SM Head Modifications on a budget
Here's a little exercise I often do with a new valve job that helps me see the relationship between the valve angles and the seat angles. If you zoom in close you can really see the venturi that is being formed. This drawing is very close to the valve job I cut this morning - as close as I can measure.
On the computer I can move the valve to any lift and look at the actual curtain area. In this case the lift is 0.050" which forms an actual opening of 0.035" wide for the air to pass through. If we know the curtain size and the diameter of the curtain, the curtain area can be calculated. Bernoulli says 350 fps is the maximum theoretical velocity (it really isn't since this is a venturi), so multiplying the curtain area by the maximum velocity gives us CFM.
This drawing also demonstrates the complexity of defining curtain area with a simple equation. As the valve lifts higher, the point on the valve and the point on the head that define the curtain width (the two points closest to each other) change. I'm sure the math can be done, but most discharge coefficient numbers are based on a curtain area estimate that is not real accurate.
In this case the math says 31.7 cfm and the flow bench says 32.5 cfm. Pretty good correlation.
@273 this is especially for you since you like the math and have talked about the fact that flow at low lifts is always about the same on any head. Since the velocity is limited to 350 fps (or slightly higher due to the venturi effect) then cfm is totally dependent on curtain area. Curtain area is a function of valve seat angle(s) and valve diameter.
Here's a little exercise I often do with a new valve job that helps me see the relationship between the valve angles and the seat angles. If you zoom in close you can really see the venturi that is being formed. This drawing is very close to the valve job I cut this morning - as close as I can measure.
On the computer I can move the valve to any lift and look at the actual curtain area. In this case the lift is 0.050" which forms an actual opening of 0.035" wide for the air to pass through. If we know the curtain size and the diameter of the curtain, the curtain area can be calculated. Bernoulli says 350 fps is the maximum theoretical velocity (it really isn't since this is a venturi), so multiplying the curtain area by the maximum velocity gives us CFM.
This drawing also demonstrates the complexity of defining curtain area with a simple equation. As the valve lifts higher, the point on the valve and the point on the head that define the curtain width (the two points closest to each other) change. I'm sure the math can be done, but most discharge coefficient numbers are based on a curtain area estimate that is not real accurate.
In this case the math says 31.7 cfm and the flow bench says 32.5 cfm. Pretty good correlation.
@273 this is especially for you since you like the math and have talked about the fact that flow at low lifts is always about the same on any head. Since the velocity is limited to 350 fps (or slightly higher due to the venturi effect) then cfm is totally dependent on curtain area. Curtain area is a function of valve seat angle(s) and valve diameter.