PCV/Blowby

[DENALIHD66]

I'm asking if someone who really knows what they are talking about to please help me understand something.  I've always set up my cold air intake kits to spec, but the tubes that are positioned on the CAI tube that run into the crankcase from right before the throttle body... what is the purpose of these?  Are they pulling air in from the intake tube, or is that where blowby gasses vent out to, and then recirculate back into the intake manifold?  

 

Is that the only/best way to run a CAI?  I've seen where guys will just put breathers on the top of the crankcase vents or something.  What would you use to then plug the holes that came pre-fabricated on the CAI tube then?  Is there a loss of vacuum by running it the way the CAI engineers designed it?  

 

https://www.knfilters.com/video/posters/l/63-3082_install.jpg

[sdog4127]

The tube is an air intake source for the positive crankcase ventilation (PVC). It should be installed before the throttle, but after the air filter. The other option is, as you have seen, is to install a filter directly on the valve cover to filter the air being pulled into the crankcase.


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[michigan2500hd]

42 minutes ago, DENALIHD66 said:

I'm asking if someone who really knows what they are talking about to please help me understand something.  I've always set up my cold air intake kits to spec, but the tubes that are positioned on the CAI tube that run into the crankcase from right before the throttle body... what is the purpose of these?  Are they pulling air in from the intake tube, or is that where blowby gasses vent out to, and then recirculate back into the intake manifold?  

 

Is that the only/best way to run a CAI?  I've seen where guys will just put breathers on the top of the crankcase vents or something.  What would you use to then plug the holes that came pre-fabricated on the CAI tube then?  Is there a loss of vacuum by running it the way the CAI engineers designed it?  

 

https://www.knfilters.com/video/posters/l/63-3082_install.jpg

I know the manufacturers run the blow by gases into the intake to burn them, thus reducing pollution. Not sure it works but that's the reason I've heard. I believe a catch can helps keep the intake clean by trapping the junk ( liquid and solid) and recirculating the gases. 

[sdog4127]

Absolutely correct. A catch can is installed between the filtered air inserted into the intake tube. The engine fumes are pulled into the intake. A Catch can condenses the oil vapor and water and “catches” it before being pulled into the intake for reburn. If you have a simple air filter breather on the valve cover, there is no use for a catch can.


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[DENALIHD66]

Okay so THAT'S the purpose of a catch can.  Basically an extra filter between the tube and the INTAKE tube to filter out the particles, but the gasses pass through so they can re-burn into the intake manifold.

 

If the catch can purpose is to "catch" the particles, then putting a filter breather on doesn't allow them to escape?  Unless that filter breather needs to be replaced/cleaned constantly?

[sdog4127]

Okay so THAT'S the purpose of a catch can.  Basically an extra filter between the tube and the INTAKE tube to filter out the particles, but the gasses pass through so they can re-burn into the intake manifold.
 
If the catch can purpose is to "catch" the particles, then putting a filter breather on doesn't allow them to escape?  Unless that filter breather needs to be replaced/cleaned constantly?

If your engine has s simple vent filter on the valve cover, it is pulling unmetered air vapor from atmosphere. It will never re-enter your intake and coke your valves.

Now... all modern engines monitor intake air flow extremely close to accurately prescribe fuel injection ratio. So if your engine uses a mass airflow sensor (AFS) you will absolutely need a catch can if you want to condense crankcase vapors before they are introduced back to the intake manifold.

Simpler fuel systems, like a conventional carburetor, typically do not have electronic feedback monitors for fuel ratio, so a simple round pleated filter cap will do.


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[DENALIHD66]

Okay wait a minute.  Is the valve cover pulling in air, or allowing air back out?  What's the point of a catch can then if its only pulling in air?  I guess I'm just trying to understand how exactly it works and what it does.

[swathdiver]

1 hour ago, DENALIHD66 said:

Okay wait a minute.  Is the valve cover pulling in air, or allowing air back out?  What's the point of a catch can then if its only pulling in air?  I guess I'm just trying to understand how exactly it works and what it does.

My experience has been that both valve cover vents (whichever kind employed) direct blowby out of the crankcase. 

 

From the 2007 Suburban Restoration Kit:

 

Crankcase Ventilation System Description

A closed crankcase ventilation system is used in order to provide a more complete scavenging of the
crankcase vapors. Fresh air from the throttle body is supplied to the crankcase, mixed with blow-by
gases, and then passed through a crankcase ventilation valve into the intake manifold.
The primary control is through the crankcase ventilation valve which meters the flow at a rate depending
on manifold vacuum. To maintain idle quality, the crankcase ventilation valve restricts the flow when
intake manifold vacuum is high. If abnormal operating conditions arise, the system is designed to allow
excessive amounts of blow-by gases to back flow through the crankcase vent tube into the engine air inlet
to be consumed by normal combustion.

Filtered fresh air is routed from up-stream of the throttle blade to the front of the right rocker arm cover via ~
a formed rubber hose. To reduce the potential of oil pullover into the throttle bore area due to back flow of
the ventilation system, the fitting in the right rocker arm cover is shielded from the rocker arms. From
there fresh air and gases are routed through the crankcase and up to the opposite rocker arm cover
where the positive crankcase ventilation (PCV) valve is located. Gases are then routed through a hose to
the intake manifold.

[DENALIHD66]

Seems more complex than I figured.  But of course, everything seems regulated/controlled electronically anyway depending on conditions.  My initial thought was, put breathers on the crankcase vents, so that they are not pulling air from the intake tube that was destined to be sucked into the throttle body, therefore more power?  Obviously the intake tube holes would then be sealed over.

[Old School Mechanic]

The two tubes from the CAI are to allow filtered air into the engine. Vent. It's not pushing air from the engine into the CAI.

Engine vacuum then pulls the air (and other gasses/oil [blow-by]) out through the pcv valve.

This stuff is then sucked into the cylinders through the intake valves and burned.

Since direct fuel injection sprays fuel directly into the cylinders and not sucked in through the intake valve like a carburetor or throttle body injection the intake valves don't get "washed" off by the gasoline, therefore they get build up from blow-by from the pcv system.

A catch can could come in handy, reducing the amount of stuff that gets sucked back into the engine.

Hope this helps

[swathdiver]

On 7/28/2018 at 12:02 PM, DENALIHD66 said:

Seems more complex than I figured.  But of course, everything seems regulated/controlled electronically anyway depending on conditions.  My initial thought was, put breathers on the crankcase vents, so that they are not pulling air from the intake tube that was destined to be sucked into the throttle body, therefore more power?  Obviously the intake tube holes would then be sealed over.

I'm going to cap that off and put two K&N breathers on each side too.  

[DENALIHD66]

Is my theory right in saying that the intake would then pull in more air because it loses less vacuum from the other hoses?  And probably cleaner air overall going into the throttle body.

[swathdiver]

It'll be cleaner and cooler air with those capped off.

[Old School Mechanic]

On ‎7‎/‎29‎/‎2018 at 2:09 PM, swathdiver said:

I'm going to cap that off and put two K&N breathers on each side too.  

Yep, they done that with older engines for years...the trouble was not many serviced those "air filters",

which were more like a sponge material, the real old ones were in the oil filler cap, nobody changed those.

They became clogged and then the pcv wouldn't work right which caused crank case pressure then oil leaks.

Keep em clean

[swathdiver]

1 hour ago, Old School Mechanic said:

Yep, they done that with older engines for years...the trouble was not many serviced those "air filters",

which were more like a sponge material, the real old ones were in the oil filler cap, nobody changed those.

They became clogged and then the pcv wouldn't work right which caused crank case pressure then oil leaks.

Keep em clean

And the ones older than those just vented to the ground, no filters!