But the thing that hangs me up, for a given pressure differential across an orfice, you can calculate the airflow in cfm.MadBill wrote: ↑Tue Apr 02, 2019 10:35 amYou're looking at it backwards. At a given operating condition, say 7,000 RPM WOT, the engine will draw into the cylinders a fixed volume of air*. The more restrictive the carb, the greater the pressure drop into the manifold and the lower the resulting all-important mass of air/fuel ingested. *The air meter measures said volume at atmospheric pressure, so a greater carb restriction will draw in less mass and thus show lower atmospheric pressure CFM.
The pressure drop across the valve varies hugely with crank angle and valve lift but can be anywhere from a few inches of H2O to 100" or more. In fact good software programs like Dynomation show inflow continuing as cylinder pressure rises above port pressure as the intake valve closing event is approached, as charge inertia can for a short time overwhelm the rising negative pressure drop.
The higher the pressure differential, the faster the air is moving, thus more cfm for a given orfice. It takes more work though(pumping lose)
To me from a theoretical standpoint, addition vacuum(pressure differential) would be beneficial up until the point that it meets the cfm demand of the engine because like you said, the engine is pulling a fixed volume of air. It seems to me that the pumping loses are from the engine speeding up the air more.
How can the carb flow more air with a lower pressure differential? This is the thing I am stuck on. From a theoretical standpoint, unless there is some other force at work here, a 1050 dom will not flow enough air to keep up with the demand of my engine at .7 inches
I get that the addition pressure differential create the pumping loss, because it has to do more work to get the air in the engine, but it comes with the gain of faster air movement and cfm. But like I said, it seems this would only be beneficial up to the point of meeting engine demand.
Again I am trying to understand this stuff and I may be out in left field.