Think of you engine as a 4 inch air pump. With a 2 bbl carb, and single exhaust, your pump has a 2 inch inlet, and 2 in. outlet. Add a 4 bbl, dual exhaust, now have 3 in inlet, 3 in outlet. Add cam, heads, porting, hi-rise, now have a 4 in. inlet, and still have 3 inch outlet. Add pistons, stroke it, now 5 inch pump with 3 inch outlet.. hope it helps
