[dataway]

Anyone know if the typical 68-72 breather tube from valve cover to air cleaner base is the same for both 2bbl and 4bbl applications? I know the 2bbl air cleaner is just a tad smaller in diameter ... probably not enough to require a different tube.

I've got a 2bbl tube lying around I want to use on my 68 GTO.

[poncho447]

Same tube, you are good to go.

[hotcharity95]

what affect does it have to use an aftermarket air cleaner and have no tube, just a plugged valve cover?

[dataway]

Originally Posted by poncho447

Same tube, you are good to go.

Thank you.

[salem1912]

Originally Posted by hotcharity95

what affect does it have to use an aftermarket air cleaner and have no tube, just a plugged valve cover?

If you plug the valve cover you will have no crankcase ventilation on hard acceleration.

[hotcharity95]

A breather on the other side, instead of a fill cap, should do the trick right? The PCV to carb is intact also.

[dataway]

I think, technically, the crankcase is supposed to be under a slight vacuum. Installing a breather cap instead of an oil filler means the PVC can pull from outside air, rather than the slight negative pressure inside the air cleaner base.
Although ... millions of people use valve cover breathers and I don't think it's a problem.

[Kenth]

From 1968-on all vehicles uses closed PCV venting crankcase inside air cleaner.

Nice feature to reduce crankcase fumes into the drivers area.

And, i hope you´re having atmospheric pressure inside the air cleaner.

[dataway]

There has got to be a tiny bit of negative pressure in the air cleaner base ... assuming the engine is running and pulling air through the snorkel ... in order to move air, there must be a area of higher pressure and an area of lower pressure. Higher pressure is ambient, lower pressure is in the air cleaner base created by the engine pulling air into the carb. Carb suction creates negative pressure in the air cleaner, air cleaner pulls in air through the snorkel.

[Bruce Meyer]

Originally Posted by dataway

There has got to be a tiny bit of negative pressure in the air cleaner base ... assuming the engine is running and pulling air through the snorkel ... in order to move air, there must be a area of higher pressure and an area of lower pressure. Higher pressure is ambient, lower pressure is in the air cleaner base created by the engine pulling air into the carb. Carb suction creates negative pressure in the air cleaner, air cleaner pulls in air through the snorkel.

The negative pressure comes from the PCV valve that's hooked to direct manifold vacume. All the tube does is let air in. A breather does the same thing.

[Kenth]

VACUUM

Definition: The absence or reduction of air pressure. Vacuum is created in the intake manifold by the pumping action of the pistons. Air is pulled out of the manifold into the cylinders faster than it can be replenished by air bypassing the throttle plate. The throttle creates a restriction that allows vacuum to buildup inside the manifold. This is necessary to help pull fuel through a carburetor.

Here is a simple description of the principle, worth pondering, that should make things a little clearer.
The engine is an air pump - it ingests air and compresses it.
It´s often assumed the reason air enters the cylinders is because the piston sucks it in - it creates vacuum and that pulls in the air.
But, atmospheric pressure - about 15 psi, will push air into a void, provided its passage is non-restrictive.
When a carburetor is large and its throttle is wide open, atmospheric pressure helps fill the low-pressure area above a piston on the intake stroke.
Close the throttle to a small opening, and atmospheric pressure forces a partial charge of air past the restriction. The result is less-dense air in the consuming cylinder, therefore less power.
Consequently, a small venturi and throttle opening restrict the amount of air atmospheric pressure can push to the cylinder.
For most street and highway driving, a small venturi carburetor is most efficient.
If your goal is maximum power, however, and low- and mid-range economy, driveability, and torque are secondary, then think "big".
Think "big" all the way.
Bigger intake ports, a higher-lift camshaft with longer duration, bigger intake ports and an efficient exhaust system to properly discharge burned mixture.
Restrictions in the induction path keep atmospheric pressure from driving in a full air charge, so the engine wil not perform to full potential.
The more air that atmospheric pressure can force into the combustion chamber, the more horsepower.
From "Rochester Carburetors" by Doug Roe

FWIW
__________________

[dataway]

My point was, compared to an open breather in the valve cover, the breather tube sees slightly less than ambient pressure because it's connected to the air cleaner base, which has slightly less than ambient pressure since the carb is drawing air from the air cleaner housing.
At wide open throttle the air cleaner housing sees considerably less than ambient pressure as air is being drawn in through the restriction of the air cleaner snorkel. Which means a breather tube connected to the air cleaner will attempt to maintain more vacuum in the crankcase than would a open to atmosphere valve cover breather.

Since the crankcase should operate at a slight negative pressure, a valve cover type breather that is open to atmosphere will pass far more air than the PCV valve can pull and the crankcase will NOT have a negative pressure. It WILL be ventilated very well, but it won't be able to maintain any negative pressure in the crankcase.

The advantage of a valve cover breather is at high rpm, when the blow by gasses are far more than can be handled by the PCV valve, the valve cover breather will allow for the escape of those gases rather than blowing oil past the valve cover gaskets, rear main seal, distributor gasket etc. But since most engines spend little time at WOT typically the breather tube provides sufficient ventilation ... flowing from the crankcase the air cleaner at high rpm, and flowing from the air cleaner to the crankcase at low rpm.

[Kenth]

A picture can tell more than a thousand words,

[dataway]

Exactly

[salem1912]

Right, when on heavy throttle PCV stops working and the negative pressure in the air cleaner will then draw from the breather side so there is always a vacuum in the crankcase.

[Kenth]

There is NO vacuum in aircleaner or crankcase since they are vented to atmosphere at all times without restrictions. With engine running vacuum is present in intake only (unless throttles are wide open on heavy load).

What you see is atmospheric pressure rushing into the void under the partially closed throttle plates. This atmospheric pressure pushes fuel through the carburetor circuits and crankcase fumes from the CC vent tube for combustion.

HTH

[dataway]

Yes there is negative pressure in the air cleaner. Negative pressure is anything below ambient. In order for air to be drawn through the air cleaner snorkel, there MUST be lower than ambient pressure in the air cleaner. And yes there is negative pressure in the crankcase at low rpm. The PCV attempts to pull a vacuum on the crankcase, a small negative pressure is created in the crankcase which then pulls air from either the valve cover breather tube, or a valve cover breather open to atmosphere ... air does NOT move unless it sees a pressure differential. You cannot have ventilation of any kind without a pressure differential.

I think there is a confusion of terms here. Any pressure level less than ambient pressure can be considered a degree of vacuum.

It's as simple as putting your hand over the intake snorkel of an air cleaner while the engine is running ... but leave a space for air to enter .... you can feel the vacuum develop... yes the snorkel is still open to ambient pressure, yet the air cleaner develops a negative pressure inside. The more your restrict the opening, the more negative pressure develops in the air cleaner as the carb attempts to evacuate the air from the air cleaner.

Since the snorkel is designed to smooth and shape air flow, it does provide a restriction to the air that enters the air cleaner, thereby creating a slight negative pressure in the air cleaner. At high rpm this negative pressure in the air cleaner increases as the throttle plates open, drawing more air, but the snorkel opening remains a fixed size.

At high rpm when crankcase pressure tries to go positive, and the air cleaner negative pressure is at a high point, it will help evacuate gases from the crankcase through the breather tube.

[Kenth]

Crankcase Ventilation Controls.
Draft Tube System.
Any piston engine creates blowby (para 3-6) as it operates. The
pressure created in the crankcase by blowby must be
relieved by venting.
a. Draft Tube System (Fig. 7-6). Older vehicles
used a very simple system that vented blowby to the
atmosphere through a draft tube. The draft tube extends
from an area of the crankcase that is above oil level to a
point of exit that projects straight downward under the
vehicle. The outlet of the tube Is cut on a slant upward
toward the rear of the vehicle. With this shape outlet, a
suction Is created by the forward
movement of the vehicle. Circulation of fresh air will
occur In the crankcase with the addition of a breather cap
also located at a point on the crankcase above oil level.
(1) The negative pressure created at the end of
the draft tube will cause air to be drawn In through the
crankcase breather. A wire mesh filter is built into the
breather to keep dirt out of the crankcase.
(2) The draft tube contains a sediment
chamber and a wire mesh filter at the point where it
attaches to the crankcase. Its purpose is to trap any oil
that tries to leave through the draft tube and return it to
the crankcase.
(3) By strategic location of the breather cap and
draft tube and the use of baffles, a complete purging of
crankcase blowby fumes is ensured.
The draft tube system is obsolete now because it
discharged excessive hydrocarbon emissions directly into
the atmosphere. It also did not keep the crankcase as
clean as the positive crankcase ventilation system
described in the next subparagraph. This is because it
relied on the movement of the vehicle to activate it. As a
result of this, draft tube-equipped engines were very prone to
sludge build up.

Positive Crankcase Ventilation (PCV) System
The positive crankcase ventilation system
utilizes manifold vacuum to purge the crankcase of blowby fumes.
The fumes are then
aspirated back into the engine where they are reburned.
(1) A hose is tapped into the crankcase at a
point that is well above the engine oil level. The other
end of the hose is tapped into the intake manifold or the
base of the carburetor. It should be noted that if the hose
is tapped into the carburetor base, it will be in a location
that is between the throttle valves and the intake manifold
so that it will receive manifold vacuum.
(2) An inlet breather is installed on the
crankcase in a location that is well above the level of the
engine oil. The inlet breather also is located strategically
to ensure complete purging of the crankcase by fresh air.
(3) The areas of the crankcase where the
vacuum hose and the inlet breather are tapped have
baffles to keep the motor oil from leaving the crankcase.
(4) A flow control valve is installed in the line
that connects the crankcase to the manifold vacuum. It
is called a positive crankcase ventilation (PCV) valve and
serves to avoid the air-fuel mixture by doing the following:
(a) Any period of large throttle opening will
be accompanied by heavy engine loads. Crankcase
blowby will be at its maximum during heavy engine loads.
The PCV valve will react to the small amount of manifold
vacuum that also is present during heavy engine loading
by opening fully through the force of its control valve
spring. In this way, the system provides maximum
effectiveness during maximum blowby periods.
(b) Any period of small throttle opening will
be accompanied by small engine loads, high manifold
vacuum, and a minimum amount of crankcase blowby.
During these periods, the high manifold vacuum will pull
the PCV valve to its position of minimum opening. This
is Important to prevent an excessively lean air-fuel
mixture.
(c) In the event of engine backfire (flame
traveling back through the intake manifold), the
reverse pressure will push the rear shoulder of the
control valve against the
valve body. This will seal the crankcase from the
backfire, which could otherwise cause an explosion.
(5) The positive crankcase ventilation system
can be the open or the closed type.
(a) The open type has an inlet breather that
is open to the atmosphere. When this system is used, it
is possible for a portion of the crankcase blowby to
escape through the breather whenever the engine is
under a sustained heavy load. This is unacceptable on
later automotive equipment and, as a result, the system
is no longer used.
(b) The closed type has a sealed breather
that is connected to the air filter by a hose. Any blowby
gases that escape from the breather when this system
is used will be aspirated into the carburetor and burned.
This Is the system that is currently used.

[dataway]

Yep, all that agrees with what I am saying.

The key thing to remember is air NEVER flows without a difference in pressure to create flow. Whether the difference is ambient to vacuum, or higher pressure to ambient. It always requires a difference in pressure to move air. ANY place that air flows, there will be a higher pressure at the source of the flow, and a lower pressure at the end of the flow.

So crankcase gases vented to the air cleaner .... which is at a slight negative pressure compared to ambient atmosphere will maintain a slightly lower than ambient pressure in the crankcase on a closed system compared to the older open systems at low rpm. At high rpm the crankcase almost always has a higher pressure than ambient unless you have some kind of additional evacuation system ... which some race cars do that run extended periods at high rpm.

Back in the day they would plumb the valve cover breathers to evacuation fittings in the exhaust .... and yes the right type of fitting in the exhaust will pull out crankcase gases during the negative exhaust pressure pulses.

F1 cars go so far as to try to pull high vacuums on the crankcase so pistons on the down stroke only have to push "rarified" air.

I'm not sure if you are understanding the point I'm making. A crankcase vented to the air cleaner in a closed system will maintain a lower pressure in the crankcase because the "Ambient" pressure it sees is air cleaner pressure .... which is at a slightly lower pressure than "open to atmosphere". The air cleaner HAS to be at a lower pressure than outside air, or air would never move into the air cleaner.

All this assumes a normal engine. A worn engine will produce so much blow by that the PCV can never draw it all out, and it will always have a pressurized crankcase ... and it will have mung and oil flowing out of oil filler cap, the breather tube, the air cleaner will be full of oil residue and the valve covers will probably leak.

[1968GTO421]

When using an open element aftermarket air cleaner, most have a cut-out in the base for the crankcase ventilation tube/hose. Here is a link to the item that can affix the breather tube/hose to the air cleaner base:


http://www.edelbrock.com/automotive/...tilation.shtml



Hence one can go with a stock type set-up or use an open breather in the valve cover.