Imprezer

iTrader: (9)

Can someone explain in detail what PCV does and why it does it. I know the basic idea behind it, but I am trying to figure out if I should use one on my car or not.

For performance, is it better to have it venting into the atmo or into the intake manifold?

How do I know if my valvecover/crankcase pressure is too high?

Kevin M

iTrader: (12)

It helps to prevent blow-by I think.

Imprezer

iTrader: (9)

Mulder

PCV= Positive Crankcase Ventilation

There is always some amount of blowby gas in the engine that gets past the rings. Back in the old days engines had open crankcase venting, basically just a hose from the valve cover that ran down and vented these crankcase gases out to the atmosphere. As emission controls became required this was unacceptable so the idea of a closed PCV system was developed which used engine vacuum to pull the gases from the crankcase into the intake so they could be burned in the engine rather than released into the air.
It's hard to tell for sure that you have excess crankcase pressure, but if this occurs (usually due to either a bad/stuck PCV valve or really worn rings) the result can be leaking seals and/or gaskets (as the pressure finds a place to escape) and/or increased oil consumption.
I suppose if performance is the only consideration, and emissions be damned, an open vent will keep the gases out of the intake and prevent any effect on fuel mixture or combustion, and also cut down on the sludge that can result from the blowby entering the intake. Of course this is not very environmentally friendly and not the best idea for a daily driven street car, especially one that has to pass an emission inspection.

Imprezer

iTrader: (9)

The positive crankcase ventilation (PCV) system prevents air pollution which will be caused by blow-by gas being emitted from the crankcase. The system consists of a sealed oil filler cap, rocker
covers with fresh air inlet, connecting hoses, a PCV valve and an air intake duct.

In a part-throttle condition, the blow-by gas in the crankcase flows into the intake manifold through the connecting hose of crankcase and PCV valve by the strong vacuum created in the intake manifold. Under this condition, fresh air is introduced into the crankcase through the connecting hose of the rocker cover.

In a wide-open-throttle condition, a part of blow-by gas flows into the air intake duct through the connecting hose and is drawn into the throttle chamber, because under this is condition, the intake manifold vacuum is not strong enough to introduce through the PCV valve all blow-by gases that increase in the amount with engine speed.

Group B

iTrader: (3)

If you do not use a pcv, and vent it to atmosphere, via what is called a "road draft tube", with or without a filter element on the end, it's not SMOG legal, FWIW.

There is also some risk of the motor grenading if you utilize a "road draft tube", if it does not flow well.

However, the drawback of a PCV system is that oily gasses flow into your intake, and can be nasty.

From what I've heard, the AVO catch can is the only catch can system that cleans the gasses & collects oil in its can, while still flowing in a circuit similar to that of the stock system.

Group B

iTrader: (3)

Positive Crankcase Ventilation (PCV)

Positive Crankcase Ventilation is a system that was developed to remove harmful vapors from the engine and to prevent those vapors from being expelled into the atmosphere. The PCV system does this by using manifold vacuum to draw vapors from the crankcase into the intake manifold. Vapor is then carried with the fuel/air mixture into the combustion chambers where it is burned. The flow or circulation within the system is controlled by the PCV Valve. The PCV Valve is effective as both a crankcase ventilation system and as a pollution control device.

PCV systems have been standard equipment on all new cars since the early sixties. Prior to 1963 PCV was only used in California. There are a variety of PCV systems used on various makes and models of cars produced since 1963, but all function essentially the same.

PCV systems can be described as either open or closed. The two systems are quite similar. However, the closed system in use since 1968 is more effective at air pollution control. The systems differ in the manner in which fresh air enters the crankcase and excessive vapor is expelled.

Open PCV Systems

The open system draws fresh air though a vented oil filler cap. This presents no problem as long as the vapor volume is minimal. However, when the crankcase vapor becomes excessive it is forced back through the vented oil filler cap and into the open atmosphere. The open PCV system, though successful at removing contaminated vapors from the crankcase, is not completely effective as a pollution control device.

Closed PCV Systems

The closed PCV system draws fresh air from the air filter housing. The oil filler cap in this system is NOT vented. Consequently, excess vapor will be carried back to the air filter housing and from there into the intake manifold. The closed system prevents vapor, whether normal or excessive, from reaching the open atmosphere. The closed system is very effective as an air pollution control device.

The PCV Valve

The most critical part in the PCV system is the flow control valve, commonly referred to as the PCV valve. The purpose of the PCV valve is to meter the flow of the vapor from the crankcase to the intake manifold. This is necessary in order to provide proper ventilation for the crankcase, while not upsetting the fuel/air mixture for combustion.

Blow-by gases and vapor should be removed at about the same rate they enter the crankcase. Since blow-by is minimal at idle and increases during high speed operation, the PCV valve must control the flow of vapor accordingly. The PCV valve is designed to compensate for the engine ventilation needs at varying engine speeds. It is operated by manifold vacuum which increases or decreases as engine speeds change.

For example, at low or idle engine speeds manifold vacuum is high. This pulls the plunger to the extreme forward position, or manifold end of the valve. Due to the shape of the plunger, vapor flow is reduced to a minimum. The low rate of the flow is adequate for ventilation purposes and will not upset the fuel/air mixture ratio.

At high speeds manifold vacuum is decreased. The plunger is only drawn to a point about midway in the housing. This allows a maximum flow of vapor. Since the engine needs more fuel/air mixture at high speeds, the introduction of more vapor does not affect performance. In the event of a backfire, pressure from the intake manifold forces the plunger to the closed or engine-off position. This prevents the backfire flame from reaching the crankcase and exploding the combustible vapor.

A neglected PCV system will soon fail to function and the result can be expensive as well as troublesome for the car owner. If the crankcase is not adequately ventilated, the motor oil will quickly become contaminated and heavy sludge accumulations will begin to form. Internal parts, not protected by the motor oil, will begin to rust and/or corrode due to the water and acids that will become trapped within the crankcase. If the PCV system is not functioning properly, the flow of crankcase vapor into the intake manifold will not be properly metered. This, in turn, will upset the fuel/air mixture for combustion and cause rough idling or even stalling of the engine. Furthermore, intake and exhaust valves, in addition to spark plugs, may well be burned and rendered useless, prematurely affecting performance and requiring expensive repairs. To assure trouble-free performance of the PCV system and, in turn, the engine and vehicle, routine maintenance of the PCV system is absolutely recommended and required.

Imprezer

iTrader: (9)

This is what I found.

Thanks Mulder.

Imprezer

iTrader: (9)

So ideally, I want to vent the crankcase and valvecovers back into the intake manifild/intake system, but not the oily vapours, right?

Group B

iTrader: (3)

Your welcome Scully!, and you do that through the use of a baffled oil catch can, but one that vents properly like the AVO, and unfortunately, not the Cusco...

Imprezer

iTrader: (9)

I am not using CUSCO catch tank. It's "nipples" are too small.

The thing is, I am using STi AVCS heads. They have 2 breather outlets ON EACH head.

Then, of coruse, I have the normal crankcase vent that has a "Y" on the end.

In stock configuration, both heads and one end of the crankcase Y go to the intake (before the turbo).

The other end of the crankcase Y goes to the PCV valve which is on the manifold.

So...

Here is what I am thinking to do.

1. Run all 4 outlets from both heads and one crankcase Y end to a baffled catchtank and then run a very FAT FAT FAT hose (~1-1.5") from the catch tank to the intake before the turbo.

2. Run second crankcase Y end to the PCV to intake manifold.

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Is that the best way to do it?

Peaty

iTrader: (2)