I have seen a few instances on cars with cheapo headers (ie non equal length tubes and improper collector length) actually produce more horsepower through the exhaust then with open headers!!!! This was on a sbc in a Camaro, had a really nice exhaust system all mandrel bent tubes with an X pipe. Made 11 more horse on the dyno with the exhaust hooked up vs. a set of open Blackjack headers....... Interesting, huh????