this is my weak, worn out 318 with dynomax coated headers, $80 summit 2.5" X pipe and Hooker Aero Chambers(Hybrid of absorption and chamber, supposed to be pretty quiet... ha):
I have them on my '79 Magnum with Headman headers on a stock low compresion 400 from '78. There quite until approx 60 mph. I don't know what it tachs, but it has 3.55 w/245/60/15's.
Below that, I can have a convo with the stero on. My 2-1/2 pipe is "H"ed and goes to the rear bumper.