I know there is enough debate over wether true duals net any gain over running into a single pipe so im going to stay away from that and just say true duals cant hurt. now for the questions

#1 people say that a 3" single back pipe is more then enough and if your car is crazy serious then maybe a 4" single back. So then why do a lot of people talk about running 3" pipe with true duals thats 6 inches of pipe!!! Wouldnt running a 2 1/2" or maybe even a 2" be more then enough?

#2 if youre running true duals do you really get any benefit running a pipe with a larger diameter then the collector on the header? If you think so then why? I would think that if the collector was say 2 1/2 inch and you ran a 3" pipe that the bottle neck in the collector would negate any gains from a larger exhaust.

Give me your thoughts people

 

you need to look at the area of the crosssection of pipe, not its diameter

A = Pi * R^2



size area
2" 12.5
2.5" 19.63
3" 28.2
3.5" 38.48
4" 50.27




see the drastic diff between the 2" and 3"? two 2" pipes would be slightly less then a single 3"... so they use the closest size... usualy a 3" system, but some people do 2.5.....

 

im stupid yeah i should have realized that. Still need an answer to my second question.

 

the amount of backpressure will increase with tube length...


take a straw, cut it to 1" long and blow thru it..

then take 50 straws, attach them end to end and blow thru it.
alot harder to blow thru, huh?

 

I once read this book on how headers and exhaust systems work, i dont remember who wrote it though. it was a few guys. It was all about the science involved in it. This book, btw, was based mainly on the sbc engines, but sometimes the big block as well.

Anyway... they say that for an engine from about 300ci-360ci, there really isn't any gain by using a dual exhaust system over 2.5" because the engine doesn't need the extra flow potential very much, but there is alot of velocity lost by going to a bigger system, which also kills the scavenging effect of the headers.

Oh yea... also from about 300ci-360ci, there isn't much need for headers with more than a 3" collector and 1-5/8" primaries, for the same reason as stated above. The engine can breathe perfectly fine with that size, but bigger than that will lose you alot of velocity.

IMO, most people get dual exhaust just to have dual exhaust. These cars are perfectly happy with a stock-style system with a 3" I-pipe and smooth bends and a high-performance muffler.

Dual exhaust does sound cooler though. The sound of the two halves of the engine don't get mixed together, so the frequency that the pipes "speak" at is much lower (133 pulses/sec @ 4000rpm), with means it doesn't sound nearly as smooth and tame as the same engine with a single would (266 pulses/sec @ 4000).