For the life of me, I can't figure out why all of the current aftermarket exhausts for the 350Z utilize the Y-pipe design. Why is this?

I would imagine that a true dual exhaust would produce more power since you eliminate the Y-pipe design. Think of it this way: when two highways merge, there is more traffic than if you had two independent highways. Even if the Y pipe outlet is 200% bigger than it's two inlet diameters, there will still be most resistance than if you had two independent exhausts for both headers.

 

The Stillen system is not a Y, it actually uses an X design to help extract gases. A "Y" isn't inherently bad for flow if you increase the diameter of the collector pipe.

In terms of weight, it is generally lighter to run one larger pipe than too smaller diameter pipes.

Don't forget that you NEED some back pressure on an NA engine, without adequate back pressure, you will loose bottom end torque. Where the torque to hits the torque curve is a design function.

There are many things to consider when designing an exhaust system,and many trade offs. A great system for the track may not be optimal for the street.

 

I understand the importance of backpressure on a NA motor, but with two smaller exhaust tubes, I would expect a lot better results than what they're receiving now.