As you can see here you've gotte two answers already with a 40 degree difference in recommendations. A huge difference. Truth betold, there is no ballpark temperature range to look for. All engines are different and run at different temperatures and perform different even from the same manufacturer. Ambient tamperature makes a difference, but so does humity and altitude and direct sunlight or shade in addition to air quality index. And that doesn't even factor in the actual tune you have on the motor, how hard you drive it, or what pipe you have on it.