In an internal combustion engine, the difference in pressure between the combustion chamber and the crankcase often results in blow-by, or gasses from the combustion chamber leaking past the piston rings into the crankcase. If not addressed, this can result in a number of problems. High crankcase pressure creates resistance to the normal upward and downward motion of the pistons, which reduces the net horsepower produced by the engine. If the crankcase pressure is high enough, it could cause also gaskets and seals to fail. Blow-by gasses also contain some water and elements of fuel, and if the gasses are allowed to condense in the oil pan, they can cause the formation of acids and contaminate the oil.

A normally-aspirated engine provides for crankcase ventilation by the use of a PCV (Positive Crankcase Ventilation) valve, commonly mounted on a valve cover. When the valve is supplied with manifold vacuum, evacuation channels in the head and block allow crankcase gasses to be drawn upward from the crankcase through the head and past the PCV valve into the intake manifold, where they can be burned again. The PCV valve is both a one-way check valve and a metering device. Maximum intake vacuum occurs during idling and deceleration, but this is also when blow-by is at a minimum. During these periods, the PCV valve is mostly closed to prevent the system from creating too much vacuum, which the ECU would register as a vacuum leak. Excessive vacuum in the crankcase could also cause seals to leak backwards, drawing in contaminants from the outside air. During periods of heavy acceleration and higher engine speeds, intake manifold vacuum is low, but this is also when blow-by is at its peak. In order to compensate for the reduced intake vacuum, the PCV valve opens fully when engine vacuum is low. This works very well in a normally-aspirated engine, because the intake is always supplying sufficient vacuum to clear the crankcase of blow-by gasses.

But in a forced induction system, the manifold is only providing vacuum (negative pressure) at idle, low speeds, and during deceleration. At higher engine speeds and periods of boost, the system is actually under pressure. If not controlled, the increased manifold pressure will flow backwards through the valve cover into the engine, creating positive crankcase pressure. Most PVC valves are designed so that the sudden high pressure of an engine backfire closes the valve, but even the best valves become contaminated with oil and dirt so that they no longer completely block off the manifold pressure. This problem can be addressed with the use of a one-way check valve. The check valve would work with the PCV valve to limit air flow back into the crankcase. However, the gasses must still be evacuated, and this requires the use of a second evacuation channel.

In the setup described below, the second evacuation channel is actually provided by the first part of the primary intake channel. This is only possible because that part of the system is bidirectional, allowing air to flow in both directions. When the intake manifold vacuum is high but supercharger intake vacuum is relatively low, air is pulled through the crankcase in the direction of the manifold. But when the manifold is pressurized under boost, the check valve completely closes off this part of the system. When that happens, positive crankcase pressure pushes the blow-by gasses in the opposite direction. Almost simultaneously, the air flow at the supercharger intake increases greatly. The increase in airflow results in higher vacuum at the supercharger intake, which draws in the crankcase gasses.


Crankcase Ventilation System, Modified for Fixed Displacement Supercharger

This schematic illustrates the basic setup

Installation notes

· Because the air used to ventilate the crankcase will eventually be burned by the engine, it must be measured by the MAF (Mass Air Flow sensor). If the air is not reported to the ECU (Engine Control Unit), the system cannot maintain the proper air/fuel ratio. Therefore, the air must be pulled in after the MAF sensor, but ahead of the supercharger intake.

· The catch can positioned on the PVC/check valve side of the system can be unidirectional, but the catch can used in this part of the system (on the right in the above schematic) must allow air flow in either direction.

· Ideally, the valve cover connections should be as close to the rear as possible. This is because in a hard braking situation, oil is thrown forward within the valve cover. Under such conditions, any connections at the front of the valve cover could draw in liquid oil. If a new connection is made at the rear of the valve cover, it is advisable to also add an oil baffle to the inside of the cover at the connection point.

· With the creative use of check valves, it is possible to use a single catch can with 2 intake ports and 1 output port, but unless the tank is rather large it will require more frequent draining and cleaning. High-quality inline valves are available for about $20 each, but cheaper alternatives are available. Older imported small cars and trucks used them in the vacuum line to the power brake booster unit (American cars switched early to a booster that incorporated the check valve internally). On a real tight budget, these can be found at the junk yard for a dollar or two. The valves must be oriented properly to allow flow in the correct direction.

Air/Oil Separators (Catch Cans)

Catch cans are considered a requirement with any forced induction engine, including those with supercharger systems. Whenever the crankcase ventilation system is used to circulate crankcase gasses through the intake, the need to filter the air becomes even more urgent. Dirty oil vapor can coat the entire intake system, including the valves, with an insulating layer of contaminants that may also contain corrosive elements. This layer may interfere with proper cooling and heat dissipation as well as gumming up the works.

Most importantly, oil vapor in the intake stream lowers the octane of the fuel/air mixture. This could cause engine-damaging detonation and all of the problems associated with it. Therefore, the intake of crankcase gas should always be filtered through an air/oil separator. It is important to use cans that contain oil baffles and/or some sort of filtering element. Those that require periodic draining are recommended, as the oil contained therein is contaminated and not well suited for re-use.

Alternative Crankcase Ventilation Methods

Vent tubes/Breathers

Breathers and vent tubes are the simplest type of crankcase ventilation, and because the crankcase gasses are vented to the atmosphere they require no additional filtration. But they are not very environmentally friendly, and in most cases they will render the car incapable of passing any sort of air quality test. This type of ventilation system is very effective at relieving excessive crankcase pressure, but is not as good at removing contaminated air out of the crankcase. Most of these systems are considered passive ventilation systems rather than positive ones.

Vacuum Pumps

Aftermarket vacuum pumps are another way to provide crankcase ventilation. In addition to supplying consistent and reliable vacuum, they allow low-tension piston rings to be used in the engine building stage. These rings produce much less friction against the cylinder walls while providing an improved seal, thus decreasing blow-by even further. With a big enough pump, it is possible to create such negative pressure in the crankcase that horsepower is increased.

But they can be expensive and problematic, especially if they create too much vacuum. Anything above 14-15 inches begins to pull oil away from critical engine parts, and can even pull unfiltered and unmetered air into the crankcase from outside of the engine. The use of a dry-sump oiling system with oiling jets can overcome this obstacle, but the cost of such a modification is likely prohibitive to all but the most serious engine builders. The use of a vacuum pump further increases the complexity of the system and adds to the engine’s overall parasitic power loss.

Exhaust Evacuation

One very inexpensive way to increase crankcase ventilation is to install an exhaust evacuation kit. These kits typically involve installing a breather on each valve cover. The breathers are connected to the exhaust stream, where exhaust flow creates a vacuum to pull in crankcase gasses.

The most obvious problem with this type of system is that it will cause the vehicle to fail any emissions test. This may not be a problem, but these systems also deposit a large amount of oil into the exhaust system in a very short period of time. And finally, many vehicles rely on exhaust system backpressure for proper EGR (Exhaust Gas Recirculation) function. Such systems cannot provide adequate vacuum or air flow under positive pressure.

DISCLAIMER: An improperly functioning PCV system, or one that has been modified improperly, can lead to emissions violations or even damage to the vehicle. As always, the authors of this article and the owners of this web site bear no responsibility for injuries to persons or damage to vehicles sustained as a result of the information contained in this article. Any modifications to your car are performed entirely at your own risk.

IMAGES: All images are the work of the author and the property of this web site.