I've been studying David Vizard's exhaust chapter in his Max Performance Chevy Small Blocks on a Budget book. But his article in PHR http://www.popularhotrodding.com/eng...h/viewall.html
is a good summary.
I can now understand why the exhaust system sucks in our cars.
1. Poor flowing exhaust port
2. Low flowing cats.
3. 2.5" stock exhaust does not flow enough CFM
4. Restrictive muffler.
Unless you port the heads and prep the valve (in my area I was not able to find someone to perform the radius the chamber side of the exhaust valve trick found in the David Vizard Book), but if ported, a Chevy exhaust port can flow close, if not 200 cfm.
If you're going to use the stock iron manifold, port the top and sides as per the books guideline, as this will add about 15hp. There were a few other places and nuances to heed in the book about porting manifolds.
But it did recommend headers. See the sizing chart. Ideally, you need 1-5/8 or 1-3/4 diameter depending on how well your exhaust port is flowing. After this, you're looking at primary length.
Here is the problem. If you're going to go shorty's (ie. Dyno Don's, Hedman's, Hooker's) you're not getting the recommended 24-36" pipe lenght that is recommended, despite getting the pipe diameter right. I know that the Hooker Super Comp Long Tubes meet this requirement. However, they do hang pretty low for ground clearance, and you gotta do some fitment to get them around the hydraulic clutch cylinder in manual cars. But theoretically, according to Vizard, these are the "correct" pipe lengths at 24" and diameter (1-3/4) for 3rd gen Camaro's making over 375HP.
Our cars just have fitment problems for headers, so if you're using shorties, they ARE better than the stock iron manifolds as they improve flow and separate the exhaust ports thus reducing pumping losses. The collector/secondary should be 2.5" for street performance, but for motors over the 375 HP mark, a 3" collector is recommended, and preferably longer collectors, or merge collectors. This isn't always possible with what is available on the market unless you're going with custom headers.
There is a lot of debate regarding headers on the board. I'll let you read the article and books and make a decision.
As for everything behind the headers:
You need a dual cat set up. http://www.samracing.com/HR-Oct00_pg3.html
According to S.A.M's testing with a 4th gen converter, let's assume that the stock converter does only flow 304cfm. Divide that by the magic 2.2 figure and it's only good for 138HP! A dual set up then, is only good for 276HP! Remember the 350 TPI's? They only had 245HP at most, so the dual system was ok. But what is really needed is 2 CATS that can flow for the HP you need. One example would be Vibrant's race cat that flows 578 cfm according to their catalogue. P/N 7101 fits a 2.5" inlet and 2.5" outlet. 578+578= 1156 cfm / 2.2 = 525 HP limit.
The problem is that it Y's back into 2.5" usually on the stock y-pipe. If you can have something custom made, you'd have everything exiting into a 4" set up.
What do you need for a cat back?
The Mufflex 3.5-4" system according to this: www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/#easy
The 3rd Gen's were cursed with a single pipe exhaust system. As you can see, the stock 2.5" system behind the cats would only flow 509cfm and that's good for 232HP. But you if you're replacing your exhaust, go big. Mufflex makes a 3.5 and 4" system for a no loss back pressure system (from the pipe's end.)
Something that flows 2.2 cfm times your flywheel horsepower. So if you're up at 450HP, x 2.2 = 990 cfm. The Dynomax ultra flow series mufflers flow 1195. Good for 543HP.
The Super Turbos were great in the 80's, but only flowed around 600 cfm. Walker still lists that system in their catalogue and it's cheap. But it's not good enough for a third gen pushing more than 232HP. (The pipe diameter is the restriction. The muffler restricts at 272 HP. And their dual cat set up restrict at 355HP.)
So in summary:
-Hooker Super Comp Long tubes if making more than 375HP, otherwise 1-5/8. If you hate the ground clearance scraping sound all the time-especially on speed bumps and driving up your drive way if it's on an incline-shorties will be better than stock iron. You can debate which brand all you want. But whatever you go with:
-Dual Cat set up with vibrant or random cats that flow in the 500cfm range per cat so that you get a system that will flow past 1000cfm. You're may tear your hair out looking for a dual cat system off the shelf!
-Mufflex 3.5-4" system. You may have to trade off the drone of the pipes for performance. That's just what you have to pay.
-Dyno max Ultra Flow or any other muffler that will flow over 1000 cfm.
If you're looking at the max out of your exhaust, do it right once, or pay to do it twice!
As for the pressure wave termination box on a third gen---if you figure it out, let me know. I think the cats will do a good enough job and that there will be no more room left.
I almost considered the Walker stock replacement system, until I realized that I would pay again to upgrade the system later. It's not a bad system as long as you realize that it limits power to 232HP (which was good enough for the stock LB9.)