I started a renovation on a 73 240Z October 2023 rapidly approaching 2 years I replaced all the smog took all the small off and concerned about the size of my exhaust pipe I think it's too wide two and a half inches diameter anyone have any information as to the correct diameter that I should be using please? Seems that the two.1/2 inch may be too large.for the correct exhaust flow.. thank you

 

There are many things to consider here. If you are using the original cast iron header the size is 1 3/4. If you do upgrades like cam and headers you can put 6 into 1 tuned or 6/2/1. I used 6-1 with 2 1/2 and a Supertrap muffler which can be tuned by adding or removing baffles.