The reed valve is really nothing more than a cork that functions automatically. It opens, and closes the intake track at the most opportune time, and helps to maintain crankcase pressure which compacts the air/fuel mixture by preventing any blowback through the intake track. If you compact the air fuel mixture, then you can get more air, and fuel into a given space. That being the crankcase, and transfer ports. It's a bit like having a supercharger, but without the drag on the engine that the pulleys, and belts would produce. You can further increase the pressure by decreasing the crankcase volume by utilizing "case stuffers", and stuffed crankshafts. It all really depends on how much time, and money that you want to spend on your motor. There is a trade-off however, and that being faster wear due to the increased pressures, which is mitigated somewhat by the cooling effects of a greater volume of fuel, and oil injected into the areas of wear. The expansion chamber does essentially the same thing, but with the exhaust track as compared to the intake track. The length, and shape of the header, chamber, and stinger work in concert to perform this function. The trick, and hence the science is to balance all this activity into a perfect series of exchanges in the combustion chamber, crankcase, and transfer ports in order to get the very most out of the given space.