,000 ohms resistance

The third plug is a Champion REM38E massive electrode spark plug. It was installed in 2009, and has 450 hours in service. The resistance is a whopping 200,000 ohms! This plug was constantly fouling after landing, and also during taxi. Replacement with a Unison spark plug solved this persistent problem.

Champion REM38E, 3 years old, 450 total hours, 200,000 ohms resistance

This next plug is another Champion REM38E, from the same engine as the previous plug. It was constantly causing trouble with both pre-flight and in-flight magneto checks. Here you’ll see it has an astonishing 1.024 megaohms of resistance- that’s over one million ohms!

Champion REM38E, 3 years, 450 hours total, 1.024 megaohms resistance

The last plug is a Unison REM40E. Notice the corrosion-resistant nickel finish. This is highly beneficial in South Florida, where corrosion is a constant concern. This used plug has 1454 ohms of resistance, right in the middle of the “new” specification. We have several planes running these spark plugs, and even after several years and hundreds of hours, the resistance remains steady, between 1300 and 1500 ohms.

Unison REM40E, 3 years, 450 hours in service, 1454 ohms resistance- the same as a NEW plug!

The Tempest (formerly Unison/Autolite) design spark plugs use a fired-in resistor. It’s completely encapsulated so corrosion is never a factor. This proprietary resistor remains stable during the life of the spark plug. I’ve personally run Unison spark plugs in the right engine of my Piper twin since 2002. That’s ten years and over 900 hours in service, with no change in internal resistance values, no fouling issues, and minimal wear of the electrodes.  They still are within new resistance limits at 1450 ohms. Even more noteworthy, the massive electrodes are still within wear limits as well. I’ve rotated the plugs every 100 hours, and they’ve outlasted competing spark plugs by a two to one margin. (The Champion REM38E spark plugs shown here came from the left engine of my Piper twin.)

Here we test the actual resistors removed from two Champion RHB32S fine wire spark plugs. The first resistor has 15,300 ohms by itself. Once assembled in the spark plug, the assembly has 35,000 ohms total resistance.

Champion resistor with 15,300 ohms

Here is another Champion resistor that came out of a completely dead plug. The resistor has five million ohms of resistance!

Champion resistor with five million ohms of resistance

Unfortunately, Champion doesn’t have any guidance regarding acceptable resistance values for their spark plugs.  We do know that plugs with great enough resistance fail to work in the engine. On the other hand, Tempest has published a guide to evaluating spark plugs that can help solve the persistent CHT and fouling problems that you may be experiencing. (The publication is reproduced here, courtesy of Tempest.)

How Many Ohms of Resistance Are Your Spark Plugs Creating?
Tempest Spark Plug Cutaway 
What?  Spark plugs deliver spark energy, they don’t create resistance to energy…do they?  Well, they don’t exactly create it – or do they?  Read on to find out why more and more A&P’s and IA’s are now checking the resistance as part of their scheduled spark plug maintenance.
For the past two years Tempest has been educating the general aviation industry on the importance of checking the resistance of spark plugs.  Here are some common myths and misconceptions about spark plugs and resistors:


1. Resistors are used in spark plugs to reduce radio noise.


False - resistors, although they may help with radio noise reduction, are used primarily to reduce the electrode erosion effects caused by capacitance after-fire.  This is a known after-firing of the plug caused by the residual energy built up through the harness leads and magnetos.


2. Spark plugs are considered in good condition if they spark in a tester.
False - Bomb or bench testers can not adequately simulate the conditions of the engine cylinder, altitude and condition of the magneto.  It is a fact that plugs with high resistance have tested as “good” in a bench test environment.


3. The resistance in the spark plug doesn’t matter, because my magneto has the ability to put out more energy than is required.


False - The typical aircraft magneto will put out about 20 - 25kV. Let’s go back to science class for a moment and revisit Ohm’s Law.  It takes 1 Volt to push 1 Amp through 1 Ohm of resistance. Based on Ohm’s Law a magneto can not deliver adequate energy to the spark plug if the system resistance is above 20-25kΩ. Tempest recommends any spark plug over 5kΩ needs to be replaced with a new plug.  Experience suggests that any resistance over

5kΩ ohms causes the voltage to bleed through the path of least resistance rather than ionize the gap.


4. All aviation spark plugs are made the same, therefore there is no difference in resistance.


False - Tempest uses a 21st century proprietary FISS resistor design which results in consistent resistance values of 1 – 2kΩ. Our competitor uses an old style stack up design which can cause extremely high and inconsistent values, resulting in misfiring and a rough running engine.


But don’t take our word for it…please read the attached article “A Tale of Two Sparks” by Norm Howell.  He tells his story of a rough running engine(s) on his Aerostar and the results he found with regard to high resistance of his spark plugs.


“A Tale of Two Sparks” by Norm Howell


Click here if you would like a tool box reference card for checking the resistance of your spark plugs.


  Tempest Spark Plug Resistance Reference Card



For specific product information on Tempest spark plugs and oil filters, please click the link below.


Tempest Oil Filter & Spark Plug Brochure

Tempest Logo Primary

Tempest is the leading brand of pneumatic, filtration, and ignition components, including a complete line of OEM dry air pumps, oil filters, ignition components, spark plugs, specialty tools and PowerFlite® starters.  Manufacturing facilities are located in Gibsonville, NC, and Lakeview, MI with sales and marketing offices located in Greenville, SC and Atlanta, GA. 

For more information regarding Tempest products, please go to

Recently, Tornado Alley Turbos published a service bulletin, SB11-05, calling for removal of Champion fine-wire spark plugs. TAT has experienced many failures of these spark plugs when installed in the turbocharged Cirrus SR22, among others. They are recommending removal of these plugs to head off potential engine failures due to detonation / pre-ignition. Here is the link to the service bulletin:

We don’t fully understand why spark plug insulators crack at this point in time. What we do know is once an insulator cracks, the spark plug may become a glow plug under the right conditions. This leads to destructive pre-ignition that can destroy a piston in a matter of minutes. This danger alone is reason to consider replacing your spark plugs today.

While TAT is only calling for removal of fine-wire Champion plugs due to cracking, we are also experiencing cracking of Champion massive electrode plugs. I’ve seen a recent failure that nearly caused a piston to fail. The aircraft was flying overwater, but fortunate to be close enough to land, so being able to reduce the power stopped the damage before the piston failed completely. It did melt the edges, and resulted in a cylinder replacement. The pilot saw an elevated CHT reading, just like the one depicted in the TAT service bulletin. The temperature rise is rapid, and left unchecked, may cause failure of the affected cylinder, resulting in engine failure in less than 10 minutes. Since many planes in South Florida operate over open water, this can be a serious safety of flight concern. Here is the spark plug, and the piston with melted edges:

Champion RHM38E massive electrode spark plug, with cracked insulator

Cirrus SR20 piston, cracked spark plug caused pre-ignition, resulting in melted edges.

So if you’d like a smoother-running, more reliable engine, I would recommend installing all new spark plugs during your next maintenance visit. Tempest has taken the time-proven Autolite / Unison design and brought it back into production. Tempest spark plugs are available for all aircraft, in both fine-wire and massive electrode types. (Fine-wire types are considered to be 1-2% more fuel-efficient, and they last 2-3 times longer than massive electrode plugs.  Plus they are highly resistant to fouling, offering lower total costs over their life span.)

Platinum has Tempest spark plugs in stock, and we can fine-tune your complete ignition system for maximum performance and fuel economy. They are in limited supply, so contact us to reserve a set for your airplane today.

See Part Two



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16 Responses to Spark plugs. Why you should check yours, today!

  1. Marc-André Théorêt says:

    HI Glen, I flew few days ago with Nathan. I think he mentioned to you that I had changed my Champion for Tempest on my SR22 GTS. Engine is rough at idle but gets better when it warms up. I put my last flight with Nathan on CirrusReports. It is listed under CGBBG. Do you think you can have a look and let me know ?



  2. Hello Marc,

    It will need a slight adjustment to the fuel pressures. It is probably too lean between idle and 1700 rpm, if it gets better when warm. If it were too rich, the idle would get worse when warm.

    Also, the take-off rpm is low at 2680, that needs to go up to 2700, and the cruise rpm is a bit low at 2480, it should be 2500.

    Best regards,


  3. Victor / Executive Air Transport says:

    Platinum, That’s pretty interesting. I went out in the shop just now and tested about 49 spark plugs( 33 Champion and 16 Autolight, we don’t have that many Autolights, I tested everyone we have in the box) from our pile of removed plugs.

    All the Autolights tested good, and within a few hundred ohms of the others, If anything they were mostly on the low side, maybe around 1K, except for one, which was about 30,000 ohms. Might be an anomaly, or perhaps it was dropped and has been sitting in the box corroding away for the last year or two.

    I got 27 good plugs(both brands) at 2K ohms or less and 22 bad plugs (21 Champions and 1 autolight) at over 2K ohms or right up to 135,000 ohms.

    This is the first I’ve heard of this. Incidentally, of my 6 cracked spark plugs I have sitting around, two tested at 1700 to 1900 ohms, one at 2300 ohms and the other three were 11,000 to 27,000 ohms. I don’t know as that means anything.

    I suspect that impact would cause the insulator cracking rather than heat. As in a vibration, or some sort of pre-ignition or detonation issue that causes a pounding inside the cylinder. Why all of a sudden? Don’t know why this has become an issue, but it’s here.

    I wonder if Champion has heard of or addressed this issue (high internal resistance) in print anywhere?

  4. tom says:

    I’m confused. I bought a promo set of Slick harnesses and Autolite fine wire plugs from Unison in 2008. Since then Champion bought the Slick line from Unison. Didn’t they also buy the Autolite spark plug line or was that spun off to Tempest? Is there a way to look at my plugs to determine if they have the cast resistor or the insert?

  5. Pingback: Spark plugs, part two | Platinum Aviation Blog

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  7. Mike Mladejovsky, PhD EE says:

    The Ohms Law analysis presented here is flawed. The current would be proportional to the applied voltage if the resistance was linear. In the case of a corroded dissimilar metal interface where the spring contacts the plating on the chip resistor, the resistance is highly non-linear. When reading the resistance with an Ohmmeter, the applied voltage is a few Volts, and a few nano-meters of oxide will cause a high reading on the Ohmmeter. When the plug fires, the voltage forcing current through that junction is tens of thousands of Volts, and is sufficient to break through a few nano-meters of oxide, and the current will be limited only by the intrinsic resistance of the chip resistor…

    I have been flying behind plugs which show infinite resistance as measured by my Fluke meter for hundreds of hours with normal CHTs and EGTs.

    • Bob Shettel says:

      Dr. Mladejofsky is absolutely correct. That non-linearity of resistance is common for most resistors. I always taught my automotive students that a DVOM could tell you a component was bad; it could NOT tell you if it was good… I always had them measure live voltage and amperage in a circuit and then calculate a component’s resistance from Ohms Law. Looks like I was wrong on my first count, too… ;-)

  8. You are absolutely right. The right instrument should be a dielectric rigidity Ohmmeter, this type of equipment was not in reach of Glenn Juber.
    Never less Glenn Juber really noticed sort form of “decay” in the resistor compound, he tested more than 200 spark plugs..

  9. fran shoreys says:

    If the high resistance is caused by corrosion / oxidation, then will running the contact surface over some sand paper restore the value?

  10. JP says:

    I just began hearing about this problem and checked all of the plugs in our two Pawnees and our 180 HP Cub. NOT ONE was anywhere near spec!

    I’m done with Champion plugs and ordered Tempest plugs for all of our aircraft.

    Champion seems to have a monopoly with most of the larger parts suppliers – screw that!
    There are plenty of smaller suppliers that I can patronize that sell Tempest. I have emailed the Champion-only suppliers that I will no longer do business with them because of that. If enough people would do this, the “Champion problem” would be fixed pronto!

  11. Pingback: Spark plugs.... Interesting read. - Pelican Parts Technical BBS

  12. Brad says:

    I am replacing my Champion REM37BY plugs in my C172, O-360, 180hp STC from Penn Yan. Considering Tempest but what about going with the iridium fine wire plugs (UREM38S) from Tempest?

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