I want to buy an aftermarket Y pipe but I need some help from you guys. I have Alphawerks headers which I'm not to sure but I think the outlet pipe is like between 2 - 2.25 inch and then I just bought a new Greddy Ti-C single exhaust that the mid pipe is 2.75 inch. I heard that when you are matching exhaust parts that you don't want to mismatch diameters because you create a bottle neck effect which takes away performance and worsens the sound. Guys, is this true or I'm just full of sh**. The majority of the Y pipes out there would not work for me because they are or 2.25 > 3 " or 2.5 > 3 " which creates that bottle neck effect on my car. Any comments are very appreciated.

 

I would prefer to go from a 50 mm inner diameter pipe to 60 or 70 mm. Not the other way around. There is a adverse effect on exhaust volume and velocity. Whether you consider the effect trivial of significant is debatable.

 

The effects are debatable as davidv says, as long as your N/A. When you're NA, the design, not size, matters. For example, a poorly designed (bad placement of the joint) Y pipe will obviously make less power than an optimally designed X pipe regardless of piping diamater, as long as its not too small or too large. 2.5'' dual or 3'' single is probably best for this car, based on testing different exhausts.

 

? the size matters just as much as the design

 

I said after a certain size. 1.85'' is obviously too small even for treu dual. For single anywhere between 2.5 and 3.5 is fine and for true dual anywhere between 2.0 and 2.5 is fine. I think size has less of an effect than design.

 

I think it's clear. Matching diameters perfectly is a good thing. If you cannot for some reason, you should go smaller to larger, not larger to smaller.