Temperature to horsepower

Hey I'm doing a report in my Mechanical Engineering class on the effect of temperature (in celsius) to the efficiency of engines. Does anyone know about how much horsepower increase to each degree of temperature in farenheit or celsius? A guesstimate would be fine also, but please no BS.

I'm pretty sure it's somewhere around a 1hp increase for a drop in 5 degrees F, really depends on the engine though, especially naturally asperated vs forced induction.