Plenum volumes are critical to RPM range.. I have used a stock 2.0 plenum on 2270cc engines up to 145HP,above that its pretty worthless. The 2.0 Bus plenum has proven to be more well designed to me, in back to back comparisons on the same engine.
With a plenum based system camshaft specifics are absolutely critical at low speed and idle. Cams set up for a very high manifold pressure will give the best vacuum signature thats necessary for a MAP based common plenum to be effective and tunable.
Generally the lobe separation is the key..
That said, George's system is SDS driven with some CB hardware. I almost had a chance to tune one for him a month or so ago on my chassis dyno.
BTW- Its not so much a "restriction" or "constriction" as much as being less than optimum for certain RPM ranges.
I just finished the revitalization of the engine for my 912E. Its a 2056 that is being built for MPG, smooth running and 90 miles per day commuting. I optimized this engine for air conditioning and to utilize a stock 2.0 common plenum. I am using a new system that Electromotive has developed for us that is generally used on motorcycle engines for my PEFI solution. This coupled to the RAT/ Extrudabody throttle body and my seriously modified fuel delivery arrangement that utilizes a fuel distributor rather than fuel rails should fit the bill quite well.
I changed the cam in this engine from a RAT 9530 with a 105* lobe separation to the same cam with a 112* lobe separation.
The previous version supplied me with 160K miles of trouble free, reliable performance. It was worn but had much more life in it when I tore into it as you can see here.
http://www.aircooledtechnology.com/index.p...&Itemid=112