Reason for PCV Systems
Internal combustion engines always produce crankcase fumes.
primarily come from combustion byproducts leaking past rings. Secondarily, they
come from other smaller-area lower-pressure leakage paths, such as leakage
through valve guides or gasket leakage. Crankcase fumes are rich in acidic,
dirty, combustion byproducts. These combustion byproducts, or fumes, are not
good for engines. They prematurely contaminate motor
oil. Crankcase fumes also condense into a hard varnish, coating parts inside the engine
with a hard, dark, coating.
Keeping combustion byproducts (crankcase fumes) out of the crankcase
greatly extends oil
and engine life. Engine oil remains cleaner longer, and less varnish forms
inside the engine.
It is impossible to increase ring and valve stem sealing enough to prevent combustion leakage
into the crankcase and valve cover system. The only removal option is to displace
fumes with fresh, clean, air. We call this crankcase ventilation.
In modern engines, crankcase ventilation is almost always accomplished by
at one end of the
crankcase. At the other end of the crankcase system, away from the vacuum source, the
ventilation system allows clean filtered air in. The clean, filtered, air replaces the harmful
fumes as the harmful fumes are drawn into the vacuum source.
It is important to replace crankcase fumes with clean air. Some supercharger kits
like my original Vortech kit, and
many other high performance
modifications, allow unfiltered air to enter the crankcase. Worse yet, some
systems completely remove vacuum-forced
ventilation, allowing harmful, corrosive, byproducts to collect in the oil, valve covers, lifter galley, and crankcase. When this
happens, engine life deteriorates. Engine internals can even become coated with a hard
History of PCV Systems
Road Draft PCV Systems
PCV systems started years ago with road draft systems. In a road draft
system, a tube from the upper engine area hangs down near the road. Air moving
across the open tube end draws fumes out of the crankcase. That air is replaced
fresh air, typically brought in through coarsely-filtered valve covers ‘breathers”. In the 50’s and 60’s, it wasn’t unusual to see oil-burning cars
driving along, merrily streaming blue smoke out their below-car breather tubes.
They would often have as much bluish-white smoke drifting out from their draft
tubes as was streaming out their tailpipes.
Draft tube PCV systems required the vehicle be in motion, and still provided
very limited air exchange. The result of such poor air exchange was rapid oil contamination and build-up
of hard brownish-colored varnish on engine internals. It was fortunate when
a road-draft PCV system car lasted 50,000 miles without burning oil, and most
older engines (even with regular oil changes) had varnish inside.
Modern PCV Systems
The modern PCV system is a “winning system”. The PCV system does
not decrease horsepower or economy, while it keeps the
environment, engine internal parts, and engine oil clean. The PCV system
is very reliable, often lasting a hundred thousand miles or more without
maintenance. The only parts to fail are the PCV check valve, hoses, and
Modern systems commonly use engine vacuum to draw contaminated air into the intake
system. These byproducts, rich in hydrocarbons, are mixed with and burned with
the regular air and fuel. The typical American V8 evacuation path is out through
a one-way PCV valve at the lifter galley rear. The check valve prevents
air-gasoline fumes from back-flowing into the crankcase when the engine is
stopped, and the valve also acts as a flame arrestor or “backstop” in the event
of a backfire. The check valve is usually a small steel ball in a housing, with
gravity often forming the “closing spring”. This check ball does not
regulate pressure, it just blocks any reverse flow.
Gasses (hopefully no oil) are drawn out of the engine through the PCV
valve by intake manifold vacuum. This causes a vacuum in the entire crankcase area,
including valve covers. To replace gasses lost through the
vacuuming, clean filtered air is drawn into the engine through the valve covers.
This air normally
comes from the regular carburetor or fuel injection system air filter. The regular air
filter is a very high quality air filter, much better than older standard
breather filters with their coarsely-woven metallic fiber filters used in road
Since PCV systems usually depend on manifold vacuum, supercharged or turbocharged
engines create a unique problem. In supercharged or turbocharged engines, the
intake manifold operates at positive pressure during open throttle conditions.
When pressurized or under boost, the intake tries to force air
backwards into the crankcase. Compounding the problem, boosted engines
produce more ring, gasket, and seal blow-by. All of this works to build
crankcase pressure. This can push oil out of engine seals or tubes.
Many modern racers use a variation of the old road draft system, using exhaust
collector vacuum to pull air out of the engine. Most of these systems don’t even
provide a filtered air inlet, so they are essentially nothing more than a
crankcase pressure vent. Without a clean air inlet, they are one step down from
old road-draft PCV systems.
Drag Racing PCV Systems
Racers sometimes use a tap
into the side of a header collector to pull a vacuum. This vacuum would be
similar to a sprayer siphon system, where a large volume of cross-flowing liquid
pulls a small amount of liquid through a siphon tube. Certain paint guns use the
same principle, where a
cross-flowing air through a
venturi draws paint
up into the air stream. Carburetors function in a similar manner.
In tests here, little vacuum
could be created. The vacuum was so low that any backpressure easily overcame
the draft. With 500 CFM side draft flow at 0″ SP in a mocked up 3″ collector
system, less than 2″ negative Hg/in vacuum was created. This was with optimum
hole sizes, siphon depth, and siphon tube angle. There was insignificant vacuum
at low exhaust flow rates..
Also, as a general rule, these
systems lack a clean air inlet. Even the crude road draft systems of the 1950’s
and earlier had filtered air crankcase inlets. Without a clean air inlet,
crankcase contaminants are not purged.
My PCV System for Supercharged 363 Ford Engine
I mostly drive my supercharged Mustang on the street. To keep oil clean and
moisture-free, maximizing engine life, I wanted a modern PCV system. I not only
planned a PCV system that works, I also learned what did not work for me!
PCV System Design Background
A typical non-boosted (no supercharger or turbo charger) PCV system always
has some vacuum on the intake manifold plenum. A restricted portion of intake
vacuum, which often runs
-15Hg to -20 Hg, is used to draw engine crankcase air out of the crankcase. The
pulls crankcase fumes out of the crankcase through a check valve. Clean
filtered air flows into the crankcase, generally through the valve cover or
valve covers. This fresh air prevents fumes and moisture from contaminating oil
and damaging internal parts. When
an engine has boost, or if an engine has worn piston rings, the crankcase tends
to pressurize. This crankcase pressure must be vented or oil can blow out seals.
pressure, while still allowing non-boost vacuum to pull clean air into the crankcase,
requires a little planning. (It took me a while to get this right.)
Some car builders use vacuum pumps to pull a vacuum, but I didn’t want to do
that. A vacuum pump can be
expensive, requires wiring, and it also takes up space. A vacuum pump decreases system reliability, and
if rings are not sealing or a gasket is bad, a pump
system might not keep up with crankcase pressurize under open throttle
I wanted to purge crankcase gasses with “free” engine vacuum, while venting any positive crankcase pressure to air without restriction whenever positive
crankcase pressure appears. My first try used inexpensive vacuum
check valves, one or two inexpensive plastic Wal-Mart Fram fuel filters, and some standard vacuum
hoses and fittings. The only machine work was adding a single hole to an
unvented valve cover, and tapping the hole for a standard 3/8 inch barbed hose
My Initial Experiment
Someone described, in a conversation on a Mustang Corral forum, venting his PCV
system into his supercharger inlet air. That seemed reasonable to me, because
the supercharger’s inlet pipe is always at some vacuum.
I decided to try that system.
This simple system, suggested on the Corral, used blower inlet vacuum to draw
from the PCV valve. Filtered air went into the normal valve cover vent location
through a small fuel filter that was open to the air. The fuel filter cleans the
air into the valve cover, and catches oil going out.
Using a negative air inlet pressure point, such as the supercharger inlet,
sounded good at first. On my car this method had two very significant problems:
1.) At low speeds or idle, there is very little vacuum at the supercharger
measured less than -0.5″ Hg pressure at my supercharger inlet at cruise, and
even less at idle. This connection point did not have enough vacuum to purge
crankcase gasses at slow speeds or idle.
2.) At open throttle, crankcase pressure can push oil out the vent line into
the supercharger inlet. That’s bad, because
fuel octane. Oil in the intake system makes a mess and, in sufficient
quantity, can actually hurt the engine.
After running this system for a few
hundred miles, my SC inlet was wet with oil. I decided to
not use this system.
First Solution Experiment
(This might work in some systems. It did not work long term in mine. )
I decided to use fuel filters to filter air. Fuel filters are cheap, and have
very fine filtering elements. They also fit vacuum hoses and are easy to mount.
Clear filters are readily available, allowing visual inspection for water, oil,
or dirt. The downside is, if an engine has blow-by, fuel filters will very
quickly load up with oil.
Since a fuel filter is very fine filtering, oil contamination will significantly
reduce air flow through the filter. Watching the filters for oil contamination
is a necessary part of maintenance, and the clear plastic fuel filters make that
I planned on an oil separator on the outlet lines to catch any oil prior to
the bi-directional inlet/outlet filter. This also turned out
to not work well over time. These small fuel filters plug too easily with oil,
and require constant cleaning with gasoline or solvents to restore operation.
I had to wash them out with gasoline or some solvent regularly.
In the picture below, the vacuum line comes from the TFS intake plenum. The
plenum pressure in my 363 cu in engine ranges from -25″Hg to +15″ Hg under
boost. A check
valve allows PCV system airflow only in one direction, with crankcase air drawn out of
the right side valve cover near number one cylinder to the
intake plenum. The filter by
the valve cover is optional, and I eventually replaced it with a second check
valve. I used that filter mostly because it was already hanging there, I
wanted some intentional vacuum restriction, and I wanted to see how much oil
comes out that way under non-boosted conditions. I’m going to try a second,
redundant, check valve located where the filter is.
If vacuum isn’t restricted a little, and if the oil fill cap is opened, engine
idle will be affected. I was initially worried about the unmetered air, but my
concern turned out to be unfounded. Airflow through the crankcase is so small
compared to metered air through the normal path to the MAF that the unmetered
air turns out to be of no concern at all.
I found two different types of check valves at the local junk yard.
They were used on power brake systems in little foreign trucks and cars. I think
they were Toyotas and Nissans (they all look alike to me). In the foreign cars
and trucks, these check valves isolate the power brake vacuum reservoir
from the engine vacuum source. American cars have them, but they are normally
integrated as the hose fitting on the power brake booster canister. The foreign car
and truck valves are double-ended with hose fittings. This makes them
ideal. You should be able to blow
through the correct part in just one direction, as indicated by the arrows. I bought ten of
these for $5 at a U-pull-it yard.
One word of caution! Do not use a normal PCV valve at the rear of the intake manifold. To get
proper crankcase (lifter galley) airflow, use a non-directional coupling. Use
an open adaptor that fits the PCV grommet, like the fitting pictured below. You
should be able to see right through the proper PCV valve substitute:
The picture below shows the bidirectional outlet and inlet system. This
outlet sums both the VC vent and intake vent through a T connector. It has to flow freely in both directions.
This system connects to the
crankcase vent (intake manifold PCV valve grommet) and the left side valve cover.
The fuel filter is critical, since it cleans crankcase inlet air. If your
engine blows a lot of oil, you will probably want to use an oil separator after
the T and before before
the filter. So far, my engine does not need one. I haven’t found any oil in this
filter, even after runs at 15 lbs of boost.
I added this valve cover tap to be sure clean air entered the valve cover. I
placed the tap between the front two cylinders, where rocker arm oil would not
splash and where there was maximum clearance. It is a standard brass 90-degree
barbed hose fitting for 7/16″ hose. Even though this fitting is located in a
clear spot between cylinders 5 and 6, I shortened the pipe thread end so the
fitting thread protrudes less than 1/4 inch inside the valve cover. The
vent can be run to the SC air inlet, through an oil separator if you are blowing
oil. If your engine does blow oil, you will want to separate the oil and air
BEFORE any filters.
Front view of atmosphere vent side of PCV system, below:
Keeping the gas filters clean was a real nuisance. I also did not like the small
amount of oil pulled into the intake system over time.
After a few months of running, my system evolved into this one. My manifold
vacuum comes through two check valves (to prevent ever accidentally pressurizing the valve
cover). This system pulls full manifold vacuum on the right side valve cover. The manifold
vacuum is pulled in the normal spot high on the oil fill tube, so the vacuum
does not draw excessive oil. This is the ONLY crankcase vacuum port. The
manifold vacuum line runs directly to the manifold vacuum port through
foreign-car power-brake check-valves retrieved from a local junk yard.
Needing an inlet for vacuum and an outlet for boost, I allowed
bi-directional airflow into and out of the crankcase at the PCV valve location
and at the LH valve cover. First the valve cover:
A second inlet comes from a bidirectional fitting in the old PCV valve location.
This is now crankcase air inlet and outlet:
Finally, the SC air inlet is fed from a dry port (high position) on an oil
separator. The LH valve cover and crankcase bi-directional flows go to the wet
ports of my oil separator. Since the wet ports are at the very bottom of the
canister, if any oil collects, the oil eventually draws back into the LH VC or
crankcase lines by crankcase vacuum. The dry port goes to the filtered air at
the SC inlet. The dry port is a barbed fitting up high on the separator
can, while the wet ports have siphon tubes extending
below baffles to the very bottom of the catch can. This is so the wet ports can
suck any collected oil back into the engine.
SC inlet line is bi-directional after the oil separator (dry port) for clean air inlet and any crankcase boost
pressure outlet. The SC air inlet line is not labeled, but it is the upper black
hose. It goes from the SC elbow to the dry port on my oil separator.
Schematic of final system:
materials © 2013 W8JI