Hi Steve I had 2 inch hooker super comp headers on the nova at 18 lbs and it worked ok,Lemons made a set of 2.5 inch and they definetly made more power at 23lbs of boost.They told me that on these boosted engines you cannot go to big.
Depending on what car its in, you can usually buy 'off the shelf' headers up to 2-1/4". Most aftermarket BBC heads need a 2-1/8" header just to clear the exhaust port and not block it. That's what I'd go with if it were me and price was an issue. If you want / need something custom built, like Lemons, Kooks, etc., then you might want to go a tad bigger but I'd ask the builder first. Its not like losing a little low end torque is a big deal.
I have 2.5 inch with a 509 but more boost. If you are buying/building and plan on bigger in the future, I would go 2.25 minimum. Hell I had 2" on a 347 SBF and they worked well and was still an animal down low. Like 68 Bird said bigger the better and loss of down low torque isn't usually a problem on a blower car.
Hey I am sorry about that, the smiley face wasnt supposed to be in there...........(928) 692-0111.
I know this sounds untrue but I will bet you his headers will gain horsepower over any off the shelf and most any shop headers. You would have to be on a dyno to see it but I only say that because I have seen it.