I doubt it is a rule, more of an outcome related to practicalities of typical designs such as ratio of valve area to piston area, Mach choking, pumping losses, runner lengths and finish and so on. 115% is more typical.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
There is an actual physical limit although I suspect 124% has been obtained empirically. The additional cylinder fill is supercharging courtesy of wave action and like any supercharging, it requires energy to pump that extra air into the cylinder. In this case the energy comes mostly from work done by the piston during the intake stroke to create a negative pressure wave, which reflects from the runner mouth as a positive wave, which arrives at the valve just before it closes. The limit is imposed by the amplitude of that negative wave. The descending piston is pulling air in through the intake valve. Lets say it can produce a pressure of 0.5 bar absolute in the port (this is close to reality in a high VE race engine). That will return as a positive wave of approximately 1.5 bar absolute. If the compression process is isentropic and this pressure could be transferred to the cylinder at EVC, the density will be 1.33 times atmospheric. Of course cylinder volume at EVC will be less than at BDC so VE will be less than 133%.
Further VE improvement by producing a negative wave less than 0.5 bar absolute would be problematic.
- the intake runner length is determined by the time required for the wave to return
- reducing the runner diameter to increase the wave amplitude will reduce cylinder filling during the intake stroke - offsetting the increased wave supercharge.
Some energy is added to the wave from the exhaust system during valve overlap but that is secondary and also has limits.
