To elaborate a little more on this a SX and SXR have the same motor. A triple cylinder single piped 700 as srxxx86 said. SRX is a completely different motor that has powervalves and triple pipes. All three sleds are SX chassis though.
As far as your question "not sure which mountain max I have". All Mountain Max sleds are based on the SX chassis also. Mountain Max is more suited for mountain riding than a SX, SXR, or SRX obviously.