they kinda R...where do U begin when ur on a circle? 
I think of it more as a 'pressure equalizer' (w/oil separator in back & air cleaner of air in frnt) than a circular thing but both ideas work...
There's 'blow by' and 'crankcase pressure' bein taken care of.
I've never seen a 'draft tube' that I know of, the pvc's predecessor. I think the addition of the ERG valve (w/the pvc) was next iteration.
Use of incorrect PVC has created tune up type problems (rough runnin)...
I think of it more as a 'pressure equalizer' (w/oil separator in back & air cleaner of air in frnt) than a circular thing but both ideas work...
There's 'blow by' and 'crankcase pressure' bein taken care of.
I've never seen a 'draft tube' that I know of, the pvc's predecessor. I think the addition of the ERG valve (w/the pvc) was next iteration.
Use of incorrect PVC has created tune up type problems (rough runnin)...