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Something that I was wondering about for a while is why people say you should get a a 3" exhaust unless you have a turbo or some kind of forced induction. So why would a 3" exhaust hurt performance over a 2.5" exhaust when you are still naturally aspired? The 3" exhaust would give you less back pressure which is the whole point of putting an aftermarket exhaust. Less back pressure, the less your engine has to work, so performance increases. Besides the added weight of the 3" tubing, why shouldn't I just throw a 3" exhaust on my car? Is their another factor I am forgetting?
(Starting to research my options for an exhaust)
Anyone who can straighten this out for me I would be very appreciative
 

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really a 2.5" eliminates all the back pressure our cougar engine produces, with a nice straight thru muffler to aid. 2.5" is big enough for our small displacement engine, and really is no need to go 3". In my experience i would say go with 2.5 and dont even think about 3" with out forced induction, really there is no need for it.
 

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Actually IIRC 2 1/4 was optimal for our engines..
 

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It depends on what you are doing with you cougar and the HP you are trying to achive.

If the pipe is too small, then there is not sufficient flow for all the gas to escape.
If the pipe is too large, then the exhaust gas velocity drops and it does not create enough vacumn in the combustion chamber - which helsp suck more air into the cylinders.

For a stock engine, it has ben determined that a pipe size of 2 1/4 is optimal (Thanks LostRacer!). For a modified 2.5L, a 2 1/2 pipe would be good. I would not move to a 3" pipe unless you were running a supercharger/turbo or a heavily modified 3L engine.

FWIW, I have just an intake and exhuast with a 2 1/2 pipe and I notice that the torque suffers under 2,500 rpms.

Hope this helps...

-Nick
 

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just to make sure im on the same page,
thats, a high flow 2.5" y/cat then the catback tubing 2.5" then the muffler. sorry if that sounds dumb but im trying to learn off others.
thats what i would like to get my car smells so bad and my muffler fell off the other day so im gonna redo some airflow, i do the headers later
 
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