Speedmaster SBM head with 2.02 intake valve on the flow bench. Let's assume this head is going on a large stroker motor and we're looking for high flow. I'm trying to find the flow limits of the head and understand why flow stops increasing. Throwing all the cheat tactics I can think of at the head, flow has peaked at 308cfm. Why does it stop there? Where is the choke point? Checking velocity at the pushrod pinch and short turn apex shows some insane velocities. These are shown in the first chart under Test 568. I've convinced myself that increasing the size of the PRP has got to help. Lowering the velocity through the PRP will keep speeds down approaching the apex. Surely enlarging the PRP will also relieve a restriction in the port. If you pinch off a water hose the flow goes down. Relieve the pinch and flow goes up. Doesn't work that way with airflow in a port. As test 569 shows, increasing the size of the PRP does not always help. Flow in Test 569 is lower than flow in Test 568.
Everything from here on out is speculation on my part. I'm thinking out loud and sharing, not teaching.
I've run into this a few times before and it can be quite confusing. The PRP is definitely a restriction in the port. Why does relieving that restriction not increase flow? I think the answer has to do with laminar flow vs turbulent flow. In the water hose, flow is laminar, so reducing the restriction increases the flow. But in the intake port flow is not laminar and changing the PRP affects what is happening down stream. Look at the drawing of flow bench Tests 568 and 569. Both drawings depict 0.650" valve lift. In both tests the inlet of the port has a pressure of 0 inches H20 if read on a gauge. In both tests the pressure in the cylinder is -28 inches H2O. I don't know what the actual apex pressure is, but let's assume in Test 568 that the gauge at the apex reads -3 inches of H2O. That gives us a 3 inch pressure drop through the PRP. In test 569, the PRP has been enlarged and the gauge pressure at the apex would now be higher. Let's say it increased to -2.5 inches of H2O. So now there is less pressure drop through the PRP and a higher pressure at the apex. That should be more pressure available to push air through the port. Flow should go up, but it goes down.
What happens down stream is flow separation. At high valve lifts the flow cannot stay attached to the short side. As flow increases, separation increases and shuts down the available flow area. So what might be happening here is this. Even though flow in 569 is less than 568, the potential for flow is greater. The higher apex pressure in 569 is TRYING to push more air through the port but flow separation decreases the flow area so flow ultimately drops.
So here's my takeaway. In general, for an 18 deg SBM head on a large motor, a larger pushrod pinch should be a good thing. But if there are other problems down stream, enlarging the PRP can expose them and even compound those problems. Pittsburghracer has said many times that the PRP is not the problem on the SM/Edelbrock head. I'm starting to understand what he means.
As mentioned before, this is an opinion and a theory. I welcome others.
Everything from here on out is speculation on my part. I'm thinking out loud and sharing, not teaching.
I've run into this a few times before and it can be quite confusing. The PRP is definitely a restriction in the port. Why does relieving that restriction not increase flow? I think the answer has to do with laminar flow vs turbulent flow. In the water hose, flow is laminar, so reducing the restriction increases the flow. But in the intake port flow is not laminar and changing the PRP affects what is happening down stream. Look at the drawing of flow bench Tests 568 and 569. Both drawings depict 0.650" valve lift. In both tests the inlet of the port has a pressure of 0 inches H20 if read on a gauge. In both tests the pressure in the cylinder is -28 inches H2O. I don't know what the actual apex pressure is, but let's assume in Test 568 that the gauge at the apex reads -3 inches of H2O. That gives us a 3 inch pressure drop through the PRP. In test 569, the PRP has been enlarged and the gauge pressure at the apex would now be higher. Let's say it increased to -2.5 inches of H2O. So now there is less pressure drop through the PRP and a higher pressure at the apex. That should be more pressure available to push air through the port. Flow should go up, but it goes down.
What happens down stream is flow separation. At high valve lifts the flow cannot stay attached to the short side. As flow increases, separation increases and shuts down the available flow area. So what might be happening here is this. Even though flow in 569 is less than 568, the potential for flow is greater. The higher apex pressure in 569 is TRYING to push more air through the port but flow separation decreases the flow area so flow ultimately drops.
So here's my takeaway. In general, for an 18 deg SBM head on a large motor, a larger pushrod pinch should be a good thing. But if there are other problems down stream, enlarging the PRP can expose them and even compound those problems. Pittsburghracer has said many times that the PRP is not the problem on the SM/Edelbrock head. I'm starting to understand what he means.
As mentioned before, this is an opinion and a theory. I welcome others.
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