I am making a custom exhaust for a 2007 350z hatchback, (vq35hr) and i was wondering what the optimal diameter would be to go with? currently its a stock exhaust manifold going to Berk resonated test pipes, going into a Megan Racing y-pipe, and I want to go from the y pipe back adding about 2 resonators from there in order to not get rasp.. what would be the optimal way to make N/A power right now, (but also possibly switching to forced induction/E85 later) but optimal diameter exhaust for a VQ35HR with stock headers going to berk resonated test pipes, into a megan racing y pipe... and if thats not optimal, what would you change (IE: 370z headers, different y-pipe for less bottleneck, etc) Completely new to this exhaust thing, needing some pointers

 

If it's just a street car, not for track, then I wouldn't go to the trouble of installing headers on it. The Y pipe has an impact on performance. I believe the Z1 single exit that I bought for my 350Z is 3 in at the y-pipe, from there to the muffler. there was no trumpety sound at FOT but it was hella loud!! One day at the track and driving home I actually experienced hearing loss for a few hours. so I inserted the insert in the tip to muffle it a little bit more and it's tolerable that way.

Z1 has a new touring exhaust that might appeal to you that I believe has resonators built in. If you already have the Berk mufflers then never mind.

I haven't had good experience with Megan racing. their short shifter was complete garbage. I would assume their exhaust products are similarly bad.

 

I havent had any issues with the megan racing y pipe, the welds seem to have held up really well, but i do think the diameter is somewhere around the 2.25/2.5 mark

 

Dual 1.5" exhaust with no restriction will provide over 500cfm of airflow for an engine requiring only 350cfm at redline.
Hope this helps