so im currently taking fluid mechanics and the material were covering is contradicting everything i was thought to believe from this site about exhaust systems. Now maybe ill learn something later to show my thinking is wrong but right now im learning about bernoulies equations and others that show when a pipe has a larger area the velocity is less and the pressure is actually MORE..so a 3inch exhaust would have more pressure than a 2.5" exhaust..this is weird cus i bought a 3 in downpipe in order to put less back pressure but i actually have more..does anyone know where im wrong cus according to my book thats how it is..which kinda makes sense but also doesnt..lol

 

it gets complecated, but yes, the biggest thing is keeping exaust velocity up, and going to large will slow it down reducing the scavenging effect and creating exess pressure. however, you also need to keep in mind too small will cause exess pressure when you are flowing lots of air as in a highly modded motor.

 

Look at it this way, try blowing threw a straw then threw a 2" pipe which is easier and allows you to move more air?

 

i dont think that analogy applies...what im looking at is the pressure diffrences instead of flow..sure you obviously can get more max. flow through a larger pipe but it would have more pressure in it compared to a slightly smaller pipe...and correct me if im wrong but increased pressure increases EGT wich we dont want i thought..

 

I'm pretty sure its relative to the rate at which the exhaust is flowing out of the motor.

Maybe larger is less restrictive at high rpms, while smaller is less restrictive at lower rpms. (or maybe its the other way around? either way I still say its relative and not a constant)

Blowing gently through the straw would probably get more air from one end to the other than blowing gently through a 2" pipe. Blowing hard the the straw would probably not move as much as blowing hard through the 2" pipe.

I think the question is would you rather have that restriction up top or down low? I'd say it matters more up top, therefore supporting claims that a larger diameter exhaust is "better".

It's 3am and I'm no expert on the subject but I just thought I'd throw in my $.02

 

sounds pretty good..kinda like for a certain amount of flow this size is more efficient but once enough flow is created at some point the lack in size is more damaging then the actually amount of pressure..so it switches over to the other..that could be why when i punch it down low my car goes no were on the 3 in dwnpipe but up top it screams..*************so its actually to much pressure down low from 3 inch exhaust that causes loss in power not the lack of back pressure.....(interesting)***********

 

Hmmm, yeahhh..
Exhasuts are a bit more than only Bernoulies law...
Indeed fast moving air has less pressure than slow moving air, but then you also have pressure build-up due to flow resistance...

 

bernouli's equasion assumes your flowing the same volume of gas through both systems...
if the only force the gas is releasing is in the direction of the pipe, then the faster the gas moves, the less pressure there will be, as it would almost be making a vacuum from traveling so fast (this causes scavenging...the vacuum cant suck in anything from the sides (theres piping in the way), so it sucks from behind (the cylinder chamber)

however, the gasses want to expand, causing them to exert pressure towards the exit, towards the piping walls, and towards the cylinder if it is not moving fast enough

i hope this helps, and i hope its right...

 

You still have a litte way to go on understanding fluid mechanics

but you'll get it