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Unplanned IO-390 spark plug test

bmarvel

Well Known Member
Friend
For the entire 455 flight hours on the IO-390 in my -14A, it turns out I?ve been using short reach spark plugs in long reach adapters that came with a P-Mag I installed on the new engine. These short reach plugs are recessed in the adapter about a half inch so their electrodes do not protrude into the combustion chamber at all.

Why this arrangement? Well, it was unintended. When I ordered a single P-Mag for the engine I specified an IO-390 and it came with long reach adapters because this engine uses long reach aviation spark plugs. Since no specific automotive plugs were recommended in the P-Mag manual but several different ones were said to work OK by P-Mag users, I simply bought the least expensive and installed them. It did not dawn on me at the time that those plugs (NGK BR8ES) were too short for the adapters.

In two words, their performance in this configuration has been ?ops normal?. They are inexpensive and all 4 get discarded each annual. There has been nothing irregular about this setup even though one would think the flame front would be in some way inhibited by virtue of the electrodes being ?captured? inside the adapter. I?ve operated everywhere from sea level to 18,000 feet at a wide range of temperatures, density altitudes and power settings and the aircraft and engine don?t seem to know the difference. CHT, EGT, lean of peak operation, engine smoothness, climb performance, speed versus power, fuel consumption and every other parameter I?ve seen have been normal and consistent with what other 14 owners report.

Will I now install the correct long reach plugs? Nope. My mission is long distance travel and these plugs have served me well. The more expensive long reach iridium plugs might produce another horsepower (or not) but I cannot believe such a change would make any significant difference.

Just a heads up as to my inadvertent experience.
 
Will I now install the correct long reach plugs? Nope. My mission is long distance travel and these plugs have served me well. The more expensive long reach iridium plugs might produce another horsepower (or not) but I cannot believe such a change would make any significant difference.

1. heat transfer

2. reduced thread load
 
Installing long reach plugs now will require a good cleaning or replacement of the adapters.
 
Hey Bill, you can bet there wouldn't be any more HP on the Iridium or Platinum- maybe less. Copper plugs are a better conductor than the Iridiums or Platinum plugs and therefore could/should perform slightly better. The advantage or Iridiums or Platinum plugs is longevity only.
 
Hey Bill, you can bet there wouldn't be any more HP on the Iridium or Platinum- maybe less. Copper plugs are a better conductor than the Iridiums or Platinum plugs and therefore could/should perform slightly better. The advantage or Iridiums or Platinum plugs is longevity only.

Most Denso and NGK plugs use a copper core, only the center electrode tip is Platinum or Iridium. Ground electrodes are usually a nickel alloy.
 
1. heat transfer

2. reduced thread load

1. Heat transfer? How so? This would be a problem only if the spark plug were longer than the adapter. In Bill's case, the entire length of threads of the plug is in contact with the adapter, so the plug transfers heat as designed.

2. Again, the short plug is completely threaded into the adapter, so its thread load is same as if it were threaded into a short adapter. And, the outer diameter of the adapter is threaded along its full length into the head, so no distress there either.
 
Could the exposed threads of the adapter create a glow plug situation?

That would certainly be a consideration if one was pondering possible negative outcomes of this installation without having done it.

But the purpose of my post was to describe what actually did happen in this situation and there is no evidence of any distress whatsoever. Aside from all engine performance indications being normal, oil consumption has been stable, oil samples good, compression high 70s and filter clean every time I change oil.

That said, had I realized early on that long reach adapters were intended not only for long reach cylinders like the IO-390 has but also intended to house long reach plugs, I would not have done this. But because of the way I read the P-Mag install manual I used short reach plugs and wanted others to know the results.
 
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