1997 4.6 Sohc Compression Ratio?

I bought a 1997 mustang gt and the owner didnt know a whole lot about it and he said that the owner before put some high compression pistons in it. i did a compression test on it and i got about 175psi for all the cylinders (roughly) and i cant figure out how to convert that into a compression ratio. i dont know what has been done to the car but i have a list of what stock specifications if that helps. Im just trying to figure out if i have any aftermarket parts that are upping the compression ratio so if someone can figure out what 175 psi equals to in compression ratio that would be great!
1997 4.6 SOHC 215HP Mustang Engine

Throttle body: 65mm (2.559 inches)

Intake manifold: composite, cast No. F6ZE-CG

Cylinder heads: cast No. RF-F5AE-AE

Combustion chambers: 49.77cc

Valves: 1.752/1.339 inches

Rockers: roller type, w/1.80:1 ratio

Camshaft lift: .473/.473-inch lift

Compression ratio: 9.30:1

Pistons: hypereutectic w/.153-inch dish