simply, yes, work out the flow pah if the butterfly is closed.I don't understand. What does "long" mean in this context?
The manifold length is cut shorter when the butterfly closes?
Almost, but it might be the way i'm reading your answer. I think you have the gist of the way it works tho which is good.....The butterfly eliminates the long runner disadvantage at high revs, at high revs you want a short intake path, at low revs you want a longer path to allow the air velocity in the intake to assist in getting more air into the cylinders.The "long" intake runner effect on bottom end hp is achieved by the expansion chambers. This runner length effect however is a detriment to fuel demand at high rpm air flow needs. That's what I'm reading.
So the short version is, the butterfly valve eliminates the "long runner" advantage at high rpms. Is that about right?
I can read your statement the other way and understand why you used the word advantage, but whichever way the main thing is long at low revs for more bottom end power, short at high revs. If you have a long intake at high revs, you can actually starve the engine for air as the intake pulses can set up "standing" waves which actually act as "plugs" preventing air flow. This was a regular thing on the early carbed drag and racing engines, leading to the invention of the dual plane manifold for bettter performance.