The problem trying to get a gain without really changing much was the engine actually ran really well prior to our change, valves were sealing. We did gain part throttle power that you can feel by the seat of your pants.
A friend of mine years ago, put a good dual exhaust system on his car and a good quality open air filter. We dialed it in the best we could with the tools and knowledge we had at the time and his car ran great for a 318 in a 1970 charger.
