I build my own headers and even mufflers because I have access to a fab shop and my own tig welder at home.
This gave me some opportunities to play with a lot of different tubing sizes and lengths.
The question people should be asking, is what pipe for _______ track?
My general observation, bigger inside diameter moved the power "UP". Smaller diameter came off the corners better but choked on the straight. I started with a piece of 1"x .080" (.840 inside diameter)tubing for my 1st header, then made a smaller one and couple bigger ones.
Jetting changed drastically between the different headers. My engine needed to be richer with a weenie sized header(5/8" inside), because it retained too much heat and was pinging. We're open pipe so that one went in the scrap bin. Next, I fabbed up a biggie, 1-1/8 x .060. The engine wouldn't hardly run until I put replaced my 38 main with a 40, then it was still a little lean. Off the corner it died, down the straight it flew! Next up, 1"x .060". ended up going down to a Honda main jet that was kind of in between the 38 and 40. A better balance coming off the turns and straightaway umph.....but ultimately our track has 19 turns to exit and only two straights of any distance. I ended up putting the original back on. Something I see a lot of, is head pipes that terminate into the flange at 90*. Well, the exhaust port is not 90* to the flange surface. Think about it. I angle cut the end of the pipe and weld it on to the flange such that it's a straight shot.
For mufflers, you'll laugh, but I make my own little glass packs. I use .049 perforated metal with 1/8" holes, which I form around a chunk of 1" round bar in a vise, then stainless pop rivet into a tube. The tube then gets 1" washers tapped onto, then welded to each end. I then wrap the perforated metal tube with header wrap and secure at each end with thin hose clamps. Depending on what the o.d. of your washers are, this core will slip into a chunk of 2" to 2-1/2" exhaust pipe, sometimes grinding a little bit off of the washers outside diameter. I slip the core about 1/3rd of the way into the pipe, then pack it with muffler packing and wiggle it the rest of the way in.....then weld it up. They sound good, it quiets the engine down a lot and takes the rasp out of it, but still flows great. The header wrap used inside keeps the packing from burning up and blowing out.
