Resistor spark plugs, resistor spark plug caps, the effects of using a resistor spark plug and a resistor spark plug cap on ultralight aircraft engines.

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Spark plug safety tip

From: Rainier Lamers <>

Subject: Was: Spark plug safety tip

Date: Fri, 26 Feb 1999 16:35:56 +0200

Recently an interesting thread on this newsgroup discussed the relative merits of resistive plugs as used on many Rotax engines.

In order to get to the bottom of this Spark plug vs. resistance story I decided to do some experiments. I am a electronics engineer (when I am not flying :-), so I have the tools for the job.

First a coil was rigged for the job to create the high voltage required for the plugs. I used a standard car coil combined with an electronic interrupter/generator. Spark plugs with and without resistors as well as caps with and without resistors where obtained. The plugs where all equally gapped to 0.4mm. I used a scope to monitor the voltage at the plug tip as well as at the coil (via high impedance probe - scopes are expensive !). The result was quite interesting.

As expected, the voltage at the cable (before any resistor) rises at the same rate regardless of resistance until the point of firing. Thereafter however the picture changes.

The coil generates a certain amount of energy. This energy wants to go somewhere. At a voltage of about 7KV the plug fires (irrespective of resistance). Until that happens NO CURRENT FLOWS. Whether you have resistance or not does not matter. A paltry 5KOhms does not do anything when compared to the nearly infinite resistance of the gap itself (until the plug fires that is).

Once the plug fires the resistance comes into being. The coil cannot get rid of its energy in the shortest possible time due to the resistance.

This reduces current flow in the spark and it takes quite a bit longer until the energy in the coil has expelled itself via resistor and spark gap. This results in a longer spark. However the spark is weaker due to energy loss in the resistor.

The scope shows this very nicely and it also gives reason why the resistor helps to suppress RF. With the resistor changes in the rate of current flow are much less resulting in "flatter" edges on the voltage vs. time curve that the scope shows you. This implies less high frequency components of the signal in the cable feeding the spark plugs.

Conclusion: It is quite safe to insert a resistor. It will not stop the plug from firing at all. High values will however lead to a weaker spark -but it is going to spark, no matter what. 

However, contaminate the plug with fuel (even just a little) and a high value resistor will cause the plug not to fire. 10K plugs as are used on some motorcycles now are about maximum I would guess. 

I do not recommend you use resistance plugs or caps at all on inverted Rotax installations due to the possibility of plug contamination by oil and fuel.

As further experiment I increased the resistance value to see what effect this is going to have. I tried values 10K, 47K, 100K, 220K, 470K, 820K and 1M.

Even with 1M the plug (dry, not contaminated) still fired but noticeably weaker and longer (you can actually see it firing longer !). But contaminate this plug even slightly (a little moisture by exhaling onto it) and you get no spark.

Finally, a disclaimer: All of the above has been found out by a little experimenting. Use the results at your own peril. Don't get back to be with a heap of bent metal that used to be your plane claiming it is because I said you should put 1M resistors into your spark plug leads. I did NOT.

Rainier Lamers.

Editors Note:
A brief recap of the two types of caps that are currently being used on our engines. Originally Rotax engines were equipped with breaker point ignitions and Bosch spark plug caps. A breaker type ignition is not the hottest spark producer at low cranking speeds and at low RPM’s.

Because of this a spark plug cap with a 1000 ohm resistor built inside was used along with a sheet metal type shield on the outside of the cap. The intention of these two items was to reduce EMI, “electro-mechanical interference” or RFI, “Radio frequency interference”.

Because of this a spark plug cap with a 1000 ohm resistor built inside was used along with a sheet metal type shield on the outside of the cap. The intention of these two items was to reduce EMI, “electro-mechanical interference” or RFI, “Radio frequency interference”.

The latest engines are equipped with stronger dual electronic ignition systems and use a 5000 ohm NGK resistor cap, these may be either black or magenta in color, mechanically these are identical.

Along with these resistor caps the newer engines also are equipped with resistor spark plugs. All of this just to reduce that annoying hum or whine you may hear in your headset while your engine is running.

NGK Resistor Spark Plug

NGK cap 90.jpg (10155 bytes)

R in the Spark Plug number designates a Resistor spark plug. Also note that our aircraft spark plugs have a solid STEEL spark plug TIP not an aluminum one that screws on.
Ultralight Engine Info should have their resistor spark plug caps checked for continuity with an ohmmeter, periodically, as they are prone to corrode inside from the elements, especially if the engine is run inverted.
This can make the engine run poorly, as well as being hard on the ignition system.


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