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PCV Systems

When tubocharging or supercharging an engine, extreme pressure can be created in the crankcase. Modifications need to be made to the PCV system to vent pressure, and prevent boost pressure from charging the crankcase.

On naturally aspirated engines, the PCV system is not designed for running under positive boost conditions, and damaging amounts of pressure will build in the crankcase without preventing boost from entering it. There are a few methods for making the PCV system work under forced induction. First, you will need to prevent boost from entering the PCV system.

During a vacuum condition, the crank case gasses are drawn from ports usually on the valve cover(s) through the PCV valve and into the intake manifold. Gasses are literally sucked out of the crankcase here.

If you were to introduce boost without addressing these ports, boost pressure would back through the PCV valve and into the crankcase. You are sure to damage gaskets or seals if this condition should occur. (Don’t ask me how I know about that)

One solution is to place a one-way check valve between the PCV valve and the port(s). This valve should be positioned so that during vacuum conditions, the PCV system operates normally. The check valve is forced open, and crankcase gasses are drawn into the manifold. During boosted conditions, the check valve is forced closed, and pressure is blocked from entering the PCV ports.

The check valve alone will only prevent boost pressure from entering the crankcase. It will not allow the crankcase to properly vent during boosted conditions because the PCV valve is blocked by the check valve. Other changes need to be made to allow the PCV system to vent under boost.

There is usually an additional port on the valve cover(s) with a hose that leads to the throttle body. This is the PCV fresh air tube. As the manifold pulls gasses from the crank case, fresh, filtered air is drawn into the crank case through this tube. As with the PCV valve port on the intake manifold, boost pressure will enter the fresh air tube if it is not modified.

One solution is to block the port on the throttle body and re-route the fresh air tube to a port between the air filter and turbocharger inlet. The port near the air filter will provide fresh air into the crank case when there is no boost, and a vacuum under boost to help draw positive pressure out of the crank case.

A typical PCV system.

PCV system modified for turbocharging.

Catch Cans

On some systems, oil can blow by the PCV system into the manifold. Oil that is blown out through the PCV system can be captured in an oil catch can. This is a device that separates oil from the gasses that escape through the valve covers, and prevents it from entering your intake.

A catch can be placed in-line with the tube that runs from the front of the passenger’s side valve cover to the air filter near the turbocharger. Fresh air will be drawn into the PCV system when not under boost, and gasses and oil will be drawn out and into the catch can while under boosted conditions. The gasses will be drawn back through the PCV system, but the oil will remain in the catch can. The can should be emptied from time to time to prevent overfilling.

The PCV systems described above are only a few of the methods for venting the crank case for turbocharged applications. Many other solutions exist including removing the PCV system altogether and simply placing breathers on the valve covers. But most people agree that keeping the PCV system and modifying it for forced induction is a better method. Other solutions include running vacuum pumps to draw gasses out of the crank case.