The first time I looked at the late model downdraft intake , I thought that they seriously bumped their head . There was no way that the top cylinder could get air. After all it would rush in and bypass the top , let alone make the sharp 110* turn and form a column and go thru the reeds uniformly .
Then after taking the time to measure the inside volume of the box , I found out that it held a larger capacity than the actual size of the motor by close to 30 cubic inch's .
So in essence , it is somewhat of a large "still air" box . The motor will never reach a percentage of volumetric efficiency over 100 to tax the amount of air on hand .
And just like a four barrel over a single plane intake manifold , they don't pull an intake charge all at once . Each cylinder follow's the rotation . So the air ducting really only needs to refill the "box" at the rate and volume in which it is depleted .
Those slope lid manifolds have some thought put into them . The entrance has to be big enough to house the air valve . The cross section has to be big enough to both keep the column moving slow enough to where the first cylinders in line don't have to reach out and grab a fast moving target , turn it and fill the runner . As well be the conduit to support the back cylinders . The shape also keeps the air from tumbling as it moves to the back .
We have come a long way from the old "Ramchargers C/Altered intake ... >
https://tse1.mm.bing.net/th?id=OIP.A...=0&w=202&h=152
And the state or the art stuff that my hero John Marcella builds today ... ;)
https://tse2.mm.bing.net/th?id=OIP.O...=0&w=230&h=174