Textbook engine thermodynamics predicts that SI (Spark Ignition) engine efficiency ? is a function of both the compression ratio CR of the engine and the specific heat ratio γof the working fluid. In practice the compression ratio of the SI engine is often limited due to "knock". Knock is in large part the effect of end gases becoming too hot and auto-igniting. Knock results in increase in heat transfer to the walls which negatively affects efficiency. Not to mention damages to the piston. One way to lower the end-gas temperature is to cool the intake gas before inducting it into the combustion chamber. With colder intake gases, higher CR can be deployed, resulting in higher efficiencies. In this regard, we investigated the efficiency of a standard Waukesha CFR engine. The engine is operated in the SI engine mode, and was operated with two differing mixtures at different temperatures. First was Air + Methane at room temperature, second O2 + Argon + Methane at room temperature, third and last case study was O2 + Argon + Methane at sub-zero inlet temperature. A sweep of the spark timing to locate highest efficiency point, while taking into consideration KLSA (Knock limited spark advance), at each compression ratio. It is to be noted that along with increasing the compression ratio, advancing the spark timing was found to increase efficiency as well, however the process is limited by knock phenomena, especially for the Argon mixtures. Emissions were measured using an AVL SESAM-i60 FTIR analyzer. The results showed that the Argon mixture enhanced engine efficiency quite considerably from the standard air mixture by almost 5%. However, the cooled Argon cycles showed no improvements from the Air or Argon mixtures at ambient conditions.