I decided to treat the crankcase vent hose as an orifice, largely because the tube is the pathway between the pressure source (crankcase) and the destination (Either atmosphere or filter box).
I found a flow calculator at:
http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm#calc...Which calculates the flow rate based on pressure differentials. Plugging in some numbers, it gives how much fluid (such as air) can move through a specified orifice diameter.
I reasoned that the vent hose nipples are about 1/2 inch in diameter. I used a source duct area of 5 inches, as that is at least the diameter of what feeds the crankcase vent nipple.
We don't know the average pressure of all SOHC4 engines inside the crankcase. But, I plugged in some values to get some idea of flow capability.
1 PSI differential allowed for 20.1 CFM through such an orifice.
2 PSI differential allowed for 28.5 CFM through such an orifice.
4 PSI differential allowed for 40.3 CFM through such an orifice.
5 PSI differential allowed for 45.1 CFM through such an orifice.
As can be seen, the more pressure differential exists, the more volume can be transferred. I don't think this should surprise anyone. It's just nice to have a formula that verifies what we already know. If we then assume the vent is a road draft type, the flow is totally dependent on what pressure builds inside the crankcase. Motors with ideal piston ring seal, won't build any. However, I don't know of any SOHC4 engines with perfect piston ring seal. If you plug the vent tube and oil begins push past the seals, you know it is building up more pressure that outside atmospheric, right? Further, if you see any visible vapors exiting the tube, it is either blow by or heated gasses expanding from temperature rise, which also creates pressure. (Ever see a steam kettle?)
What about Honda's recirculator?
For reference, Honda specifies an average 16-24 cm hg is present in the intake duct between carburetor and engine cylinder. This equates to a 3 - 4.5 PSI (vacuum source relative to outside atmospheric source). Anyone who has vacuum synced their carbs has seen the vacuum change with both throttle position and engine RPM. During operation, outside air pressure is constantly trying to equalize the pressure differential. Which is what gives you air flow in the duct. But, it is easy to understand that if there is air flowing, there is pressure differential to varying degrees within the duct between the engine cylinder and the orifice that is the air intake to the engine. So, it is fairly obvious that the filter box air pressure will be lower than outside atmospheric, in a range from a bit less than 0 to -3 or -4 PSI (could be more at higher RPMs when that inlet orifice is taxed to provide more air flow).
What this tells me, is that Honda's recirculator works far better at crankcase evacuation than a road draft tube in all operational modes as it presents a lower pressure to the crankcase than the outside atmospheric pressure can provide. Whether it actually lowers the crankcase pressure of your engine depends on how fcuked up your rings are. But, certainly the opportunity exists to have it lower than without the recirculator. Beyond that, this exercise also shows that the more your engine tries to build crankcase pressure with noxious fumes, the better the vent hose actually vents noxious fumes. And, the recirculator will work better than outside air to draw it out because it presents a lower air pressure source.
I know that this is not a smack-your-forehead revelation.
But, writing it was more interesting than painting another section of eaves on the house, today.
