spark plugs with black at bottom, white up top

What does it mean when your spark plugs are black around the base, but the metal tip is white? Here are some pics of the plugs:

http://www.phastnet.com/users/shawn/images/ngk-1.jpg
http://www.phastnet.com/users/shawn/images/ngk-2.jpg
http://www.phastnet.com/users/shawn/images/ngk-3.jpg
http://www.phastnet.com/users/shawn/images/autolite-1.jpg
http://www.phastnet.com/users/shawn/images/autolite-2.jpg

The NGK FR5 plug had about 5-6k miles on it, while the Autolite 3924 only has about 100-200 miles on it. My guess is that there is enough fuel (the black part), but the temperature at the tip is too high causing it to turn white? If that's the case, should I try a colder plug?

Some background info, in case it helps:

94 Z28 T56
383 c.i. (11.25:1 scr)
62cc Trickflow LT1 heads
230/242 (.510/.540) 114lsa comp cam
1.6 roller rockers
Comp 'R' lifters
Jet-hot long tube headers
etc

The car has been running good, it just has a random miss at low rpms that I can't seem to figure out, and it will also occasionally backfire out the intake if I blip the throttle from an idle. But otherwise, it runs very strong and smooth throughout the rpm range.

Thanks in advance for any help!
Shawn

Wow, I'm surprised you haven't gotten a reply yet. I'm not an expert plug reader (especially with EFI), but what you said sounds reasonable. Try plugs one heat range colder and see how that works. Getting plugs of the right heat range is pretty much just guess-and-check.

Hmm, backfiring out the intake is usually an indication of a lean condition. If it doesn't change with the plug change, you might try adding a little power enrichment (or whatever EFI term is equivalent to accelerator pump shot).

Haha, ya, I was about to post again asking if I was in the wrong forum :)

I guess the next issue is trying to find a step colder than the FR5, since Bennett Auto tells me that they don't make FR6 or FR7 and it seems to be impossible to get ahold of elitesparkplugs.com, which happens to be local to me.. Oh well, someone's bound to have something..

And ya, I had read on these forums awhile back that the intake backfire was probably a lean condition, but there doesn't seem to be a way to specify extra "pump shot" in LT1Edit or Tunercat, so I guess we're supposed to be relying on the computer figuring it out on it's own when it sees MAP or MAF changes.. The only way I can affect it is by adjusting the throttle body idle screw, but it never seems to go away completely.. Hopefully the colder plugs will help some..

Thanks for the reply!
Shawn

Use the regular copper Autolite 104 gapped at around .040 if you're using an MSD.

My 396 didn't like NGK plugs for some reason.

Mike

Except that these Trickflow heads require a gasket seat on the spark plug instead of tapered seat like stock, so Autolite 104 won't work :(

It looks like the Autolite 3923 or 3922 would be my best bet, so I'll call around tomorrow and see who has some.

I do have an MSD, so I'll try .040 this time.. I think I did .045 on the previous plugs, which is probably too large (especially for nitrous), although it worked..

Thanks again for the replies!

Autolite makes good plugs.

Hmm, didn't realize the Trick Flows take gasket seat plugs. Do you have decent plug access, or does your car suck that way just as much as any other 4th gen?

Ya, same deal as any other LT1 with the plug access.. The Trickflow heads look identical to stock (once you put the valve covers on :). The plugs are pretty easy to change though with the Jet-Hot long tube headers and a 3/4" ratcheting wrench on the back of the spark plug socket.. I need to shorten the socket a bit though and I could then change all the plugs within 20-30 minutes instead of 45 or so.

Anyhow, I got a set of Autolite 3923's (one step colder) and installed them yesterday.. Not sure if it's better or not though, since my TPS sensor just crapped out and it definitely doesn't feel normal now :( What crappy luck, the TPS sensor was one of only 3 original electrical parts left on this engine! lol