Your only performance gains will be psychological. (Maybe 5-horsepower at best. And ditto for torque)
And the manufacturer's claims are just that, claims.
The only time you'll get a big performance gain out of replacing exhaust systems (Not counting headers), is when your engine was built to make a lot of power, but the exhaust is too small to allow it. But Nissan did a good job in matching your exhaust with the potential output of it's motors.
Now if you were to install headers and a dual exhaust with high-flow converters, then you'll actually feel a "seat of the pants" difference. But even that won't be as big as you may think. Because most of your gains will come at high r.P.M.S.
If you were to install high-flow intake, larger injectors, and remap the computer to deliver more fuel, then your high-flow exhaust will make a bigger difference.
Tuesday, February 23rd, 2010 AT 5:34 PM