Trying to find correct header primary size

Looking to build a custom set of headers for my a-headed 521 nitrous motor. I know ppl have different ways of figuring the correct primary size. Just wanted some input. Thanks.

2 1/4 - 2 3/8 likely. I have a set of 2 3/8 stepped to 2 1/2. I'll be swapping them out for a 2 1/4 stepped to 2 3/8


Ace

How much nitrous? My definition of nitrous motor may be different.

Dwoodside wrote:Looking to build a custom set of headers for my a-headed 521 nitrous motor. I know ppl have different ways of figuring the correct primary size. Just wanted some input. Thanks.

It will depend upon more than just engine size and NA or not.

RPM range wanted is the biggest - nitrous ONLY or NA with sometimes nitrous and exhaust camshaft duration are other parameters to satisfy.

Here's a general rule of thumb

http://www.fabshopheaders.com/header-sizing.html