While both engines have round throttle blade size the blacktop thottle blades are larger at 45mm compared to the 42mm of the silvertop. The casting of the throttle housing is also different as unlike the silvertops round openings the blacktop has more of a heart shape to suit the different manifold (see "Inlet manifold"). The throttle bodies are not readily interchangable as they have different port shapes on the exit and although they would bolt on the airflow would become quite turbulent. The mounting points are also wider apart for the trumpets on the blacktop engine