It has been a while since I have bought any so my information might be out of date. As I remember holley only offered the highflow in 2 sizes I always used the 6.5 but I think there was 10.5 also. I didn't go with the size of the window to identify them but instead used the amount of side to side play as the indicator. with a standard flow and a high flow in hand you will notice that the high flow has much more room for the needle to move in relation to the seat. If you use the standard PV instead of the high flow it can make the engine very lean. (ask me how I know) But using a high flow instead of a standard flow will not change the tune, unless you need something other than a 6.5 PV, Good luck.