I had drone on my 73' Duster 340 using Flowmaster 40's, I did some research to find the best way to get rid of the drone, I found that Dynomax VT has a flap in the muffler that opens and closes with the pressure that is applied against it. This opening and closing of the flap removes drone. This is the same process that is used in SRT Chargers and in the HellCat. I put these on the Duster and drone went away and it sounds great. http://www.jegs.com/p/Dynomax/Dynomax-VT-Mufflers/1316158/10002/-1