I have to take a stab at this, you mean the original 1968 style mufflers? I would say the restriction would be very high and the flow capacity of the factory style stuff would be very low. Scotty Brown used to have a flow chart with that stuff up there, but I can not locate anything similar to that today.

I did find this little site it has some information on flow testing but they only go up to a 2.5 pipe size...hope it helps some

http://www.broaderperformance.com/muffler_flow_tests.htm

I liked the sound quality of the DynaMax, they tend not to drone like the Flowmasters and such.


What is it you are looking to do?