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Spark Plugs!

The BR9EIX Iridium plug:

The Iridium-coated central electrode of the "I" model does not burn away like the steel standard plug's does (the "S" model).

Furthermore, the "X" denotes a Booster Gap which delivers a stronger spark (this is part of what pushes the price up. plus of course the cost of the iridium coating, which is a platinum compound)

The central electrode has quite a sharp point which helps develop a cleaner, more concentrated spark for better combustion.

Note that the temperature range of the plug does NOT change the operating temperature of the engine. It is the just the temperature of the central electrode! The plug must get hot enough to burn itself clean at normal cruising power (especially after a long idle where it tends to foul up). It must not get so hot that the central electrode burns away. Steel burns in the presence of oxygen at high temperatures. Iridium can handle MUCH higher temperatures.

So if you normallly use a B10ES or BR10ES, then drop down to a 9 (one step hotter) when you change to iridium. (e.g. BR9EIX).

In very cold climes, you can drop another number, to an even "hotter" plug, e.g. to an 8 in the above example.

Once you use an iridium plug, you will probably never go back to standard steel plugs again, except in emergencies when your favourite plug is not available.

If you do not want to spend to much on an iridium plug, then try the premium Platinum plug "Denoted by a "P" after the number.

So, how are the NGK type plugs named?

e.g. NGK's BR9EIX

B = Thread diameter and pitch (yes, there are other dimater plugs!)

R = Resister, i.e. the RF Noise Supressor (to reduce noise on your radio)

Number = heat range, lower numbers are hotter. This is merely the heat-path along the ceramic surrounding the central electrode. If its deeply cut-away, the heat must travel a longer patch to escape via advection to the cylinder head, so it gets hotter.

E = Extended length thread - to prevent the soft aluminium threads from stripping at high temperatures - Solo210 pilots with tuned exhausts will know all about blowing plugs out of the head in flight due to the standard thread length.

S = Standard Steel. I = Iridium.... and many others. Some denot ethe shape of the side electrodes.

X = Booster Gap for more powerful spark.

Try one, you will never look back again.

Diego offers these for discerning pilots.

Regards, Keith Pickersgill




Hi all,
I did some spark plug comparisons recently. I wondered if the iridium plugs, really do improve the performance of our motors. I conducted the tests with the same fuel mix, on my Miniplane Top 80 with a 19/73 gearbox and a MAH 125 carbon prop, at sea level. This first test was done on the ground, not in the air (too windy this day).
Note: This Top 80 has the coil with the black molded cap, with the resistive wire.
NGK B9 ES       gap @ .028                 9600 RPM  [BEST]
NGK B9 ES       gap  @ .030                 9510 RPM
NGK B9 ES       gap  @ .026                 9470 RPM
[Iridium Plugs]
NGK BR9 EXI  gap  @ .028                9380 RPM
NGK BR9 EXI  gap  @ .022                9500 RPM
The stock non-resistor plug, the B 9ES (gap @ .028) was the best, with 9600 RPM. As you can see, when I increased the gap, it ran worse, the same when I decreased the gap, it ran worse.
The iridium resistor plug, (gap @ .028) was the worst of all. So it seems that having a resistor plug and a resistor cap (or in this case, a resistive wire) was too much resistance, for our little coil, to make a good spark. Decreasing the gap (to .022) helped some, but was still not as good as less resistance and the properly gapped stock plug. 
A few days later I did another test, this one conducted in the air, (took off, full throttle climb, record data, land, repeat). I had ordered a different iridium plug to test, (Denso IW 27), which is non-resistor, in the same heat range as the NGK BR9 EXI.
NGK B9 ES        gap  @ .028               9500 RPM  [BEST]
NGK BR9 ES      gap  @ .028                9450 RPM
[Iridium Plugs]
NGK BR9 EXI     gap  @ .022                9480 RPM
Denso IW27        gap  @ .022                9470 RPM
So, based on these tests, the iridium plugs actually hurt performance on this engine that has the coil with a resistive wire. Since it’s not possible to change the wire, (molded into the coil) I’m going to fly it with the stock NGK B9 ES. They ran the best, at 9600 RPM on the ground, and 9500 RPM in the air, and they are cheaper so it will cost less to do plug cuts (for diagnosing mixture problems). It is still possible that an older Top 80, with the red plug cap (that does not have resistive wire) could run better, and gain some benefit with an iridium plug.
Note: I could not find an auto parts store locally, that stocked the NGK B9ES, (only BR9ES), so I had to special order them and wait a few days for them to come in. Best to plan ahead and not get stuck having to use a BR9ES.
Happy Flying!