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Old 09-30-2013, 09:55 PM   #11
AMCjavelin74
Scooby Newbie
 
Member#: 368199
Join Date: Sep 2013
Chapter/Region: MWSOC
Location: Mitten State
Vehicle:
2004 Impreza 2.5 5sp
Red

Default

/\ My factory manifold is cracked, so I figure it's a good time to upgrade.

Quote:
Originally Posted by RaceFaceXC View Post
Not exactly. Most cars come with a cheap to manufacture exhaust manifold that insulates or has a high capacity for heat so the cat works better. They also tune the manifold for the performance desired, usually great midrange efficiency, which is a compromise to higher rpm performance and efficiency. If a properly designed header that is tuned for higher rpm performance, like TWE or rallisport racing EL merge collector design, along with a good tune on the Ignition timing, decent gains can be found. Like 20 hp/tq across a large part of the rev range.
I think you're pretty right for aftermarket exhaust. Some newer vehicles are a pretty good compromise of the whole cost/fitment vs performance and efficiency battles (LSx exhaust manifolds, most modern cars too). A lot of cars though have garbage for exhaust (ever look at the stock exhaust manifolds on a GM 3400v6? Looks like it belongs on a farm tractor.)

So, where the OEM cuts corners, the more reputable aftermarket companies put in the effort to make a quality product that offers benefits over stock. While I don't see 20hp in my n/a basically stock 2.5 as possible gains, I think there are some mid/top end to be found, and going with smaller primary tubes (the smallest that will not restrict the exhaust flow) there will be enough velocity for exhaust scavenging= keeping low end torque.
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