Isolating the Restriction
 

Just want to get a sense for this: Is the stock header a bigger restriction than the overpipe/front pipe? Where is the bottleneck? The easy answer is "all of it" but I was just wondering how restrictive each of these components are by themselves. Is the primary cat the biggest culprit?

I can't seem to remember but I think someone made really good power with just replacing the header and leaving the rest of the exhaust stock. I think it was FA20club header but I can't be certain.

It's not a restriction per se, but The best ones at removing the torque dip seemed to be the equal length headers - nameless, p&L, fa20club, etc. The dynos didn't show big gains, just flatter curves. This is a handy thread:
http://www.ft86club.com/forums/showthread.php?t=31555

Biggest restriction is easily the cats. An aftermarket header removes the cat, or replaces it with a hi-flo cat.. that's where people are seeing the biggest gains in WHP and what many interpret as "removing restriction".

Replace/remove the 2nd cat, and you'll remove additional restriction and get more WHP.

Perrin saw almost no change by cutting out the front cat and replacing it with a straight pipe, this was with the rest of the exhaust already changed.

The fact that no amount of tuning with the stock header in place can get rid of the torque dip and the fact that most headers with a tune are able to almost eliminate, if not completely, the torque dip... I would say point to the stock header being the biggest bottleneck in bottom end torque.

Once you get into the upper rpm's you have to do the whole exhaust from the block back to get the biggest gains above 7,000 rpm.

Of course it's more complicated than replace this for that... the key is to maintain the highest velocity with the lowest back-pressure in the rpm range you want the biggest gain...

I think moving forward, you will see people doing the header before anything else, when they are after area under the curve anyway and not just sound.

Dynoed my car yearerday. With the stock tune, the Borla UEL header did not increase peak hp and torque, but it did increase significantly in the torque dip area.

The cats have nothing to do with a restriction... you may see 1-3 whp difference from catted vs catless. The restriction is a combination of the stock header and a tune which increases the flow drastically. The header alone frees up some horsepower, but not near as much without the tune as well.

Yeah, I honestly think the order from most restrictive to least is Header, axle back, front / over pipe, midpipe. Then again it could be Header, front / over, catback. Header is deff first on the list though.

Header > Overpipe/Frontpipe > (Pick your flavor of catback here with a non restrictive/open muffler)