I have this engine in both a Sunbird & a Skyhawk. The shop manual says a negative back-pressure (N) valve is required, but I've run them only on positive back-pressure (P) valves, with no ill effects. My understanding is that both valves would stay closed at idle, and that positive exhaust pressure combined with manifold vacuum will allow the positive back-pressure valve to open and allow inflow of exhaust gas at higher speeds. It seems that the negative back-pressure (N) valve might allow valve to clse at highre load for gereate performance, but would otherwise perform similarly. Any thoughts? Anyone?