Gilbert:
The 4 stroke engine operates on a 720 degree cycle. The valves are in motion for about 120 degrees of that cycle - depending on cam grind. Flow stops & starts.
There are two radically different flow systems. The Intake, which is at a little less than atmospheric pressure & cold. The Exhaust which is hot, and pumped out well above atmospheric pressure.
The exhaust gas tuning is the inverse of a two stroke. The plan is to have most of the exhaust charge traveling down a tube at high velocity when the exhaust valve starts to close. This creates a slight suction on the cylinder, removing as much exhaust gas as possible.
The intake tuning is built around maintaining a velocity & containing a standing wave. As the intake valve opens, a flow through the carburetor is established. Fuel is mixed. The air / fuel mixture is cold & dense. When the intake valve closes, this charge stops suddenly. If you have enough velocity, the charge packs in against the closing valve, continuing to flow as the valve closes. Improving the cylinder fill as much as possible.
The standing wave. When that intake charge stops against the valve, the energy has to go someplace. It reverses direction. This is why some engines need a long velocity stack. When we consider the intake tuning length we include the length of the carburetor and velocity stack (if one exists)
It is common to see a fuel fog standing above a carburetor on the dyno. VW engines with 48IDA Webers were famous for this. The fuel charge was pushed back through the carb & up the stack. Tuning had to take in to account the fuel that went past the venturi twice. If you ran the engine in a vehicle without an air cleaner wrapped around the velocity stacks, it would lean out as the fuel fog was blown away in the wind.
If you have the cam timing events for your engine and the displacement the calculations can be done. You need to know how many degrees BEFORE Bottom dead Center the exhaust valve begins to open, and how many degrees AFTER TDC the exhaust valve closes. For the intake you need opening Before TDC, and closing After TDC.
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