not be a pain but to quote me :
okay lets get something first here. you are dealing with a supercharger. not a turbo. these are 2 very different animals!!! not similar in almost any way except both pumping air out of them. sc's work on the intake side of things. thier job is to suck more air in and pressurize it i.e.= more air= more oxygen = more fuel burned= more power. simple (well explaining it is ) just like a normally aspirated engine a sc needs some backpressure to maintane good flow and turbulance. yes i said that. there is a sort of phenonminom that happens inside pipes that have air flowing through them regarding the "tuning" or "frequency" of the air many use different terms for this but its all the same. harmonics. think of a large pipe organ at a church its really very similar. air will flow at different velocities at different tuning frequencies. i build competition subwoofer enclosures all day. we deal with the in car resonance, cabin gain etc... all which gets figured in to any real competition system. if you go to an installer and he doesnt do enough testing and research on your vehicle and you are paying him a lot of money GO ELSWHERE!! if they do not know what all this stuff is you will not win plain and simple. if you go to large on a all motor car you can and will kill your power very easily. you generally also will only gain anything and if you do it will usually come at the top of your rpm range. to big with a sc is not the way to do things. call any large sc company they will tell you the same thing maybe not word for word but it will come out the same. jackson, ripp, eaton, vortech, etc......
i could get into it more in depth and into turbo's as well but that another time and place
and to quote planetslowcar1:
Amazing! That's the first time that I've EVER heard anyone online explain exhaust flow correctly. Usually its just, "Backpressure ownz joo, yo needs it fo torque."
Now if we could just get people to talk about HP and torque correctly this discussion could really be intelligent and informative.
If nothing else, let's just try to avoid using "torque" as this catch all term for low end power. When you slap on a huge exhaust, you don't lose torque. The engine probably makes more, it just makes it at a much higher RPM. Even more so than the misunderstaning of exhaust flow, its the misunderstanding of HP, torque, the power band, and how they relate that causes the exhaust size discussions to turn into mass stupidity contests.
As for STREET (that means full car length) exhaust. Just think of it like an intake manifold. The flow is optimized to make torque at a certain RPM. Smaller sizes make more down low and less up high. Larger sizes make less down low and more up high. The trick is to find the best balance of flow (biggerness lol) and velocity (smallerness). But it has NOTHING to do with backpressure making torque or the load on the engine. It has only to do with optimized scavanging and velocity and what RPM that occurs at.
Think of it this way.
A smaller exhaust will have a higher velocity (ideal gas law) but less flow (common sence). At low RPM, the small pipe is easily filled and the velocity is good. That decreases backpressure and makes good low RPM torque. But as RPMs build the engine is exhailing more than the exhaust pipe can smoothly flow. That makes backpressure and kills high RPM torque.
A large exhaust will work opposite. It will have lower velocity but more flow (at any given RPM). So at low RPM the huge pipe isn't filled enough for the gas to have good velocity (ideal gas law again) and that makes the gas tumble. That reduces flow and creates backpressure, killing low RPM torque. But at high RPM the large pipe is now filled correctly and the velocity is high. Now you have very little back pressure and that frees up high RPM torque.
So they both have their uses. Again, sizing is key. Just like sizing a turbo to fit the engine's flow (CFM), your driving style, and the operating RPM. Its the same for exhaust. But notice the VERY KEY part of all that explaination. In every case backpressure is what kills the power. This is why race exhaust (a large diameter dump pipe) usually makes gains everywhere. When the exhaust is extremely short the ideal gas law doesn't matter. The pipe is so short there's no time for the gas to tumble, back up, speed up, whatever. The only time you have to worry about tuning for velocity is when the exhaust is the full length of the car and you need to keep it flowing out a long pipe quickly.
In conclusion.
BACKPRESSURE DOES NOT MAKE TORQUE.
I hope that was all helpful and will prevent having this discussion every other week on this board (like all the other car forums).
some people have 3" some have 2.25 what is good for HP and TQ (i dont care about sound 
