My understanding is that is does not really inprove effeciancy or warm up time of the engine. What it does is make the combustion process hotter and the exhaust hotter a little bit sooner. This does not make the fuel burn better but, it changes the composition of the exhaust gasses to reduce certain cold ones and lights off the cat sooner to reduce pollution. I have read that as long as the air temp is 0 F. or higher drivers will not notice any advantage to using preheated intake air on a injected car. On a carb car the heat helps to vaporise the fuel and prevent carb icing. Injected cars do not use a venturi effect so icing is lees common

Bill D.