17.4 is just 157/3^2 ; solving for the scale factor of the throttle valve. It is a rough approximation, but sizing of a throttle valve does not have to be superduper precise. More precision would require a lot more information than has been offered.
Yes the Cv calculated WILL change with different gases because the Cv calculation has gas density as a parameter.
The Cv equation for compressible flows has nine (9) variables in it, some of which are calculated from other parameters. you really don't want to crunch through it manually. Cv is inversely related to the square root of Molecular weight. Effective MW for air is 29.
Still, I contend that the throttle plate will/should only see air. Your natural gas flow should be regulated as a function of the actual or projected mass flow through the throttle plate at its range of openings and over the range of engine speeds. NG or Propane carburetors exist that do this well, and althought I was not specific, it was my intent that you use the scaling guideline I offered to find an appropriately sized carburetor to meet your needs.
To find the gas flow you will need to determine the stoichiometric ratio of the fuel to the air, and then determine how much of the combined gases the engine can inhale. THe fuel gas will limit the amount of air that the engine ingests by displacing a part of the incoming charge.
Example: So since oxygen is 20% of air, if you burn pure methane
CH4+2O2> CO2 + 2 H2O
If the methane is warmed to the same temp as the incoming air, you would need one part methane to 2 parts O2 +8 parts N2 and other atmospheric components. 11 parts total. The necessary gas would be 9% gas by volume in this example.
DOn't know at what pressure the methane is being stored, WHst the initially regulated pressure of the methane, whether it is flowing thru a hx to normalize the temps, so it's darned hard to guess what size device would be there to regulate the fuel gas.
