Are these the best spark plugs at 9000 ft??

[4404spdcharger]

I live at 9000 ft and do most driving between 5000-7500 ft.  I've changed the carb jets and advanced the timing to 12 deg at idle.  The car is a 68 Charger R/T 440 4 spd bone stock.  Here's a pic of a plug I looked at yesterday.  It looks OK compared to the pics I've seen in books that "read" plugs.  Note it's a Champion J12YC, the MOPAR service manual recommends J11s or J10s.  Anyone have any comments or experience at this altitude?  I have this problem with all my vehicles (snowmobiles, dirt bikes, boat, 75 CJ5) so any advice would be helpful.

[TylerCharger69]

I do know that certain elevations can require plugs of either higher heat ranges or lower heat ranges....but I'm not sure which direction to go in higher elevations as I live right at sea-level.  And I know carburetor needs tweaking as well.....someone will chime in on this one....I'm sure.

[HPP]

Most of my driving is done from 5000-7000 ft and I do or don't necessarily change plug requirements from sea level depending on the compression in the engine. Compression is also a big requirement and we need to build more of it in our engines to maintain power. So, if you have more than stock compression, you can stick with stock heat ranges. If you have stock compression, you may want to step up the heat range one or two steps. The reason for this is the lack of air, or the abundance of fuel, in the charge at our altitude tends to make the combustion process run cooler. To get the plug to run at its best, you need to either lean out the mixture or step up the plug heat.

Your plug looks a little dark still, so I'd say you still need to step down the fuel mix some more. You'll need to do some experimenting to determine if that is at idle, part throttle, or full throttle. Or step up one more heat range on the plug to promote the self cleaning action better.