NGK Spark Plug for 915is?

From personal experience you DO NOT want to use the 914 NGK plug on a 915iS. I installed a set and gapped them to .024".

Everything ran perfectly for 30 hours and then I began to get intermittent lane A lights indicating random sensor failures

that reset easily. Then as things progressed I started getting sensor drive voltage alarms as well. Never occurred to me that it was the plugs because it ran good for 30 hours, Thinking it might be an ECU issue I tried another known good ECU and got the same results.

Bought a new set of NGK plugs (which I am now stuck with), but when I pulled the old NGK plugs at 130 hours and checked the gap on them I found 4 of them at .035", 1 at .037 and 3 at .040". Evidently the 915iS is very hard on the NGK electrodes and wore them down, increasing the gap quite quickly. This increased the ignition secondary resistance which required more power from the primary system to jump the plug gap causing sensor failure indications. The sensor drive voltage alarms indicated this as well. Once the plug gap got wide enough it became hard for them to spark, causing misfires and power loss under heavy load.

Installed a new set of the very expensive Rotax plugs and all issues went away. Hopefully my trauma prevents others from experiencing this. Had it not run so good for that first 30 hours I might have put 2 and 2 together and figured this out much sooner. Of note is that these NGK plugs were the real deal and not a set of the fake ones.

Cheers

HKK wrote:

Dear Glenn

Have we been screwed all this time about the thermal values of the spark plugs for different Rotax engines? No, but Now. For some Rotax engines the Rotax spark plugs are to cold. It never could be that a spark plug can substitute other plugs with differerent thermal values.

I had the same problems - contact with Rotax is nonsens - your worse your time and you be fustrated too, they never are interested in your problems, they never accept your experiance.  The guilty party is always the user.

So i did my own testrow (I made a tiny documentation about this), Rotax, NGK and NGK iridium was into test. Short result: Rotax spark plugs get to fast a black carbon, the spark ist not steady. Below 4000 rev. the spark find his way only at one side of the electrode. Rough running, Nearby the resistance of 5 kohm is very variable. This ist not really a problem but it shows the quality standard of the russion Bosch spark plugs, labeled with Rotax.

I cannot understand that experts are not louder in this case. In my opinion the livetime of your enginge reduses dramatically with Rotax spark plugs. I can understand if somewhere say "make our own decisions" but in my oppinion to put it this way is irresponsible

HKK

"I cannot understand that experts are not louder in this case. In my opinion the livetime of your enginge reduses dramatically with Rotax spark plugs. I can understand if somewhere say "make our own decisions" but in my oppinion to put it this way is irresponsible"

NGK told Rotax that they would no longer allow their plugs to be used in aviation, it was not Rotax who made that decision.  We have stated this for years but everyone thinks they know better.  It would be irresponsible for Rotax to equip an engine with NGK after being told they should not, in the USA lawyers would love that and the results would be more damaging fighting legal challenges.  

It's nice that you took the time to do all that testing. However one must remember that how the spark behaves depends on a lot of factors you can't replicate. Such as humidity, combustion chamber pressure, how well the fuel is atomized and outside air humidity to name a few. The point of my post was to make others aware of the standard NGK plug not being able to hold up under 915iS conditions and what happens to the FADEC system when the gap increases. Not sure if the iridium plugs would work better or not. At this point I'm done being a potential crash test dummy and will stick with the Rotax plugs.

Glenn is correct.

There is a misconception of what the Rotax dual ground plug works like.  Any spark plug, single ground, dual ground or some brands have even more, all will fire to the one with least resistance.  In a dual ground it always goes to one ground.  If that ground has any contamination, more resistance, it will then seek out the other side. If there is wear (increasing gap) it will select the nearest or one with the least resistance.  This is why Rotax was able to go from 200 hour replacement time to 400 on auto fuels.  (no lead)  

Just my views.

Cheers

Thanks RW I didn't know that.