I tried Flowmaster 40 series on my 69 Dart when I had a 440 motor and it was too loud. The exhaust dumped out in front of the rear wheels. I changed to Flowmaster 50 series and it sounded good, quiet enough not to get pulled over. I lost about 3 tenth with the 50 series mufflers over open exhaust with the car running in the high 10s. I installed a 512 stroker motor with Dynatech split flows and its quiet enough to meet noise rules at the track and so far not to get a ticket on the street. I no longer take them off on weekends at the track because the car runs into the 9s with them on and I am not allowed. I estimate that the dynatecs slow the car down about a tenth and a half. The weight is about 30 pounds.