Usually a header comes with a reducer and those dictate pipe size. It's probably 2" What brand of header are you using?

I've not bought it yet but most likely Hedman. Seeing how it's the least expensive route. I'm wanting to build a 2.3 to more like a ministock motor but for the street so I'm wanting to squeeze as much hp as possible out of it.

FYI, when putting on headers we often cut the head flange between cylinders, it allows the individual pipes to pull up better to seal better. If you actually remove a small section then plugs easier to change as well. Pretty much stops header bolts from coming loose too. When they do header flange after welding it they commonly use a grinder to do so and like a butcher in most case. Carefully using a like 4" right angle grinder to detail the flanges to a better flat and then cutting flange like said and you can pretty much kiss gasket leaks goodbye and for the long term. After that they were as reliable as a factory manifold was leakwise.

One glasspack will probably be pretty loud. Depending on how the louvers are punched in the core, mounting muffler one way may be quieter than the other way or flipped end for end. Four cylinders are harder to keep quiet than eights, the eights have more cylinders to interfere with each other to kill noise.

2" sounds about right.  A lot of people think bigger is better, but that isn't so.  On the other side some people say you need back pressure and that isn't so either.  What you want is velocity. That is, you want the exhaust to move as fast as possible through the pipe.   The benefit of that is it promotes a scavenging (drawing out) of the exhaust gases from the combustion chamber.

Like all things you would ideally have multiple pipes sized for all situations. That being smaller for part throttle, normal driving and larger for full throttle power.  Sort of like an inverse, progressive two barrel carburetor. Barring doing that most exhaust are sized as an appropriate average.

Lastly turbo motors seem to like very large pipes. As best I can tell the turbo kills a lot of the velocity.  Thus, the exhaust, (now having been slowed down) needs volume (not velocity) as the most expedient way of getting the exhaust out.

Tom

The wave tuning is well beyond the average persons ability to measure and tune to great effect. And Flowmaster style design aside, any mufflered car likely isn't maximizing exhaust flow period.   Thus, we are pretty much left with pipe size as a variable.  Also, typically most power gain is measured in some form of high rpm racing (ET for example) or "end result" dyno run.  I doubt that a whole lot of effort goes into maximizing the 1,000-3,500 rpm cars typically operate in.  I'm not trying to be argumentative, only practical as to a daily driven car with some performance aspiration.

I'd love to see dyno tests as pure science rather than executed with a purpose. Hey, maybe this is one for Mythbusters.

An exhaust test that always rings true for me is one done on a mild Mopar 360.  The difference between stock 318 cast exhaust and the expensive 340 Hi Po manifolds was about 5 HP at 5,800 RPM. The difference between stock 318 cast exhaust and $800 TTI headers was 10 HP at the same 5,800 RPM.  Maybe in racing it counts for something, but not on the street. Note, all cast exhaust had 2" pipes 24" long.

Tom

Another hint: copper gaskets seal much better and don't blow out.

LOL, while I prefer flat flange collector hookups I have had a slip on ended Hooker header on my Mustang II/Pinto like forever and never any leak at all. Used them 100% of the time on bikes as well and no leak at all. On car you must have some form of resilience built into at least the first exhaust hanger/mount after the collector or engine mount flex will quickly leak the joint. I've been known to mod or add slots too for better seal.

Rule of thumb as someone just suggested, give up wave tuning if you run a full to rear wheel exhaust system. You CANNOT wave tune that. Personally, at our shop (early '70s-early '80s) we considered both Flowmaster and the at the time highly favored 'turbo' type mufflers to be crap. Back then our favorite was no tailpipe setup at all and using big core header collector mufflers alone. Short enough you could easily get some wave tune benefit out of them. In fact I took a small cubic inch AMC built to spin pretty high and had it buttoned to a latest trick 2 1/2" dual piped dual turbo muffler setup, it ran like a dog after the initial engine break-in with open headers. Ran in the high thirteens capped up at the strip or horrible. Car felt dead as a doornail, wouldn't hardly break tires loose. Came back home and immediately cut all pipe off to put on Thrush header mufflers with 2 1/2" cores using same headers. Next run after that car was busting off 12.50s same day (on regular hard rubber street tires) and no other changes, car felt like motor found another 75 HP and an honest to God powerband now. Traction became a huge problem and leaving at 2500 rpm. Motor before not wanting to go over 6000rpm was now revving cleanly to 7500. Tailpipes absolutely destroy small inch high rpm predisposed engine tuning. Bigger motors the same just a bit less depending on how radical the cam timing was.

You can only wave tune when you have your tuned length and must have zero pressure atmosphere pretty close to your figured tuned length, start adding tailpipe length and the more you do, the more that tuned effect goes away. Pipe length destroys wave tuning.

'The wave tuning is well beyond the average persons ability to measure and tune to great effect.'

Measure? Can't without some form of statistical measuring. Tune? Oh, yes you can absolutely make it better. You can wave tune lower rpm as well but that usually lowers upper rpm power and we Americans are very greedy, we want all we can get. Pretty much all your FI PCMed cars now use wave tuning in the intake manifold lengths, much of it geared toward low to mid better result, you can tell by looking at the length of the individual runners. It helps at the lower rpms found using overdrives on modern transmissions. Or better mileage. And now many actually use variable cam timing and intake runner volume changes to gain low to mid as well as ultimate top end power. Like having cake and eating it too. 

Well, I stand slightly corrected. Compared to stock 318 cast manifolds it was 15 HP for the Pricy ($800) TTI headers and 8 HP for the also expensive ($300-$400) cast 340 manifolds.  RPM was 5,400 not 5,800.  I was going from memory when I posted.  Here is a link to the article with all the particulars (there is a dyno results chart):
http://www.moparmusclemagazine.com/techarticles/engine/mopp_0307_mopar_crate_engine_exhaust/viewall.html

But, going back to the initial post he was asking regarding the pipe size for a header and implied a full exhaust.  Myself and two others suggested 2". The rest of my post was to in general terms let him know why a 2-1/2" or 3" exhaust would have not been an asset.  Regardless, I appreciate your contribution.  It always allows me to learn and contemplate something new.

Certainly not correcting you, and I go off memory a lot as well. It's just that I've done plenty of stuff chasing wave tuning and can bring other things to the table.

I used to pipe and port 2 strokes as well, probably the best way to see how striking wave tuning effect can be. It can really open your eyes sometimes.

The 2 inch will work fine, I might go to 2 1/4" in some isolated occasion.

Here's the thing. If you can fake the engine into thinking the exhaust has hit atmospheric then you approach true open conditions. One way is by putting a much bigger area around the length you would normally expect to find open end of a tuned pipe, some high-perf small engine cars can do that by adding a big area converter entry that has bocou flow in it, the exhaust thinks it has hit open air with the big cross section change. Another on dual exhausts is to add a cross over pipe (old Mopar trick that still works) between the two sides right in that area. A single exhaust to dual outlet, it doubles the area. Added benefit is the extraction the other side can add by its' plug going down pipe. What Vizard's high velocity does, the speed pulls exhaust plugs behind it along for the ride.

Nobody ever does it on cars because of space considerations (except on say formula open wheel race cars), but adding a megaphone bell can really help there. It does greatly on bikes but the extra noise there has forced almost all competition exhausts to go to canister muffler type and lost power there. Problem with the meg is you are wave tuning the exhaust AND the sound too, or the incredible loud noise they bring, proof they work.

One thing that nobody has talked about, is that all glass pack mufflers that I have looked at, have a 2 inch core!!!! Makes no difference what size pipe is used, if the muffler used has a 2 inch core. There are mufflers that have a bigger core, but they are not stocked by your local parts store. Lobak makes both street and race mufflers with a core that match the pipe size. And they cost very little more than a cherry bomb. Stick with a 2 inch system. Plenty big for any street driven Pinto. And if you want a cheap header, just go to your local junk yard and get one off a 2.3L Ranger pick up. Very nice and made from stainless steel. It will out live the car. Get the down pipe too, to make a do it at home job easy. For a real nice sounding system, put a short glass pack under the car, and one at the tail end of the tail pipe at the rear bumper. NO RICER FART CANS PLEASE!!!!

Hmmm strange, if chamber mufflers are so bad wonder how I gained power and mileage in my dually???..

Never said you couldn't gain power with 'chambered' mufflers, you just won't get as much. Look at the race mufflers, you won't see one not straight through.

Once wave tuning is tossed out the window then the improvements come by reduction of impedance to flow only. Or, reduction of possible back pressure. ANY back pressure is bad for engine output if not intended for some certain effect.

Two totally different things although they can heavily overlap. Once pipe gets long you start looking to just improve flow with no waves involved. The waves pretty much destroyed anyway, the classic exhaust system is set up to do so. Waves=noise=bad.

If properly designed you can MAKE MORE power with a straight through muffler, you can only RECOVER some lost with a reverse flow muffler. That in relation to an engine with a stated amount of output before either one used.

Well that's a lot of good info! haha. I was planning on probably using the Headman header. As far as noise goes, I have a Dynomax Super "turbo" muffler on there now. Just to keep it quiet for now. I don't really like the sound of it. I did drive a Pinto a while back with a cherry bomb at the very end of the exhaust and I thought it sounded pretty good and surprisingly wasn't very loud. I also haven't decided where I wanna exit the exhaust though. I'm kind of thinking about bringing it out in front of the passenger side rear tire.

Four cylinders commonly can call for two mufflers, they are very hard to keep quiet. Harder than V-8.

Yeah, the good firing sequence there, 1-4, and 2-3, 180 degree pattern....... ......I never liked the 1-2 and 3-4 that street bikes use. You can tell the difference instantly when motor is cranked up, and some extra power there too, the scavenge is even instead of lopsided.

They used several headers like that on some of the Kawasakis used in the original Mad Max movie, you can hear the rasp of 180 firing clearly there as them bad boys rip over the flatlands there.