- Foreword - What this is all about.
- Goals - What we are trying to achieve.
- Why - Why these are good things to aim for.
- Background - What you need to know beforehand.
- The OEM Way - How an OEM system on your car functions.
- OEM Issues - Why the OEM way isn't good enough for some of us.
- Misconceptions - What probably isn't wrong with your OEM setup.
- Doing It Better - How to improve on the OEM way without regression.
- Summary - What to take away from this article.
- Credits - Who assisted in the production of this article.
What this is all about.
Very often threads pop up asking about the best way to achieve good crank case ventilation. Equally as often bad information is posted up by those that do not understand these systems. It is therefore my goal here to clearly define the objectives, problems and methods involved in ending up with a good ventilation setup.Goals :
What we are trying to achieve.
- Prevent pressure build up
- Get water vapour and CO2 out
- Allow clean fresh air in
- Generate a good vacuum
Why these are good things to strive for.
- Pressure causes leaking seals and blown dip sticks
- Pressure causes loss of power and poor ring seal
- Pressure causes more blow-by and more pressure
- Water vapour and CO2 cause base alkalinity to be depleted and acids to form
- Fresh air displaces water vapour and CO2 (oxidation of the oil is not a major concern)
- Good vacuum causes better ring seal, more power and less blow-by
- Good vacuum helps clear water vapour, CO2 and dissolved petrol/gasoline etc from the engine
What you need to know beforehand.
The OEM Way :
- What is a PCV valve? - A PCV valve is a special check valve with a variable flow profile that matches the blow-by production of the engine under moderate to low load conditions.
- What is a back fire? - A back fire is an explosion of fuel-air mixture in the intake tract.
- What is an after fire? - An after fire is an explosion of fuel-air mixture in the exhaust. After fires are often incorrectly referred to as back fires.
How the OEM system on your car functions. Your OEM vehicle probably has the following setup on it :
- One PCV valve from cam cover or crank case to intake manifold (between valves and throttle(s))
- One breather port from cam cover or crank case to the intake plumbing before the throttle, after the AFM (if present) and before the turbocharger (if present)
These stock systems have several modes of operation :
- At idle and under lift off deceleration conditions the intake vacuum is strong and the blow-by production low. The PCV is fully open and in this state restricts the flow path from crank case to intake manifold to a low level. This level is however still sufficient to draw all blow-by produced into the engine for re-burning. Flow travels from the crank case to the intake manifold in this mode.
- At low load under cruise conditions the intake vacuum is moderate and so is blow-by production. The PCV sits half way open and in this state restricts the flow path from crank case to the intake manifold to a moderate level. As above, this level is however still sufficient to draw all blow-by produced into the engine for re-burning. Flow travels from the crank case to the intake manifold in this mode.
- At high load under wide open throttle conditions pre-boost the intake is very close to atmospheric pressure and blow-by is significant. Depending on how good the breather from the crank case to the pre-throttle intake is, the PCV valve will either be closed or slightly open. If the breather is good, the PCV will be closed and all flow of blow-by will be from the crank case to the turbo inlet or equivalent. If the breather is restrictive there will be a pressure build up inside the crank case which will overcome the spring pressure in the PCV and cause it to open allowing flow of blow-by from the crank case into the intake manifold. Because the crank case is under pressure the breather will also be flowing blow-by into the intake pre-turbo or equivalent.
- During a back fire, when the engine is off or when the engine is seeing intake boost pressure the PCV valve is closed and any blow-by is exiting through the breather into the intake pre-turbo or equivalent.
- In the case of boost being present, the PCV is closed and sealed (or close enough to it to not worry about it) and blow-by is at an all time high. Because it is unlikely that the crank case pressure will exceed boost pressure all blow-by flow must exit through the breather. Because the breather is the only exit path it must flow well enough to prevent pressure build up inside the engine. Boosted engines typically produce a lot more blow-by and also are on average more likely to be driven hard.
It should be noted that during idle, lift-off, cruise and low load conditions that there is a cross flow of fresh air into the crank case from the breather and blow-by and fresh air out of the crank case into the intake manifold. This is because the PCV is drawing more gas than the engine is pushing past the rings. This style of setup is more than adequate for the vast majority of vehicle users running naturally aspirated and low boost engines in a normal non-abusive way.OEM Issues :
Why the OEM way isn't good enough for some of us.
- Under heavy load and in boost the engine is breathing significant amounts of oil vapour and blow-by.
- The oil vapour dilutes the fuel being injected and therefore lowers its octane rating and increases the chance of detonation.
- The blow-by displaces volume that could have been occupied by fresh oxygen filled air thereby reducing your power output.
- The oil vapour that is ingested by the turbo condenses on the inside of the intercooler thereby insulating it and increasing intake air temperatures and increasing the chance of detonation and decreasing power output by being less dense.
- Typically the breather hole is far too small to freely breath the blow-by generated during a sustained thrashing. This usually manifests itself as leaking seals, and dip sticks that won't stay put in the hole and keep blowing out.
What probably isn't wrong with your OEM setup.
Doing It Better :
- Breathing blow-by gases and oil vapour at low load is a non-event and nothing to be concerned about. Likewise the oil film coating the inside of your intake manifold is also not a problem and if anything keeps the air cooler as it passes through.
- PCV valve leaking boost causing pressurisation of the crank case - If there are two equal restrictions and three volumes with the following configuration : O-O-O and the red volume is at X psi while the blue volume is at Y psi the pressure in the middle volume can't exceed Z where Z = (X + Y) / 2 because of the pressure source alone. If boost = X = 15 psi and atmosphere = Y = 0 psi that makes crank case pressure due to leaking boost 7.5 psi at max assuming equal flow. Because your PCV valve no matter what condition it is in is almost certainly a much greater restriction (if leaking at all) than the breather on the other side, the chances are that any pressure build up you have is almost purely caused by blow-by that is not being ventilated freely enough through the breather to the pre-turbo intake.
How to improve on the OEM way without regression.
- The first thing to improve is to remove the oil from the blow-by before feeding it back into the engine via the intake pre-turbo. There is little/no point in doing this on the PCV line as we don't care what the engine breaths under those conditions. We care that our intercooler stays dry and free of an oily layer of insulation. This is accomplished by adding a catch can inline with the breather tube to the turbo inlet. Not surprisingly, many OEMs have already done this modification for us.
- Secondly if legal where we are, we could allow blow-by to vent to atmosphere via a catch can instead of entering the engine through the inlet. This guarantees that the intercooler remains dry and clean for maximum charge cooling and minimum chance of detonation. In some countries this is illegal as these gases are considered to be part of the overall emissions of the car and burning them in the engine makes for cleaner air.
- Enlarging the breather port is a very good thing to do if you are going to thrash the car hard for a sustained period of time. Many OEM cam covers do not have sufficiently large breather ports for heavy use or boosted use and particularly for heavy boosted use. Here is an example of a Mazda FE3 cam cover modified to have two half inch ID breather tubes instead of the single 4mm ID hole disguised in a much larger fitting. The owner reports no further oil leakage from around his seals.
- Exhaust venturi scavenging can be added such that there is active suction during high load conditions as well as cruise and low load conditions. This has some advantages over "just" venting the system well in that A it can ensure a flow of fresh air into and through the crank case and B cause a vacuum to be applied to the crank case. Again, this may not be legal in some countries.
- An electrical or belt driven air pump could be used to extract the blow-by gases from the crank case actively. This has the disadvantages of complexity and power wastage, however would make for a very clean dry engine. A potential source of these are "smog pumps" found installed on some older American and European cars. They are also used on some drag cars to increase power from better ring seal. If you add a vacuum pump, and want to actually see vacuum, obviously you will have to put a restriction on the breather to create that vacuum. If doing this be sure that the pump flows more than the piston rings do or blow-by will pressurise the crank case and cause problems.
- If you are building an all out car for the track then the chances are it will be dry sumped. Dry sumping has the advantage of producing a strong vacuum if configured correctly. The primary purpose is not to evacuate the crank case of vapours, but never the less it does an excellent job of this.
In summary I would advise to :
- Leave the PCV exactly as it is and ensure it is clean and functioning correctly
- Enlarge the outlets from the crank case for breathing purposes
- Add a good catch can inline with the breather hose(s)
- If legal vent the breather to the atmosphere through a smallish filter rather than into the turbo inlet
This will keep your engine clean on the inside, free of excess pressure and your intercooler clean and dry too.Credits :
The following people provided information and pictures for this article. Many thanks to them!
This article was written hastily and in a short space of time very recently and consequently is probably full of mistakes. Suggestions are welcome, please place them in the comments thread here : viewtopic.php?f=35&t=99
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