Sheet metal and tubing.
Matt.. a guy named Michael built a pipe for his recently and I asked him to share the dimensions he used... this is his reply >>
sure. the data i entered into javapipe was 6000 rpm, 160deg exhaust, 120deg transfer, about 160 for exhaust area (i have the exact number written down somewhere), and 450k for exhaust temp.
i ended up building it with a 7" length of .75" tube to a 9.5" cone to a 14" long 1.5" tube to a 4" cone to a 2" length of .5" tube.i think thats about right, the numbers are written down in the garage..
for the cones i used 22gauge sheet metal. for the silencer (which works a LOT better than i thought it would..) i used a 2.25" diameter piece of exhaust pipe about 4 or 5 inches long. i welded a piece of sheet metal to each oped end of the pipe and drilled .5" holes (so theirs one .5" hole for in and one .5" hole for out). i slipped one end over the end of the exhaust pipe. for the other end, i took a 3" length of pipe and closed off one end by welding some sheet metal on it. i then drilled a bunch of 1/8" holes in the sides of the pipe. i put it in the silencer so that the holes were all inside the pipe and just an inch stuck out.
all together, its a pretty damn long pipe. it just barely comes past the rear tire..and of course the welds are pretty crappy considering i just started learning basic welding a week ago..but i like my pipe. very satisfied (and it only cost me 20 bucks).