So I was looking through CBEs today and noticed all the configurations had a 2.5 inch joining to a 3 inch y pipe. Doing some simple math, 2(pi/4)(2.5)^2) gives us the cross-sectional equivalent of the 2.5 inch part. This equates to about 9.8 inch^2 while the 3 inch comes to about 7.1. Wouldn't it make more sense to go with a 2.5 going into 3.5? 3.5 inch pipe would have a 9.6 inch^2 area which is closer to the two 2.5 inch area of 9.8. Am I missing something here? Also I'm completely new to the modding scene so if I sound like a complete idiot, or if the 2.5-3.5 inch exists feel free to call me an idiot and correct me. If you guys have some recommendations please let me know as well. I have a 2013 g37x so I might not be able to use some pipes.

 

I tried to have this done but it would have costed me too much for a 2.5" primary then merge to 3.5". My current y pipe is 2.5" > 3.0" > 3.5" merge outlet. My exhaust is a 3.5" single
I haven't ran the setup yet. I'm still trying to put my engine bakc in the car.

 

My question is goal? What are you trying to achieve?
Better flow, better sound, less weight, etc
All of these play a huge part in everything.
Over time you'll realize for better power you'll be better off going to a single exhaust. 3 in seems to work however so will 3.5. The reason why you dont see much of this is availability and pricing, sound plays a huge part. After 3 inches expect to pay a pretty penny for resonating devices.

Back in my Honda days we discovered 4 inch tapered intakes into a 3 inch throttle yielded best results along with a a 4-1 collector to a single 3 inch pipe with or without megaphone would surpass anything in the top end.
The 37 will benefit from this type of thinking more than a 35 for sure. At the moment there's just not much info since folks get swept in the exhaust bolt on game and deciding between duals and singles, etc.

 

Thanks for the insight man. My goal is for more low end torque and a deeper exhaust note. However after you mentioning the 3.5 inch being very expensive, that may not be an option for me. The reason why I'm trying to change it out so soon is because I found out the dealer screwed me because one of the pipes in the factory y is completely sheared in half due to rust and they did a crap job patching it up. For now I'm just getting me some of that repair tape and patching it up right now.