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Old 1st July 2019
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Default --Vacuum Pressure --

Vacuum and (positive) pressure are the terms that describe the amount of molecules of a gas in a given unit of space.
More molecules inside the engine than outside = inside air pressure.
Less molecules inside the engine than outside = inside is vacuum pressure.

Image two scenarios:
1. The case is sealed (closed to atmosphere).
The pistons just compress and relax the fluid in the case.
2. The case has a huge passage that allows the fluid to pass into and from the atmosphere.

Which takes more power to cycle?
Possibly the second.
Because the 1st, as the pistons use power to compress the fluid;
That power is returned as the pistons rise from the stored energy in the compressed fluid pushing pistons up.

That may be what's really happening for the most part in our bikes.
Obviously, the breather opening is too restrictive to allow so much flow that the case pressure stays more constant.
That restriction is on purpose to lessen the pumping losses by being closer to #1 than to #2.

As the vacuum increases, the pumping losses decrease and the fluid is less dense.
On the flip side, less dense fluid can't suspend as much liquid (possibly resulting in more liquid drag).

Below is some terminology for vacuum measurements.
PSIG - (pounds per square inch (gauge):
Gauge pressure is pressure measured relative to ambient atmospheric pressure (approximately 14.7 PSIA).
PSIA - pounds per square inch (absolute):
Absolute pressure is measured relative to high vacuum (0 PSIA).
PSIV - pounds per square inch (vacuum):
Vacuum pressure is measured relative to ambient atmospheric pressure.
PSID - pounds per square inch (differential):
Differential pressure is pressure measured relative to a reference pressure.
If the reference pressure is one atmosphere the differential pressure range is equal to gauge pressure range.

The earth's atmosphere exerts a pressure upon us, known as the atmospheric pressure, which can be measured in a number of ways. 13)
At sea level, the standard pressure is 14.7 psia or 29.92“ of mercury (Hg) or 760 mm of mercury (Torr).
Because the barometric pressure varies, the above “sea level” pressures are used as a reference point.
There is 14.7 psia pressure being exerted on us by the atmosphere, but there is also 14.7 psia inside of us pushing out.
(given the fact that for every action there is an equal but opposite reaction)
Thus, we do not feel discomfort from the atmospheric pressure.
Another way to state this is that there is no differential pressure between the inside and outside of our bodies.

The term “vacuum” is used to describe the zone of pressure below atmospheric pressure.
Vacuum is a negative gauge pressure, usually referenced to the existing standard barometric pressure where the equipment will operate.
This means vacuum is a differential reading between the surrounding atmospheric pressure and the pressure in the system evacuated.
In all instances when given a vacuum condition, the question should be asked, at what elevation the pump will operate.
(since the barometric pressure varies with altitude above or below sea level)

Example of differential pressures (or Vacuum):


Applying the same principles, you can see the relation of positive and negative crankcase pressure in the Sportster engine.
Every piston upstroke and downstroke reverses crankcase pressure from positive to negative forces.
The pistons act as an air pump and then a suction pump respectively.
Below are drawings exampling positive and negative (vacuum) forces inside the engine.


86-90 models with cam chest breather vents:



91-97 models with head breather vents:



98-03 models with head breather vents:



04 and Up models with head breather vents:



The breather valve is necessary to keep the imbalance in the crankcase (from the 45° arrangement) at bay.
The umbrella closes when the upstroke happens. It's not mechanical. Vacuum pulls it closed.
The forces are simple in that during upstroke, a negative pressure is pulled inside.
The outside air has a higher pressure than the inside air does.
So the outside air tries to enter the engine.
If the umbrella closes, it keeps outside air from entering.
If the umbrella stays open or doesn't seal fully, the higher pressure from atmosphere enters the crankcase.


In most engines negative crankcase pressure allows less ring pressure and the combination of both means more hp.
Over the years folks have used exhaust system energy to pull pressure from the case for this reason.
Guys have won championships with an engine that had an electric pump to reduce crankcase pressure.
Crankcase pressure in these engines fluctuate wildly from positive to negative.
At some point, it can have a dramatic affect on scavenging.


Oil scavenging:
Positive crankcase pressure aids scavenging.
Negative pressure makes the pump's job harder, because the pump is fighting the crankcase vacuum.
(with little to no blow-by and a check valve on the breather system)
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Reference the XLFORUM'S Sportsterpedia
for additional technical information & advice

Last edited by Hippysmack; 11th October 2019 at 22:57..
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