I found this along time ago I just cant remember what site I got it from interesting read:
"Exhaust valve duration. This is a question of relationship as well as airflow potential of your exhaust system. Know your exhaust to intake airflow ratio on your heads. If we assume 75% is the target, and we know that your heads flow 80%, we've got an exhaust potential that exceeds our need. In that case we need to decrease exhaust duration to artificially create a better scavenging wave in the exhaust (this is widely regarded as the most important factor in exhaust design). Of course headers have a HUGE effect on this because most scavenging and pressure waves propagate from the collector area which is directly affected by tube diameter and length (do you feel the math coming on again?). For arguments sake lets say the following guidelines define the optimum header diameter for a full length header on a V8. You can back calculate these numbers to determine the appropriate diameter for 4cyl, 6cyl, 12cyl, etc.

250-325 CID 1 5/8"

330-415 CID 1 3/4"

415-500 CID 1 7/8"

500-550 CID 2"

550-600 CID 2 1/8"

ADD 1/8" for every 50 CID over 600

For every 500 RPM over 6500 go up one primary size

These are rough guidelines that are obviously affected by many factors including materials, power adder, use, etc.

Now that you know your potential you can make a reasonable stab at exhaust duration. In general a single pattern camshaft makes more average power on a properly set up N/A combination. However, given that you have probably run into some constraint already, exhaust duration should usually be manipulated accordingly. It is not recommended to run any less than 4 degrees less than intake duration (reverse split camshaft), you should reduce primary diameter or put metal back in your heads (I know it's not possible, but you now know why your head work was so important). You should also not increase the duration of the exhaust more than 4 over the intake on an N/A motor. You're power adder is the next thing to address if you've got turbo, supercharger, or nitrous your motor has a LOT easier time getting air in than it has getting it to come out. Increase your exhaust duration 2 degrees for every 100 hp extra air your motor will see on the intake. This should be limited to a maximum of 10 degrees over the intake duration. If you have a turbo car, don't go too far with this or you'll start shoving too much exhaust back into the combustion chamber (this is a great way to increase detonation potential in your motor). If you've got a turbo motor this number will also be able to manipulate EGT which is important if you want to keep from liquefying your exhaust valves."

 

Not for N/A.

 

Looks like everyone with a 4.8L should remove their pacesetters and put on some 1 5/8" headers. According to that article, even a 5.3L with 1 3/4" primaries is questionable.

 

I rest my case

 

This is actually a true statement, the NA 4.8-5.3's would be better off with 1 5/8" primaries, just that until recently none were available (still not sure who actually does make the 1 5/8's). Still, the 1 3/4" headers are better than stock manifolds and y-pipe. I'd definitely be better off with 1 5/8" with my current set-up, but you know what my future plans are

Hey, I thought you hated it when you were catless...you're going that route again? (Although some good tuning will take care of alot of the odor you were experiencing, just not all of it)

 

Hey Guys, thanks for all the comments. To answer a few questions, the headers are 1 3/4" with cats. This truck is a daily driver and will not see any other modifications except a tune and maybe a CAI, but only when I can find documented proof of an improvement. There is a nice SOTP feel to it, but at 5700 lbs it is not as nice as it could be in a lighter vehicle. Someone asked about sound??? I had an 02 Corvette with headers, X pipe and ZO6 mufflers and this truck with stock 3 1/2" exhaust sounds very similar to the Corvette. It barks when you first start it cold and has a nice deep, throaty rumble at idle. It is loud as hell when you get on it and sounds normal when cruising. It is going to be tuned by Keith McCord in St. Louis as soon as we can get our schedules to match.

GMCtrk, you mentioned 1 7/8" headers. I don't know about your application, but here is some info that I have not seen mentioned on this site. I measured the AR Header Y pipe and it is 3" OD, so actually about 2 7/8" ID. The primary pipes measure 1.732" OD (just under 1 3/4") so depending on wall thickness, they could actually be about 1 5/8" or slightly larger ID. Just right for my application, but may be a bit undersized for your truck. I guess when it comes to headers, size really does matter. It always seems to come down to that with guys.....................

 

I live in St. Louis, any chance to meet up and check out the headers, or run our trucks once you get it tuned?

 

Primaries are 18 ga and wall thickness of the y pipe is .065

 

Thanks for the write up that's an awesome looking truck you got there

 

GMCtrk, I can meet you in St. Louis when I visit the city, but I don't do any street racing. I am in transportation and preach safety every day, so it would not look too good if I got my license revoked. Will send you a PM. Oldcrow, thanks, the wall thickness is about what I thought. Tom@ARH, thanks for the comments. I have to say that your headers fit very nicely and don't have any leaks. I have installed a few headers in my day, and these are the best I have ever worked with. Nicely done!