I ran NGK D9EA's in my Jab 3300 for about 200 hrs. I switched to NGK DR9EA iridium plugs in an attempt to lessen rf noise (it helped!). Eventually, I started developing fouling on one or both #5 plugs which would clear with high rpm's. I switched back to D9EA's without improvement. Boroscope exam showed #5 with less cross-hatch than other cylinders and valves looked good - compression 78/80. I eventually had Ben at Jab USA rebuild the head to be sure valves were OK - there was slight corrosion on the #5 exhaust valve stem, so I had him replace both valves since they weren't that expensive and the head was already apart. The problem persisted, however, so I tried NGK D8EA plugs on #5 (NGK D8EA is a slightly "hotter" plug). This has completely resolved the problem and the EGT's and CHT's are extremely well balanced.
My research says one heat range change in a plug is a very minimal change and has negligible effects on combustion and EGT temperatures - the longer tip insulator simply keeps the tip hotter to burn off deposits.
My question is: Would it be OK to run D8EA's on all the cylinders? The only reason I would want to do this is it would be simpler to run all the same plugs on the engine - one less thing to keep up with as far as which plug goes where!
John L Austin
(I should add that I run 93 AKI non-ethanol mogas and the #5 fouling appeared to be oil fouling.)
John L Austin
Hi,
I can't say that I've experimented with different heat plugs so I'll have to fall back on theory and logic...
If overhauling the head didn't stop the fouling issue (assuming the guides were replaced or confirmed within wear limits) then you might have a stuck oil ring on that cylinder - but most likely with those leak-down numbers your compression rings are ok. How is your oil use?
I'd be wary of going for a hotter plug - the tip can cause pre-ignition if it gets hot enough. Oil contamination in the cylinder also tends to reduce the "Octane" of the combustion mix and can lead to similar combustion issues. Has the engine ever "run on" (kept running after the ignitions were turned off)?
But anyhow, to answer your question... anything is possible - but I wouldn't. Making sure the tuning is right would be tricky, especially as plug colour when burning unleaded fuel doesn't tend to give clear mixture indication. EGT monitoring would also only tell part of the story: pre-ignition wouldn't necessarily give a high EGT. Instead I'd recommend pinning down the source of the oil fouling.
Regards,
Doug.
My oil consumption is about 1.6 oz/hr - this has been fairly consistent over the life of the engine (575 hrs). Ironically, the engine occasionally would "run on" after shut-down, but there have been no episodes since I switched the #5 cylinder to D8EA's. Likely that's just coincidence since all the other cylinders still have D9EA's? It might be interesting to put D9EA's back into #5 and see if the problem recurs ... I suppose it is possible for a stuck ring to become "unstuck?"
Thanks so much for your insights, Doug!
John L Austin
If I've done the sums right, 1.6 oz is about 50ml: which isn't high.
Running-on can be caused by a few things... high ambient temps, high idle speed etc, so it's hard to pin it down to one particular thing. You'd have to monitor it over a long period to see if the trends are different.
It is possible for rings to unstick, if they're caught early enough and some remedial action is taken.. but in the normal course of events they usually stay stuck: Mr Murphy and his laws see to that!
Regards,
Doug.
