Hotter plugs for GT40 heads

[Jarhead67]

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?

 

[General karthief]

Why do you think they are too cold?

 

[Jarhead67]

Why do you think they are too cold?

There‘s a lot of soot on the plugs after driving. I’ve shown pictures to several mechanics in our Mustang club, and they concur.

 

[General karthief]

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?

 

[Noobz347]

What do you have them gapped at? Want to check and see if hotter plugs will help anything? Close up the gap in the plugs.

Also: Check coil saturation. What is your voltage going [into] the coil with the engine at idle? What is the voltage at the battery terminals at idle?

 

[96pushrod]

If you’re fouling plugs with 104s, there’s likely something else wrong.

 

[Jarhead67]

If you’re fouling plugs with 104s, there’s likely something else wrong.

I ask because 104’s are cold, they‘re heat range 2. That’s what’s called out for GT 40 heads though.

 

[Jarhead67]

What do you have them gapped at? Want to check and see if hotter plugs will help anything? Close up the gap in the plugs.

Also: Check coil saturation. What is your voltage going [into] the coil with the engine at idle? What is the voltage at the battery terminals at idle?

.054 is what’s called out for those plugs, but I can close them up. Voltage is 14 at both locations, I’m running a 130 volt alternator.

 

[Mustang5L5]

If you’re fouling plugs with 104s, there’s likely something else wrong.

I concur

Changing plug heat ranges is trying to cure the symptoms, but I would suspect there is something else that is potentially wrong.

 

[a91what]

dwell is set by the TFI module.
Can you post up some pictures of the plugs, the car may very well just be running rich

 

[96pushrod]

I ask because 104’s are cold, they‘re heat range 2. That’s what’s called out for GT 40 heads though.

Are you sure on that?

 

[mrspeed]

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.