spark plug can CHANGE it's heat range - Page 2

10-23-2011, 09:03 AM

#26

HiFlite999

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Quote:

Originally Posted by olddragger

this is the mazda rotary engine that was being spoken off.
He was running lean a/f's. I didnt get that part? He was getting some varience in egt due to misfires?
Rotary av guys are fanatics (most home build plane guys are fanatics and rightfully so) and a lot of good info is posted concerning their findings.

I thought it would be good info to share here, mainly to those folks that have FI on this car.
FI cars have much higher loads than the NA folks etc etc and if you track the beast, the heat range of the plug is very important. I know of some guys that dont have a dedicated set of track plugs--perhaps they should? I know myself I have gone on track (successfully) with 5K mile plugs. After reading what this fellow has shared--I wont anymore. I never realized that internal breakdown like this was possible. The last thing I want is a glowing sparkplug.

The resistance test is a good one, but there is more to the story? I have always wondered exactly why our plugs go bad so fast and why the only symptom at times is a dyno that shows lost of power up top. I know coils are part of that scenario, but plugs that look good and have good resistance findings, now I have a better idea of what MAY be wrong.

The part about the separation between the cooper core and the ceramic was particulary interesting. And the fact that you cant see it with just an eyeball inspection. The trailing plug seems vulunable here especially?

*All* long-time airplane drivers are fanatics!

Aircraft engines run in a nearly totally different environment than cars. A major difference is that they run at 75% power for hours at a time, and are certified for doing so at 100% power. Very few cars on the road will survive that without a lot of mods. Aircraft plugs are, as a consequence, relatively "cold" and foul easily on startup and at idle. They are the equivalent to "track" plugs.

Since most airplanes have variable a/f ratio controls (the 'mixture' ****), one has a chance to, and is indeed required to, play with it. It's quite amazing how much it can be varied with no misfiring at all. The most efficient cruising condition is considered to be a the peak EGT reading. However, there is considerable variation between cylinders, so one nudges the mixture control forward (richer) to give a 50-100 deg F drop to be on the safe side. Some guys put an EGT for each cylinder, to then one can run at near-peak EGT for the leanest cylinder. (However, going to full power with the mixture "leaned" may result in instant loss of $30,000 from one's bank account, plus an unplanned landing.) This, and what he's talking about in the article, would however be impossible to do on the street, unless you can find a perfectly flat highway to run 75% power at 120 mph for many minutes along.

As a practical matter, the change in plug resistance over time may be more important. A friend ran his Honda for ~100k miles on the same set of plugs. No misfires at all until $700 worth of ignition system components fried. The ignition circuits are designed to work with a specific plug electrical resistance, if that varies, it will fail in a way similar to hooking up an 8 ohm stereo amp to 2 ohm speakers.

At least $70/four plugs seems cheap to me. Good platinum airplane engine sparkplugs run $100 EACH; 2 plugs/cylinder x 4 cylinders = $800. Even stretching them to 200 hours gives $4/hour just for sparkplugs ....

One reason I don't have an airplane any longer.

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