Running some more flow tests on the SBM Speedmaster head. This port is 185cc, 2.02 valve, 50 deg seat with 40 deg top cut. Valve is sunk into the head about 0.090" to achieve a top cut of about 0.200" to 0.250" wide. As mentioned in some of my other posts, I'm trying to understand what happens at the top end of the flow curve to limit the flow. Why do flow gains ultimately stop? It has to do with flow separating from the controlling surfaces (port walls, short turn, valve, seat angles, combustion chamber walls) causing vortices that shut down/decrease the available flow area.
In this test, a normal combustion chamber is compared to a combustion chamber where the exhaust valve is covered in clay and a 39 degree top cut 'cone' is formed with a raking tool. This creates a controlling surface top cut around the entire perimeter of the intake valve that should help reduce the flow separation that occurs in the combustion chamber. This allows for a more controlled pressure recovery situation.
Looking at the results, I found a couple of things interesting. First, there is almost no difference in the flow curves below 0.550 lift. I thought there would be some loss of mid lift flow due to the increased shrouding in the areas of the exhaust valve and the spark plug. Second, the clay cone extended the 'nose over' point from 0.550 lift to 0.650 lift. This 'nose over' point is the point where substantial flow is lost on the short side of the valve and the majority of the flow shoots over the top of the valve. This flow increase could be due to two factors. It could be that the increased clay shrouding on the long side and on the exhaust valve side is forcing the air flow to stay attached on the short side until higher lift. It could also be that the increased length of the top cut (the cone) is preventing or delaying the flow separation until further down stream. This creates a more favorable pressure recovery situation in the chamber. Or, both could be partially responsible.
Think about this. On the flow bench, there is a 28" pressure drop across the entire port. At low lifts and low flows, almost the entire 28" of pressure drop is across the valve. As the valve opens further, less and less of the 28" pressure drop is across the valve. The port itself starts becoming responsible for more of the pressure drop (i.e. more of the resistance to flow). At high lifts and high flows, the flow separation over the short turn and in the combustion chamber is responsible for a portion of the 28". Let's say in the normal combustion chamber that number is 5" of pressure drop. In the clay cone combustion chamber, that number has been reduced to 4" (I'm picking numbers out of the air for illustration). Now, that additional 1" of pressure drop is available to increase flow through the port. And that is exactly what happens. The flow through the port at 0.650 lift is about 18 cfm higher in the clay cone port due to the fact that there is less pressure drop in the chamber and more in the port. It's just like cranking the air pressure at the entrance to the port up by 1". I find it interesting that the flow at 0.550 is almost identical in both chambers. I would have expected to see the clay cone chamber doing better at 0.450, 0.500 and 0.550. Something else is going on.........
Obviously, this test has little application in the real world. I think it does give a little insight into what happens on the flow bench. Just thought it was interesting and a few people might enjoy the results.
In this test, a normal combustion chamber is compared to a combustion chamber where the exhaust valve is covered in clay and a 39 degree top cut 'cone' is formed with a raking tool. This creates a controlling surface top cut around the entire perimeter of the intake valve that should help reduce the flow separation that occurs in the combustion chamber. This allows for a more controlled pressure recovery situation.
Looking at the results, I found a couple of things interesting. First, there is almost no difference in the flow curves below 0.550 lift. I thought there would be some loss of mid lift flow due to the increased shrouding in the areas of the exhaust valve and the spark plug. Second, the clay cone extended the 'nose over' point from 0.550 lift to 0.650 lift. This 'nose over' point is the point where substantial flow is lost on the short side of the valve and the majority of the flow shoots over the top of the valve. This flow increase could be due to two factors. It could be that the increased clay shrouding on the long side and on the exhaust valve side is forcing the air flow to stay attached on the short side until higher lift. It could also be that the increased length of the top cut (the cone) is preventing or delaying the flow separation until further down stream. This creates a more favorable pressure recovery situation in the chamber. Or, both could be partially responsible.
Think about this. On the flow bench, there is a 28" pressure drop across the entire port. At low lifts and low flows, almost the entire 28" of pressure drop is across the valve. As the valve opens further, less and less of the 28" pressure drop is across the valve. The port itself starts becoming responsible for more of the pressure drop (i.e. more of the resistance to flow). At high lifts and high flows, the flow separation over the short turn and in the combustion chamber is responsible for a portion of the 28". Let's say in the normal combustion chamber that number is 5" of pressure drop. In the clay cone combustion chamber, that number has been reduced to 4" (I'm picking numbers out of the air for illustration). Now, that additional 1" of pressure drop is available to increase flow through the port. And that is exactly what happens. The flow through the port at 0.650 lift is about 18 cfm higher in the clay cone port due to the fact that there is less pressure drop in the chamber and more in the port. It's just like cranking the air pressure at the entrance to the port up by 1". I find it interesting that the flow at 0.550 is almost identical in both chambers. I would have expected to see the clay cone chamber doing better at 0.450, 0.500 and 0.550. Something else is going on.........
Obviously, this test has little application in the real world. I think it does give a little insight into what happens on the flow bench. Just thought it was interesting and a few people might enjoy the results.
