Ngk TR6, out of box at .038"
6.5 psi boost on 4.8L, unleaded 90 octane, 15 degrees spark above 3000.
At what level of boost will I need to close up spark gap ?

 

The point right before it starts misfiring under load due to too large a gap. All things considered, it's too difficult to say exactly. You'll just need to adjust gap if you have issues.

 

When I was at that boost level, it was suggested to me to drop the gap to .025.

I did and it ran sooooooo much better and smoother. Just a thought.

 

My opinion.. Below 10psi run the gap .032 on a 6 heat range. 7's require a tighter gap, I run my 7's .021

A 6 heat range shouldn't require a .025 gap, if you are running enough boost to require that then you most likely need a colder plug.

At the end of the day regardless you need to learn to read plugs if you are going to boost it.

If your spark isn't being blown out with your current gap then leave it, Ideally you want as much gap as you can run without blowing out the spark.