Positive crankcase pressure is one of the many unavoidable side effects produced by the normal operation of a piston engine. As the piston travels up and down within the cylinder, a small amount of combustion gases are able to make their way past the piston rings and into your crankcase, this is called blow-by.
There are several ways to combat positive crankcase pressure and its associated effects, including; a dry sump oiling system, vacuum pump, and a scavenging exhaust tube. We will be focusing on the two most common and cost effective streetcar solutions, an open and closed catch can system. Both accomplish the same goal of preventing most of the water, oil, and fuel vapors from being reintroduced into the engine through the crankcase ventilation (CV) system. The difference between the two will be covered in greater detail later in this article, but it has to do with what is done with the air after removing the heavier water, fuel and oil particles from the air column.
For your average commuter car, blow-by isn’t much of an issue unless the piston rings have worn to the point of considerable compression loss or a positive crankcase ventilation (PCV) valve is clogged. On the other hand, for a performance engine making more power than stock or a built motor with loose clearances, that mixture of diluted vapors can become quite an efficiency killer and even damaging if left unchecked, especially if the motor is boosted.
Factory CV System
Most factory crankcase ventilation systems work by pulling fresh air into the crankcase from the intake tube using the CV port and recycling the blow-by vapors into the intake manifold, via the PCV valve. The PCV valve is designed to only open when the air pressure in the intake manifold is lower than in the crankcase, creating a vacuum to pull the vapors into the manifold and back to the combustion chamber again.
For a performance engine where more severe blow-by is more common, that mixture of air, fuel, water, and oil vapors that are recirculated will coat the inside walls of everything downstream of the inlet port on the intake and eventually clog PCV valves and cake up engine components. Depending on the type of engine, this can include several important components, such as; the throttle body, intake valves of direct injection engines, the turbocharger compressor wheel, and the internal fins of an intercooler on boosted cars, reducing its efficiency. In addition, these vapors also create a universal problem of mixing with the gasoline injected into the combustion chamber and reducing its knock resistance; possibly pulling ignition timing on ECU controlled vehicles and reducing power.
Closed Catch Can System (Emissions Friendly)
A closed catch can system is the most popular solution for streetcars, because it still recirculates the air back into the system after removing the unwanted vapors from the air and retains the factory PCV system. This allows the car to still comply with emission standards, while still working effectively on most engines with bolt-on power adders. The downside to using this setup is that the PCV valve acts as a bottleneck on higher horsepower and high boost applications and won’t be able to vent the air quickly enough, allowing positive crankcase pressure to build. Depending on the application, a second catch can may be required to work effectively.
Open Catch Can System (Vent To Atmosphere)
An open catch can system is popular on cars with built motors and high boost because the PCV valve is deleted to allow for a greater volume of air to pass through the system. Because the PCV valve is removed, positive manifold pressure can no longer be blocked from entering the crankcase like on the factory system or a closed catch can. This means that the vent ports are routed into the catch can, like normal; but after the oil, water, and fuel are captured, the air is vented directly to the atmosphere through a filter and the inlet ports on the manifold and intake are capped off.
This setup can produce a strong fuel-like smell in the cabin and outside of the vehicle when not moving, and will not pass any visual emissions inspection. If you are producing enough power to require an open catch can system, it is highly recommended to take full advantage of the PCV delete and use -10AN hose and fittings to guarantee adequate ventilation of the system.
Positive crankcase pressure and its related effects can wreak havoc on a modified performance engine. A catch can system is a cost effective solution to preventing higher crankcase pressure associated with increased power, building up due to lack of ventilation. Additionally it will prevent components from being coated in a film of oil and sludge.