As said, I've done a lot of reading and have added several books to my library on engine building, efficiency, power, etc. One of the authors that makes the most sense is David Vizard, and I have 3 of his books. According to him it was his work with Dynomax that pointed out the some mufflers flow well and some poorly. They were the first to flow test their products, and he still recommends their stuff. Others on his list include Magnaflow and Flowmaster. But, he doesn't recommend glasspacks.