'66-'69 factory 427's with 10.25 & 11.00 came with AC43N so I'd start with that heat range.

 

Thanks. The car currently has Champion RN12YC, which my googling tells me is similar to the AC44NS.

 

I have 215 heads on my 427 solid roller,using the AC R43XLS.

 

The "S" in the designation indicates an extended tip plug. I've always run the AC43N plugs in our L72 and L78 engines with no problem.

 

I am seeing carbon fouling on my plugs as I pull them out as well.

 

That's typical for a Champion plug. They are well known for being substandard plugs.

 

I am also running MSD box, coil, and distributor. Sorry for the spotty details!

 

Lose the Champion plugs. Believe it or not I have had great luck with Autolite or Motorcraft plugs. :secret:

 

What PN would a similar plug be in Autolite or AC? Thanks!

 

I'm gonna try AC R44XLS and see how she runs!

 

Make sure you need either the gasketed plugs, or the taper-seat plugs. It depends on your casting number.

 

Just so I can give good advice here....what heads are on the engine? Casting number and date please. So many non stock combos out there....

 

I am doing VC gaskets this afternoon, I'll let you know when I get home. PO claims they are rectangular port heads. Thanks again for everyone's help!