Professional Answer to Exhaust Size Question 2.5" vs 3"

David Vizard did a good article on this. It came down to 2 things, 500hp and up it was 3", but the deciding factor is also the flow of the muffler. Many mufflers don't flow enough air to support 500hp. It is critical to consider muffler CFM when making this choice. I have an 11:1 410 w/hyd roller and went with the 2.5, but used Goerlich 18" can length mufflers that flow 485cfm each. Since I'm just under 500hp, this worked out well, and fits well. TTI head pipes from their headers should fit a lot better, but they don't. I'd recommend purchasing only the TTI tails and muffler hangers if you have a talented exhaust guy (rare) in your area.