Originally Posted by
cory
I'm really sorry to disappoint you, but you're wrong. When/if we were talking about header dimensions then sure. The smaller the diameter that can flow with out restriction will create the most velocity and in turn create the lowest pressure on the back of the valve... But we're not talking about header specs here. The truth is that the exhaust cooling rapidly from the chamber all the way to the exit of the exhaust system wherever that may be. If you have a 2.25 or 2.5in collector you'll have to step that exhaust up to give the gasses enough room to move without restriction, and the further from the choke you get, the larger you will have to step the exhaust pipe to do this properly, so as you can see the longer the exhaust system is the larger it would have to be to truly offer no restrictions... There is no gain in a smaller exhaust unless there is a flaw in the design of the header or some other problem in the set up. I understand the sound thing, but I personally love the way my 3in sounded. Hey, find some graphs of back to back tests..
im not sure where you are getting your theory from but that's bogus, im sorry. the larger the exhaust the less it has to flow to escape the piping, and the cooler it gets becoming more dense and even more resistant to flow.
quoted from Mike Kojima and Sarah Frost's book "How to Build Performance Nissan Sport Compacts"
-"Exhaust gas velocity is essential to exploit the scavenging effect from tuned headers. A good velocity exhaust stream keeps the exhaust gas flow high and develops a vacuum behind the closed exhaust valve that helps with scavenging when the exhaust valve opens on the next cycle. This is most important during valve overlap, the part of the 4-stroke cycle where both inatake and exhaust valves are open.
Too-large of exhaust pipe diameter has less exhaust-stream velocity and can lead to sluggish flow at low velocity, less torque, and bad scavenging. A good exhaust for naturally aspirated engines will have low back pressure and high-velocity flow."