I found in the manual a statement that says the cold type spark plug is suitable when spark knock may occur from
1)extended highway driving, or
2)frequent high engine revs
while using the normal plug. So does this mean that people with just intake systems (not only those squirting or blowing) could use colder plugs too if they're experiencing problems? The reason why I took my JWT out was because I occasionally got knock, more than I could live with. Could I go to one step colder plugs for the summer and put the JWT back in? What are the disadvantages of cooler plugs? I don't fully understand the differences in spark plug temperatures to begin with.

 

Check out the 4th gen forum and look for Daniel B. Martin. He has explained this to someone before.

I wouldn't suggest going to colder plugs unless you have a SC, NOS, or turbo.