SM Head Modifications on a budget
I know this is theoretical mumbo jumbo and may not really belong here, but the numbers represent the current port being worked on. As in other posts, I'm trying to understand what limits the flow.
If we define the discharge coefficient as 146 cfm per square inch of minimum cross sectional area available for flow, then the red line represents 100% efficiency or a discharge coefficient (Cd) of 1. The sloping part of the red line represents maximum flow as the valve opens and the valve curtain area increases. Once the valve curtain area hits 1.86 in², the pushrod pinch becomes the minimum CSA and it limits the flow. That is the green horizontal line. The blue horizontal line represents the flow limit if the enlarged PRP area of 1.91 in² is limiting the flow. If the pushrod pinch is ignored, the throat eventually becomes the minimum CSA and it will limit flow. That's the horizontal red line at the top of the graph.
The area or vertical distance under the flow curve represents the port efficiency (pink line). The area or vertical distance between the flow curve and the theoretical flow maximum (in this case I chose the green line to represent 100% efficiency) is the port inefficiency (the blue line).
Let's ignore the pushrod pinch and assume the red line represents 100% port efficiency. At high lift the distance between the red line and the actual flow curve is huge. This is the inefficiency caused by friction, turbulence, flow separation, the valve, bends in the port and any other imperfections. These losses increase rapidly as velocity increases.
This theoretical limit of 146 cfm/in² or 350 fps through the minimum CSA is not really correct. If the minimum CSA is acting like a venturi, those numbers can be exceeded (locally, not in the entire port) if the CSA being measured is at the vena contracta (the most narrow point) of the venturi. I have seen average velocity of 375 cfm through the pushrod pinch and more than that through the valve curtain.
One last point and I'll stop the rambling. The shape of the theoretical flow curve explains why the actual flow curve is shaped the way it is - why it eventually flattens out.
Sorry for the digression. Please post corrections or rebuttals to these thoughts. Searching for the true answers here.
I know this is theoretical mumbo jumbo and may not really belong here, but the numbers represent the current port being worked on. As in other posts, I'm trying to understand what limits the flow.
If we define the discharge coefficient as 146 cfm per square inch of minimum cross sectional area available for flow, then the red line represents 100% efficiency or a discharge coefficient (Cd) of 1. The sloping part of the red line represents maximum flow as the valve opens and the valve curtain area increases. Once the valve curtain area hits 1.86 in², the pushrod pinch becomes the minimum CSA and it limits the flow. That is the green horizontal line. The blue horizontal line represents the flow limit if the enlarged PRP area of 1.91 in² is limiting the flow. If the pushrod pinch is ignored, the throat eventually becomes the minimum CSA and it will limit flow. That's the horizontal red line at the top of the graph.
The area or vertical distance under the flow curve represents the port efficiency (pink line). The area or vertical distance between the flow curve and the theoretical flow maximum (in this case I chose the green line to represent 100% efficiency) is the port inefficiency (the blue line).
Let's ignore the pushrod pinch and assume the red line represents 100% port efficiency. At high lift the distance between the red line and the actual flow curve is huge. This is the inefficiency caused by friction, turbulence, flow separation, the valve, bends in the port and any other imperfections. These losses increase rapidly as velocity increases.
This theoretical limit of 146 cfm/in² or 350 fps through the minimum CSA is not really correct. If the minimum CSA is acting like a venturi, those numbers can be exceeded (locally, not in the entire port) if the CSA being measured is at the vena contracta (the most narrow point) of the venturi. I have seen average velocity of 375 cfm through the pushrod pinch and more than that through the valve curtain.
One last point and I'll stop the rambling. The shape of the theoretical flow curve explains why the actual flow curve is shaped the way it is - why it eventually flattens out.
Sorry for the digression. Please post corrections or rebuttals to these thoughts. Searching for the true answers here.