Spark plug theory

OK, here's your chance, yet again, to show how much more you know about cars than me. The only trouble my B had doing the 1500 miles worth of Saint Louie adventures was adjusting the mixture back and forth. One of these days I'll get it right (I'm down to 1/8th of a turn on those goofball HIFs), but that minor annoyance is semi related, I think, to my theoretical question: Namely, what does the "temp" rating of a spark plug mean? Is a "hot" , or "cold" plug meant for a hotter/cooler engine temperature, or does it produce a hotter or cooler spark temperature, or what? And if you have a "hotter" or "cooler" plug, can/should the mixture be adjusted one way or another?

No expert here, but a hotter plug is used to more completely burn what is in the chamber (more spark).

So the hotter the plug, the richer the mixture can/should be? Seems like every engine would run better with a hotter plug and a richer mixture then. There must be a point then where the cyl head temp would be too much..?

Not exactly - actually, the combustion temperature is hottest when the mixture is correct - a richer, or leaner, mixture will cause the temp to go down. The plug "temperature" has more to do with compression ratios and mixture distribution in the chamber than with mixture. Unless you have significantly changed your cylinder head, the standard plugs (N9Y) should work fine.

Aircraft have a mixture control as well as a throttle - and mixture is set by monitoring the exhaust gas temperature (EGT). The mixture is "correct" just slightly rich of the highest EGT. However, rich mixtures are used during heavy engine load (climb) to help cool the engine!

Eric,
The terms 'hot' and 'cold' used for spark plugs refer to how quickly or slowly the plug can transfer heat from itself to the cylinder head, and thus how hot the tip of the plug runs. A plug that transfers heat quickly would be a 'cold' plug -- if it's too cold, it cannot burn off combustion deposits and it will foul. A plug that's too hot will not foul, but could cause preignition or even melt or cause engine damage. It has nothing to do with the quality of the spark, etc.

A buddy of mine who's into military vehicles has posted a nice page explaining the ins and outs of plugs:
http://www.garbee.net/~cabell/SparkPlugData.htm

HTH!

Rob:

That's a great web reference - a great description of spark plug theory and practice - I'm sticking with N9Y's!

Rob, thanks for that link. I'm constanly amazed at the level of engineering that goes into some of the most seemingly mundane items, and also the marvelous utility of the internet. Isn't it great how we have home access to what is basically the worlds largest library? I keep getting educated well beyond my intelligence!
Eric, I have often wondered why some of the aircraft techniques and technology don't transfer over to cars, or at least custom built cars. Seems like the EGT method would be the easiest way in the world to adjust the mixture in a car (and if turning off the key to kill the engine isn't acceptable in aircraft, why do we do tolerate it in cars?). I have fiddled with the silly little "lifting pin" on my HIFs and can never for the life of me figure out where it's "right". Probably doesn't help matters that one seems to lift the piston higher than the other! I'm getting pretty good at pulling spark plugs though!

Eric:

I do the opposite - wonder why auto technology hasn't transferred to aircraft! The reason "pull the mixture" to stop an aircraft engine is so that the engine will not fire if the prop is turned - the magnetos generate their own electricity so an aircraft engine can start even with the key "off". Unfortunately this means airplane engines have static ignition timing. Why we can't convert to electronic ignition with a backup battery is beyond me. The experimentals with electronic ignition start more easily, idle better and give more power! Oh well, I still love my little airplane - came back from northern Michigan yesterday at a ground speed of 130 to 140mph!

If you are having trouble setting your mixture I would suggest a Gunston Colortune - LLBC have them for $40. They are easy to use and take the "pin lifting" out of setting the mixture! The "Click Adjust" also works well for setting valves!

I was taught that the reason is because you don't want unused fuel running down the sides of the cylinders into the oil, and the mags are still turned off with the key, aren't they? But try and convince the FAA to allow some newfangled (1920's) technology to replace the magneto system! Hah! Good luck!
I'm pretty sure I'm very close to right on, but I really think the ultimate answer to my mixture settings is to buy the supercharger kit and toss the HIFs in the parts pile :) I just may get that colortune tool next, I got the click adjust and it does work pretty slick. I definitely had some crowned rockers that gave a false feeler gauge indication.

Eric, guess what the supercharger pulls fuel through in the Moss kit? Yep, an SU HIF!! Modified, of course, but still an HIF!!

Yea, but half as many mixture screws!

Hi guys - I've just got to throw my two cents in here on SU carb tuning.

As Sen. Earnest Hollings would say:
Theah's entarly too much tweakin goin' on out deah.
translation: there's entirely too much tweaking going on out there.

IF:
..The throttle plates are synched and locked together
and
..The needles are at the same height from the bridge
and
..The needles have the same shape
and
..Standard maintenance items are OK
(No vac leaks, dashpot oil topped up, dashpots move freely, floats and needles OK)

THEN the problem is someplace else.

hot or cold, the term used may be heat ranges....hot plug equals a lot of short range driving to burn all the fuel.

a standard plug is used for the normal on the road driving or anything over about 1/2 to more of driving.

the high heat range plug heads off the build up of carbon on the short sunday drivers.

If a high heat plug is used on a long trip the build up of heat can punch a hole in one's piston....stay with the normal plug for instance NGK BP6ES
gapped at 25-30 for normal points or 35 for electronics. have a good ride