As far as what I know from working with Honda engines, we always used to take the intake manifold off the B18C5 and put them on our B18C1's for the benefit of a straighter intake path and the fact that the B18C5's intake manifold didn't have the butterflies. We would even see good improvements throughout the entire rev range when switching to the B16A intake manifolds (they have the same intake path as the B18C1 but no butterflies). Our cars were all setup for autox and road courses also so bottom end was important, from what I've seen butterflies are pretty much useless when you're driving at ten tenths.