Adding a RB header=? HP?
 

I have a free flowing exhaust as it is, hollowed out cat and a RB muffler. How much (if any) performance gain would I see with adding a RB header? What's the real difference in air flow between the stock manifold and the header? Thanks in advance for the education!

ALOT...not sure of the exact numbers but you would definatly notice a difference. with the stock manifold the exhaust shoots out and runs into a wall, building up back pressure before it exits the manifold. headers direct the exhaust towards the back of the car before it has a chance to build up any momentum in the wrong direction. because the gasses build more momentum towards the back of the car quickly, at the end of the exhaust stroke, the fast moving exhaust sucks alot more exhaust out of the engine, leaving more room for fresh mixture and creating a low pressure zone for even more mixture. these effects are much more felt with a ported engine with a corresponding exhaust system but even with stock ports there will be a significant difference.

around 20% best money i ever spent...

Really? Wow, I forgot to mention that I have a streetport on my 13B.
Looking forward to getting that done.

Thanks!

get the street-port header. with the street-port, the thing should go like hell! i like mine and it only has a standard port

Gueeze- you have a streetport with a crappy stock exhaust?!
Go get that header!!!
The 7 is right. Paul Yaw has a great explaination of this exhaust reverberation tuning on his website. Very much worth the read.
He tends to say all headers on the market suck except his, but waddaya want- he's endorsing his product.
RB headers are excellent quality.

A hollowed cat is pointless, it's just an open chamber for gases to build up and create backpressure. Buy a test pipe with the header, you're going to need a custom cat-back or custom test pipe. Racing Beat designed their product so the easiest way to bolt it on was to buy more of their products.