I keep gettin mixed opinions form my friends. I have a 355 pushing around 400hp. Should i go with a 2-1/2 in exhaust or a 3 in?? I have long tube headers and im going to get some moroso spiral mufflers.

 

2 1/2 inch exaust would be sufficient with your motor.

 

Could you explain why so I know .

 

Generally engine with over 500 plus hp or a lot of cubic inches need the larger of the two since they move more air then an engine with 400 hp. If you run a quick search their is an awsome article that one of the great guys here wrote about exaust pulses and how an engine's exaust works. I believe it's a sticky on the top of the page titled "more then you ever wanted to know about backpressure". I would look into that and I bet all your answers will be found those and more!

 

If we are talking dual exhaust than 2 1/2" is enough.