I have a N/A 331 stroker in my 86 GT. It has GT40 heads and I’m using Autolite 104 plugs, but I think the 104’s are too cold. What would be a hotter plug I could use? I’d like to try something the heat range 4. Any suggestions?
I can't give an opinion without more info but I would investigate the rest of the ignition/timing and try a different brand plug before changing heat ranges.
104 is the stock plug for those heads?
I'm guessing you've ran into the same issue I have with my 408W. It was built for use with nitrous so a bit of extra gap in the rings and a slight overbore to begin with. After a couple hundred passes it's using a bit of oil and the plugs get sooty quickly. For non-track use I stepped up a heat range and added a quart of Rislone Ring Seal w/ the 20w-50 oil change. After that, the plugs look much better. I would freshen the engine but it still runs as strong now as it did when first built You may also want to check your air fuel ratio to make sure it's not running overly rich... Hope this helps.