Actually we have tried open headers on a 67 SS Camaro that runs the street...and it ran better running thru the Coffins than it did open headers. The car has been dyno'd at 410 to the rear wheels..Flowmasters were removed and Coffins installed and the car ran better..then we opened the cut outs and the power dropped off..
Also I have seen the inside of the flowmasters and the coffins...and I can see why they work better..
Rick
