It depends on what you have done. I have a switch wired to my ac fan that blows the heat off my headers over my valve cover and pushes to the direction of the back of the hood so whenever i go to the track or am doing autocross i just flip it on and i have written down and done tests on it and it lowerd it by a deacent 17 Degrees. However for every day i dont use it proped up cause it will give you all of like 2 degrees cooler . Now if it is an EK with your AC removed and the front grill removed its directing air into the engine and if the hod is proped it will flow out to the back. But it's up to you some people like it some people dont I say fuck it do what please you