Rob When I called MoPar Performance to see what the difference was over my OEM Imperial dual exhaust they stated the TNT etc manifolds are five to eight HP more depending on your total system. As it turned out you cannot use the 67 and newer drivers side ( A,B,C or whatever bodied ) manifold due to space design on the 1959 to 1966 Imperials. But if you happen to have a '67 or newer Imperial w/o the TNT and etc exhaust system, it will aide in the hill climbing ability or Interstate mpg figures. Rodger & Gabby Colo Spgs '47 De Soto Custom w/Fluid Drive & passenger door mounted swamp cooler '63 Le Baron w/F-Sweep trunk lid & std A/C '66 Le Baron w/dual A/C & reverbed AM/FM '70 Overlander - InterNational