Quick background is that I am designing a head for a V twin from a clean sheet of paper.  It is a DOHC, 4 valve design.  I have a mechanical engineering background and I have experience with flow simulation software and would like to utilize it to design the proper port sizes and shapes for this engine. The engine has 4.25 bore and 4.00 stroke. The rev range should be 7000 to 8000.  I would like to see it make peak power well above 6500.
Most of what I find for literature exists for 2 valve designs. 
I have found some basic differences between 2 valve and 4 valve designs though. One of the basic differences is the ability of the 4 valve design, because the lighter valve train weight, lower valve spring pressures, the lowered requirement for valve lift (more low/mid curtain area) and the resulting ability to open and close the butterfly valves faster, to require much less valve open duration for an equivilent rpm/cylinder size than the typical ohv 2 valve design. I have found that in long duration 2 valve applications you need a higher port velocity to get the flow moving at the low lifts when the piston is still moving slowly and you need high velocity when the valve is closing to prevent blow back. I believe that on well designed 4 valves the shorter duration and much faster opening and closing lessens the need for this high velocity. Furthermore this fast action (and the multi valves much higher low/mid lift flow) require a relatively larger volume behind the intake valves (less velocity). The same applies to the exhaust side, the blowdown occurs so much faster that larger ports (compared to a 2 valve and less velocity as derived from an engine size formula) are required. IMO velocity has a time element (duration) along with the engine size/port size normally associated with it. I have found 4 valve designs with smaller ports and more duration can develop more rpm but with less torque and do not perform as well. The larger port and shorter duration engines I have experience with are superior even at the lowest rpms. I have no knowledge on how this would affect any low emission requirements though.

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