I have a 400sb with a 270h cam,2.5" pipe(no crossover pipe) out the back with 40 series flowmasters. One of the best sounding cars I have had in awhile. Just recently I was screwing around and I took the Vacuum advance off of manifold vacuum and put in on ventury vacuum. Just to see if I could get a more consistant idle. Well, that made it sound even better at idle. Even after I turned the idle screw in to bring the RPM back up to where it was before. I have done this on other cars before and didnt notice much difference in the sound. The retarded timing at idle with the two cambers and tail pipes out the back is a killer combo.