FYI-
I ran across this ref. on exhaust system applications.
While its written by Caterpillar and focuses on turbo-diesel engines (oooh.... that hurt to type), it offers a good explanation on how to set up backpressure measurements on your exhaust system. There is also an equation that is simple to follow to allow you to estimate (A to B comparisons) exhaust system backpressure if you have muffler pressure drop data, gas flow rate (cfm) for your engine,and exhaust gas density at the measured AFR (lets say AFR of 12.5-13.0:1 @ WOT peak power).
See pages 21-25 for the details at the linked document below.
http://www.gregorypoole.com/products...%20SYSTEMS.pdf