CFD Online Discussion Forums - Exporting results and Mach number calculation

Good afternoon. A few quick questions, nothing life-threatening but was interested to know if anyone had any insight.

1) Mach number. The speed sound as per CFX is calculated using $\sqrt{ \frac{\gamma P}{\rho}}$
I am using air ideal gas. Pressure, I have set the reference pressure as 5500 pascal (such that for my problem the relative static pressure is 0, hence my total/absolute pressure is 5500). Gamma is 1.4. How does CFX calculate $\rho$ from the molar mass (kg /kmol ) ?
In fluent, i used to set the Mach number to what I wanted (around 6) and it gave me a corresponding velocity it used to initialize the problem. The same velocity, when input into CFX gave me a free stream mach (I found from my results) closer to 7.

I did a simple interpolation and figured out my new velocity and am running at the desired mach now but I am still curious.

2) Exporting results. I am exporting to cgns. My domain has about 5 boundaries (2 inlets, 1 outlet, 1 wall, 1 sym plane).
When I export, is it possible to export just the results of the symmetry plane instead of all the boundaries? The "export boundary values only" solves half the problem (I think) in that it only exports the boundary information, not the vast fluid domain inside. But still the results file (that includes all 80 transient timesteps) is enormous, on the order of a couple gigs. It would really really help if I could export just the sym plane solution.

Thanks for the help :)