From: Rainier Lamers <Rainier@Questek.co.za>
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
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