Table of Contents
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REF: Engine Mechanicals
IH Crankcase Pressure and Engine Breathing
See also in the REF section of the Sportsterpedia:
Crankcase Pressure
Sub Documents
In the crankcase air system;
Crankcase air pressure is mainly generated by the up and down movement of the pistons.
Additional air pressure is created by blowby from the combustion chambers past the rings and into the crankcase.
Additional air can be created by other small air intake areas as well as from gasket leaks.
Crankcase pressure also initiates the splash and mist process (the movement of the pistons and flywheels splash oil around in the engine).
The piston motions create a pulsating blast of air pressure (push pull condition as each piston rises and falls).
The pressure in the crankcase is not from oil pump pressure.
Static oil pump pressure has already been dissipated by the time it reaches the crankcase.
(although it takes static oil pressure to get the oil from the pump to the crankcase)
Crankcase (CC) pressure will have an average and constant change in velocity.
Gravity oil (from the drain ports in the heads) returns to the crankcase.
Gravity oil (from the pushrod tubes) returns to the gearcase.
Oil in the crankcase adds resistance to the air pressure generated (raising the pressure).
Role of the breather valve.
CC pressure both pushes to and sucks from the breather valve.
The breather valve allows for controlled air pressure to both leave and enter the engine to obtain what is referred to as a “slight running vacuum” in the crankcase.
If the breather valve isn't there;
The pressure generated from upstroke and downstroke would both push all the under piston air volume out of the engine and back into the engine.
(including any oil that was suspended with it.
Also that would create thicker oil suspension and more oil blowing out of the engine but not returning.
The volume between positive and negative pressure decreases as RPM goes up.
The speed of the breather valve action is important as it has to keep up with RPM changes.
See also, Example of Air Pulses Using a 1000cc Motor, in the sub documents at the top of this page.
Affects of the 45° Rod / Piston Arrangement:
Since Sportster piston movement is not equalized, we get the potato, potato sound we all love but the equilibrium in the crankcase is off by design.
This constant push / pull from offset pistons contributes to an imbalance of pressure that needs to controlled.
See, Affects of the 45° Rod / Piston Arrangement, in the sub documents at the top of this page for more information.
Differential Pressure
Vacuum and (positive) air pressure are the terms that describe the amount of molecules of a gas in a given unit of space. 1)
More molecules inside the engine than outside = inside air pressure.
Less molecules inside the engine than outside = inside is vacuum pressure.
Oil scavenging:
Positive crankcase air pressure aids scavenging. It pushes the oil to the oil pump.
Negative (vacuum) pressure makes the pump's job harder. High vacuum makes the downstroke push to scavenge less powerful.
The oil pump wants to receive more oil, high vacuum slows down the delivery of oil to the oil pump.
Ring seal and pumping loss:
Too much positive pressure is harder on the pistons on downstroke since they are having to expel that air pressure on the way down.
High positive air pressure in the crankcase is said to rob horsepower.
So it is good to have a slight vacuum in the crankcase when the downstroke begins.
A vacuum condition when downstroke begins lessens the restriction on the descending pistons and doesn't lower overall horsepower.
In most engines negative crankcase pressure allows less ring pressure and the combination of both means more hp. 2)
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 vacuum pump to reduce (positive) crankcase air pressure.
Crankcase pressure in these engines fluctuate wildly from positive to negative. 3)
However, high vacuum can have a dramatic affect on scavenging.
See, Differential Pressure, in the sub documents at the top of this page for more information.
