.................."By its very design, this means our 300-cubic-inch engine takes in 300 cubic inches of air every two revolutions of the crankshaft. Now here’s the interesting part. It does this whether the throttle is open or closed........
When the throttle is closed, very little air mass flows into the engine, so that small amount has to expand to fill our 300 cubic inches. Thus, the air will be of very low density. As the throttle opens, more air mass can flow in to fill the engine, and the density will increase. This is often called the “charge density”...........
I feel this statement might be a little mis-leading. It says it "takes in 300 cu. in." every two revolutions regardless of if the throttle is open or closed. If the throttle is closed it can't take in 300 cu. in. when the pistons goes down. It is stuck with what ever atmospheric pressure does push past the throttle and with what was there, so there is no filling or power produced.
By this it would seem to me that larger TBs would allow MORE flow of what is low density air into the motor thus allowing more O2 into the combustion chamber just as
If the throttle is wide open then the restrictions to filling the cylinder are the flow characteristics of the intake track (manifold, and cylinder head), the valve sizes and the flow past the valves and the duration the valves are open and their lift and the relationship of the exhaust closing to the opening of the intake valve which then brings the scavenging of the exhaust into play. The preceding is going to depend on the cam you picked. In other words how efficient of an air pump is the motor at any given rpm or rpm range.
Now if all of those things are flowing at their max you now bring the size of the throttle valves and the flow characteristics of the throttle body into the equation. If it is now the restriction to air flow you need to make it larger. Also all of this has to be matched together for the rpm you are trying to make hp at.
I'll bet the reason the larger throttle bodies on the stock motor can be made larger is because the manufacturer might have kept them a little on the small size to give up WOT top rpm power for better intake track velocities down low in the rpm band to make the bike more drivable. Put the larger ones on and they might make more top end hp, but the bike might not seem so strong in every day driving.
So if the motor is being starved for air by the throttle body do something about that regardless of the altitude you are running. I feel (might not be correct) that if you have throttle bodies sized for max hp at sea level on a na engine if anything they could probably be smaller at elevation and larger wouldn't do anything.
Also on a fuel injected engine the throttle bodies can be sized larger than you would size a carb cfm wise as you are controlling the fuel with the nozzles, where as if you put a too large carb on and the air flow through the carb is not high enough and the velocity is not good the carb just isn't going to work since it draws fuel with the venturi effect.
Hope someone clears up anything here that they don't agree with.