Those headers pictured are the typical 50 year old one size fits all header design. Those are good for 400 plus HP and are best used in a typical street driven hot rod with an engine of stock or slightly bigger size.
If you went to the next size header, a 1-3/4 tube, I’d call it good for better than 500 hp, but not much more.
Going up from a 1-3/4 tube header, your in race car Territory. I said that the smaller head or won’t make the power above what I list, buddy longitude Heather will do it better. You can still take the small to better higher power. The signs out had a tube size, it gets kind of funny and dicey. There’s a lot of parameters and I’m leaving out that we need to be discussed and sign stout. This I suggest, you do a lot of homework on!
