One question: Is the crank tunnel considered part of the CC volume?
It's been noted that with the Enya 45's, the difference between the 6001 and 6002 is mainly the tunnel size, and that sleeving the 6002 crank tunnel down to match the 6001 results in a much better stunt run. I would associate that with decreasing the CC volume resulting in better pumping action seen as increased velocity of the fuel/air charge into the CC.
Does that make sense?
In a static, non running system then most definitely the entire volume that is valved shut by the piston and inlet cut out or 'trapped' is considered the crank case volume, including any ports and the crank tunnel.
Why? Because a volume is defined by all points being at equal pressure, and its not hard to imagine a static model engine containing fluid that has definite limits.
But ........... in a dynamic running system I am not so sure since there is never equal pressure anywhere, the whole system resonates or pulses, the upper and lower volumes are linked during transfer and if you consider a full engine cycle then the volumes step outside of what is considered discreetly 'trapped.'
If this was not true then the engine would not have a power curve - it would be much more of a flat line.
The Enya 45's crank tunnel reduction in diameter I would say has more to do with an increase in gas speed and a reduction in condensation, since the tunnel is a high speed centrifuge that will fling sheared fuel droplets against its outer walls - the bigger the walls the more the area, the higher the surface speed and more it acts as one collecting droplets and joining them into a homogeneous mass that is more difficult to ignite.
Sure it does improve the pumping action somewhat but it is not a high performance engine that needs that a critical consideration.
Hoping to make sense, Chris.