Benefits of bigger plug gap for PMag?

[jjconstant]

I'm installing 2 PMags after the LASAR failure in my 7A. I have the expen$ive fine wire iridium plugs in my IO-360 with 9.2:1 pistons. I am committed to using these plugs. I already have the aviation harness for the PMag.

I am concerned about opening up the gap to the recommended .30 - .35 gap for the PMag. They mention that some plugs "are difficult/impossible to adjust this wide." How do I know how much I can open them up? Anyone gapped them up to .30?

I don't want to break these expensive plugs so...how much and what kind of improvements would I be giving up on either leaving the plugs at the factory recommended .16 -.21 gap or opening it up "a bit"? Thanks

Jeremy Constant

[Fred.Stucklen]

I'm running the fine wire iridium aviation plugs on an IO-360 with P-MAGS. No issues opening up the gap.....

Fred Stucklen
RV-7A N924RV 525 Hrs

Originally Posted by jjconstant

I'm installing 2 PMags after the LASAR failure in my 7A. I have the expen$ive fine wire iridium plugs in my IO-360 with 9.2:1 pistons. I am committed to using these plugs. I already have the aviation harness for the PMag.

I am concerned about opening up the gap to the recommended .30 - .35 gap for the PMag. They mention that some plugs "are difficult/impossible to adjust this wide." How do I know how much I can open them up? Anyone gapped them up to .30?

I don't want to break these expensive plugs so...how much and what kind of improvements would I be giving up on either leaving the plugs at the factory recommended .16 -.21 gap or opening it up "a bit"? Thanks

Jeremy Constant

[jjconstant]

Thanks Fred. How much did you open it up and did you just pull the gapper through to open it up?

Thanks again

Jeremy

[jjconstant]

I'm also still trying to figure out the benefits of a larger gap.

I understand the benefits of being ABLE to jump a bigger gap with a stronger spark, in the case of worn spark plugs, but why purposely make the gap bigger? What improvements would I expect to see and how would I measure them?

Jeremy

[allbee]

this is how I look at it. Yes maybe the .30gap is ok, but you'll also find that if the gap gets to lets say .40 or bigger it will cause a problem with the ignition system. So if you leave the gap at 28 which is stock on the plugs and the gap opens to 30 after a few hours(100) than you don't need to sweat it. Generally the smaller gap is suggested for the low compression engine as well. Unless you want to be removing and checking the gap all the time, I would suggest just leaving the gap alone and run them. I have the smaller gap in both my pmag and the plasma 3. Runs great.

[elippse]

If the ignition system has enough voltage to fire a wider gap at high cylinder density and pressure, the flame front will start a little bigger and progress across the cylinder faster, and in that regard, it is the same as advancing the timing a little. Get it too large for the available voltage and power and there could be uneven firing. It's usually best to stick with the ignition system manufacturer's recomendations based on engine and compression ratio.

If you do decide to try differing gaps, please maintain a log of the operating conditions such as Baro and OAT and at least a two-way GPS-measured TAS and if you find consistently better results under a wide variety of conditions, let the manaufacturer know and maybe he will then come up with new specs for his customers. Just don't rely on a faster idle rpm to say that it is operating a lot better. Don't substitute wish fulfillment for critical thinking! Just wanting it to be better is not good science.

[elippse]

I'm not familiar with the electrode orientation in the fine wire plugs, but one of the things the racers do on their engines is to get the regular ground electrode pointing toward the center of the cylinder. This is based on the idea that there is some swirl in the combustion chamber and they want to get the spark so the the mix flows past it without any inteference. They mark the side of the plug, tighten it down, and then add thin washers until the plug is oriented they way they want. Supposedly this gives them more power and more consistent power from cylinder to cylinder.

[jjconstant]

Thanks for the feedback guys. If the net benefit of a larger gap would be similar to advancing the ignition a bit, I am comfortable leaving the plugs alone and letting the PMags do the advancing based on the MAP and RPM. I'm starting with the jumpers in and the more conservative curve to begin with. If all is well, I'll remove the jumpers. If I want even more advance I'll look into programming it into the PMags, but my guess is, for the type of flying I do and the longevity I want out of my engine, I don't want to push it too much. I already have 9.2:1 pistons and variable advance...

If I start experimenting with this, I will definitely do it incrementally and do data dumps from the EFIS on every test flight as I have done for all my flight testing. I'm WAY too familiar with "that change was good for AT LEAST 10 knots!" It's a standing joke around Livermore...and elsewhere I'm sure

Thanks for the explanations and thoughts

Jeremy Constant

[elippse]

Right on, JJ!

[G3i Ignition]

I have personally run and tested different spark plug and gaps. Single spark and multi spark systems. Using automotive style Iridium Denso & NGK plugs. I did not see any improvements with larger gaps over .026”. I recommend .022” - .026” on the automotive style plugs and standard aviation plug gap of .018” - .022”, opening beyond .026” just doesn’t show any benefit. These gaps are recommended for all our G3i multi-spark systems. Increasing spark plug gap will help ignite the air fuel mixture to some point, determining the max gap can be very difficult. This may sound like a broken record, if you screw in a 25-watt bulb in a 100-watt socket, its not going to be any brighter.