PCV catch can capacity for second generation Hemi

[slo town]

Folks:

A good friend of mine inherited a project car. It's one off those sinister looking Chrysler Imperials from the mid to late '50s. It has a second generation Hemi in it.

My friend has a question about the crankcase ventilation system. The previous owner installed a catch can system. Both valve covers are vented to the catch can via 1/4" ID tubing. He has concerns that the lines are too small, which could result in positive crankcase pressure.

What do y'all think?

Scott

 

[Kira]

If crankcase vapors blow freely into the can which has a ventilated port atop -where the air filter is- then there'd be no pressure build-up.

How long has that mod been in place? With vapors being pushed out of the crankcase, I'd expect to see some evidence of oil exiting that filter.
I assume the fresh air intake for this system is still the vented oil filler cap.

 

[wpod]

There is a certain little known secred when fabbing something . The secret is copy what the manufactures use.

 

[slo town]

If crankcase vapors blow freely into the can which has a ventilated port atop -where the air filter is- then there'd be no pressure build-up.

How long has that mod been in place? With vapors being pushed out of the crankcase, I'd expect to see some evidence of oil exiting that filter.
I assume the fresh air intake for this system is still the vented oil filler cap.

You're saying the crankcase needs to have a source of outside air so it can breathe (inhale)? Makes sense, but I never thought about it before.

Scott

 

[slo town]

There is a certain little known secred when fabbing something . The secret is copy what the manufactures use.

Agree. This car was a nice find but the original builder is no long available to answer questions.

Scott

 

[JTK]

I never understood the ebay looking catch cans with the filter on top.

Do these have a check valve that only allows flow out through that filter during periods of high crankcase pressure?

 

[Kira]

You're saying the crankcase needs to have a source of outside air so it can breathe (inhale)?

Yes. The PCV valve in modern engines goes to a hose...the other end of which goes to the intake.
Engine vacuum sucks in the fumes.
That is the "positive" part of the name, "PCV". While the blowby gasses/fumes/vapors are created by combustion chamber pressure, they had been allowed to simply escape int the atmosphere. Today they're sucked into the intake system positively.

 

[slo town]

Yes. The PCV valve in modern engines goes to a hose...the other end of which goes to the intake.
Engine vacuum sucks in the fumes.
That is the "positive" part of the name, "PCV". While the blowby gasses/fumes/vapors are created by combustion chamber pressure, they had been allowed to simply escape int the atmosphere. Today they're sucked into the intake system positively.

Yes, I understand that, but are PCV systems designed such that they have to inhale clean air in addition to inhaling crankcase vapors and/or blow by? Is this necessary to prevent the crankcase from having too much vacuum and possibly inhaling outside air through the crankshaft main seals?

Scott

 

[Kira]

Excessively blowing oil out or sucking air in? what the PCV prevents?
Measuring these effects multiple ways is what designers/engineers are paid to do.

 

[OVERKILL]

Yes, I understand that, but are PCV systems designed such that they have to inhale clean air in addition to inhaling crankcase vapors and/or blow by? Is this necessary to prevent the crankcase from having too much vacuum and possibly inhaling outside air through the crankshaft main seals?

Scott

As Kira was explaining, with a traditional PCV system:

1. PCV valve, which is attached to manifold vacuum, is typically located somewhere (valley, valve cover...etc, there are several options), which only allows air OUT of the crankcase, and is setup to meter that flow, IE, it's very restricted at full manifold vacuum, while at moderate vacuum, will flow more.
2. Vent tube/breather, which is attached to the air inlet tubing from the airbox to the throttle body to allow air IN to the crankcase (behind the mass air meter so it's metered air if so equipped)

So, assuming an engine isn't worn out to the point that it's producing obscene amounts of blow-by, you get a flow-through effect where air enters the breather hose so the crankcase isn't under vacuum, while the fumes and vapours are drawn through the PCV valve and burned.

The system you are sharing with us is just a pure "vent to atmosphere" setup, with a catch can to hopefully intercept the droplets. 1/4" tubing does seem a bit small, per your query, but you do have two of them I'd probably upgrade it to 3/8th's hose just to be safe.

 

[slo town]

As Kira was explaining, with a traditional PCV system:

1. PCV valve, which is attached to manifold vacuum, is typically located somewhere (valley, valve cover...etc, there are several options), which only allows air OUT of the crankcase, and is setup to meter that flow, IE, it's very restricted at full manifold vacuum, while at moderate vacuum, will flow more.
2. Vent tube/breather, which is attached to the air inlet tubing from the airbox to the throttle body to allow air IN to the crankcase (behind the mass air meter so it's metered air if so equipped)

So, assuming an engine isn't worn out to the point that it's producing obscene amounts of blow-by, you get a flow-through effect where air enters the breather hose so the crankcase isn't under vacuum, while the fumes and vapours are drawn through the PCV valve and burned.

The system you are sharing with us is just a pure "vent to atmosphere" setup, with a catch can to hopefully intercept the droplets. 1/4" tubing does seem a bit small, per your query, but you do have two of them I'd probably upgrade it to 3/8th's hose just to be safe.

Thanks @OVERKILL and @Kira.

Scott

 

[Kantaro]

What size are the factory PCV hoses? My 3.0L I6 has 3/4" lines so I used the same for my catch can.

And restrictive catch cans can be an issue

 

[SHOZ]

Yes, I understand that, but are PCV systems designed such that they have to inhale clean air in addition to inhaling crankcase vapors and/or blow by? Is this necessary to prevent the crankcase from having too much vacuum and possibly inhaling outside air through the crankshaft main seals?

Scott

I ran my '97 V8 SHO with a catch can and sealed fresh air makeup. It developed about 5"Hg vacuum when the intake was in high vacuum. Seems the the PCV valve limited the vacuum as it never went over no matter what the intake vacuum. Made a wheezing noise at idle. No other issues.

My cat can were self draining with a spring loaded check valve that was kept closed under vacuum. When I shut off the engine or went WOT it would dump.