/\ My factory manifold is cracked, so I figure it's a good time to upgrade.
Quote:
Originally Posted by RaceFaceXC
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.