I've never heard answers like this... glasspacks? who would think that they are any good or useable... flowmasters are ok, not the best, and they are the ricer fart can equivalent for mustangs, everyone has them. The warlocks will not give you enough backpressure open, unless your car is heavily modded, 400hp+. I have a gt-40x crate motor, and the flowtech terminators without tailpipes did not give enough backpressure, with tailpipes, they sound great and I don't bog down as easily(unless i screw up). They work great in the upper rpm also, with the tailpipes on, i didn't lose any hp, so tailpipes=good, open warlock+ stock or lightly modded car=bad