Nismo Cat Back

Jessobear

I have the Nismo OEM exhaust with FI resonated HFC's and I can definitely tell you that my car does not have any noticeable exhaust rasp.

I also find it funny that everyone really gets on the piping diameters of the H pipe. Having stepped piping sizes is actually totally normal and can actually aid in the scavenging effect of the exhaust system. Think about this in terms of total cross-sectional area: passing the exhaust through two 2" diameter pipes is a greater cross-sectional area than passing through a single X or Y pipe.

The advantage of an X pipe has nothing to do with cross sectional area. It has to do with easier flow between the two sides of the exhaust due to a smoother transition compared with an H pipe. An X pipe will also have the effect of making the exhaust quieter from noise cancellation due to the easier cross flow of the two legs. The H pipe configuration is not a bad design in essence, which is why people have only seen small gains from other exhaust systems.

Here's a decent link about putting an X pipe exhaust system on a car:

Exhaust System Installation H-Pipe, X-Pipe,- Car Craft Magazine