Chainsaw Repair

Looking at Figure 5 again, and thinking about the engines used in this paper, which are 200cc and 150cc with power peaks at 3600rpm. 

What I see is that when the case volume is very large, then you can get a good delivery ratio at low rpm, but it falls off quickly once you go higher in rpm.  That makes sense because there isn't much pressure differential to move the air, but if you go slow enough eventually it fills.  Go beyond that and it falls on its face since there isn't enough time to move the air.   

However, once you get close to the peak power rpm (where presumably the port timing was optimized), and you get the case volume down small enough, then it no longer falls off with rpm.  Now the delivery ratio isn't so much a peak as a threshold. 

This makes sense to me because the relationship between the pressure differential and the case volume isn't linear.  As the case volume goes down the pressure differential goes up fast, and therefore the velocity goes up fast too.  So as the case volume goes down you can move the air much faster and then you don't have this effect where there isn't enough time to achieve the full delivery ratio. 

So if we look at our smaller engines at 10,000rpm it appears to me that smaller case volume is better, and we will not easily make case volumes small enough such that the delivery ratio will fall off at practical rpms for these engines.