40 replies to this topic

[76lxhatch]

Gregory's manual description which is reasonably succinct:
"The small amount of filtered air necessary for scavenging the system is drawn into the engine rocker cover from the bottom of the air cleaner. If the engine is being operated under hard acceleration or heavy load conditions, and the generated blow-by gas exceeds that which can be passed through the PCV valve, then the excess gas is pushed through the filtered air hose into the air cleaner and then drawn into the carburettor."

The primary purpose of the PCV system (as opposed to just a breather) is just to ensure that unburnt hydrocarbons (crankcase vapours) are not vented to atmosphere. However the positive ventilation is also a reasonably efficient means of crankcase breathing which works well for a street driven car.

[LXCHEV]

I sent a letter in to Street Machine asking about PCV's versus breathers etc.... it got printed in KB's Workshop this month (ie. the current issue as we speak).... I didn't really get the answers I wanted, but it's still interesting reading...

[76lxhatch]

Do tell for those of us who still have a month to wait for that issue

[ls2lxhatch]

doesnt some of the pressure from compression pass the rings and end up in the crankcase?

If a PCV is a check valve, which it is, it should-
1 - open if the vacuum overcomes the spring in the valve which would may create a vacuum in the crankcase - apparently that is one of the advantages of a dry sump system.

The purpose of the PCV valve is to regulate the airflow through the engine. When vacuum is high the valve reduces airflow, when vacuum is low the valve is fully open allowing maximum airflow. Even under extreme vacuum the PCV valve will not completely close.

The primary purpose of the vacuum is to draw clean air through the crankcase to purge crankcase vapours which reduces oil contamination and emissions. If you were to run breathers instead of a PCV system on a street car then you would need to change the oil more regularly.

[TerrA LX]

[quote name='Squarepants' date='08 August 2010 - 04:57 PM' timestamp='1281250621' post='528802']
[quote name='the gts' date='08 August 2010 - 03:49 PM' timestamp='1281246577' post='528793']

so why does the crankcase need fresh air?

Just to dilute the gasses coming from the crankcase through the PCV to reduce pollution and add more clean air to the inlet mixture for a better burn thus better power and economy
[/quote]

Well no it is not to dilute gasses, as Squarepants has previously stated by running two PCV and no breather, you will not get flow thru the crankcase if there is no where for the air to get in, the fresh air is simply to allow the system to flow.

[TerrA LX]

Sorry, may have mis lead you a little when I said that it does not matter where the air enters, to clarify it, if you look at the OEM picture you have posted;

You will see that air enters "one side" of the motor to "flow through" the crank case then exit out the "other side" of the motor to gain full "Positive Crankcase Ventilation".

[76lxhatch]

As above as long as it gets some flow (ideally enough to pick up vapours through all parts of the crankcase) it should be OK - if this is limited the worst that can happen is you need to do slightly more frequent oil changes to keep it all clean. I'm sure it will be OK in this respect.

My biggest concern with using one of those tubes would be sucking out too much oil - its always a fine line between removing vapour and taking bulk quantities of oil (which not only depletes the oil supply but also mucks with the intake mixture and burning too much oil can be a pain). I'm guessing you may need to set up some sort of trap and/or baffle to minimise the raw fluid movement.

[TerrA LX]

Just so long as there is positive flow thru the crankcase, i interpreted that your in and out would be next to each other, negating flow thru the crankcase.

Holden V8's do not usually suffer as much in this department as the valley has a very large opening connecting the crankcase with the valley via the cam shaft so it does not need to be directly connected to the crankcase directly.

Never played with chevs personally but assuming by your statement "the gts" that the chev heads drain to the valley like a holden?

[ls2lxhatch]

by tapping into one of those valley breather tubes the breather will be breathing directly into the crankcase just above the cam.

I think tapping into the valley breather may pickup too much oil spray. I would just position both tubes in the valley as high as possible and position them as far as possible from oil and each other.

The LS2 has a breather connected from the drivers side rocker to the intake and one connected from the front of the valley to the intake manifold.

[enderwigginau]

I still dont understand why it needs fres air into the crankcase

Seal it all up - then when you crankcase looks liek a crushed beer can, call me and I'll explain

The BEST spot for the PCV is in one (or both) rocker covers, as it doesn't do its job otherwise. If you baffle it so that it only draws air from the TOP of the rocker covera dn can't suck oil, you can put it anywhere. At the back is commonly used, but I've seen underneath, so long as you run a tube into the top of the cover.

That diagram is a bit confusing, as piston movement causes vacuum to switch across banks, which is why some run a PCV with a t-piece plumbed to both covers. One breather and one PCV is an attempt to bias the flow and create it the way the diagram shows......
Really, you need to get a real engine with only one rocker cover, then you'll be less confused

Grant..