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Good ‘engine breathing’ is usually associated with efficient intake systems e.g. high
flow air filter, a well designed manifold, etc. However, efficient ‘crankcase breathing’
is an equally important function of any engine whether Ford or not. Even in a new engine, the combustion
pressure will inevitably pass the piston rings into the crankcase. If an engine’s
breathing system should become blocked or restricted, the crankcase will pressurise causing
any one or more of the following problems:
- The oil/air mix will force its way out through any other convenient exit e.g. oil seals, dip stick, filler cap, etc.
- The efficiency of the oil control rings will be reduced creating increased oil consumption.
- Impurities such as water vapour and acids (by products of combustion) will build up and contaminate the oil causing sludging and increased engine wear.
- The adverse affect on the air/fuel mixture will result in starting problems and rough idling conditions.
- As a consequence of the weakened fuel charge, detonation or ‘pinking’ will ensue. To compensate, the ignition will need retarding resulting in further power loss.
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Prior to 1963 most vehicle engines vented their vapours and oil deposits
to atmosphere and the road surface! With increasing environmental pressures
Positive Crankshaft Ventilation was introduced whereby the crankcase vapours
were drawn up into the inlet manifold and, along with the air/fuel mixture,
burned up in the combustion chambers. To enable this system to work safely
and efficiently the ventilation from the crankcase is controlled via a PCV valve.
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To avoid upsetting the fuel/air mixture, the PCV valve must regulate the
evacuation of these blow-by gases and vapours (which will be minimal at
idling speed but will intensify as engine speed is increased). Since manifold
vacuum is highest at low engine speeds, the PCV plunger will be drawn forward
to a position that will restrict crankcase ventilation to a minimum thus
ensuring no unsettlement of the air/fuel mixture. As engine speeds are increased
the manifold vacuum will drop thus reducing the ‘pull’ on the plunger which will
slide back to a midway position allowing a greater flow rate from the crankcase.
Since the engine demands more air/fuel mixture at high engine speeds, the
escalation of crankcase vapours into the combustion chambers should not affect
performance.
The PCV valve also acts as a flame trap. In the event of a backfire,
the resulting pressure through the inlet manifold will force the plunger back into
the closed position, thus preventing an explosion of the vapours in the crankcase.
Various PCV systems are in use but they all function in essentially the same way.
Earlier systems were known as ‘open’ systems that still allowed some vapours to vent
to atmosphere via the filler cap. ‘Closed’ PCV systems have been the norm for some
time now, whereby the filler caps are not vented and air is recirculated via the air
filter. Left unchecked over a period of time a PCV system will deteriorate and may
cause major engine problems as outlined above. Regular maintenance is essential with
some manufacturers recommending the renewal of the PCV valve at every major service
interval.
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For all moderate stages of engine tune, the standard PCV system should cope
with the increase in engine power whilst continuing to control the emissions
from the crankcase. However, even on a fairly new car, the system should be
thoroughly checked and any suspect valves, hoses, etc. replaced. It must also
be borne in mind that, on all management controlled engines, any alteration
to the system may upset the sensor readings and thus create further problems
(including MOT test failure on emission levels!).
For most motorsport applications and the more radical stages of engine tune,
alternative provisions for engine ventilation will almost certainly have to
be made. With higher combustion pressures, higher oil pressures and higher
engine speeds, the demand for adequate crankcase ventilation will also be high.
This situation is further aggravated by the radical cam profiles used, which
will drastically reduce the available vacuum required to purge the crankcase.
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However, before you rush out to buy the biggest size breather pipe kit you can lay your
hands on, many other factors need to be taken into account and the following points should
be observed:
1) On ‘wet sump’ engines the sump must never be overfilled and it should be properly baffled
to minimise oil surge. If the crank and rods are allowed to plunge through an oil bath at
every revolution, apart from the drag and power loss factor, it will also create an even
greater volume of oil spray to contend with. This will result in oil loss through the breather
system and also past the oil control rings, the latter causing further problems e.g. plug
fouling, power loss, etc.
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2) Any filler or breather aperture should be baffled, especially if it is above
or adjacent to rotating parts. As an example many filler caps on OHC engines are
directly above the camshaft lobes which, when rotating at speed, will flick the
oil with such force that a considerable amount can be lost up the breather pipe.
N.B. Always consider this factor when deciding where to drill a cam / valve
cover to locate a breather take-off union. |
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3) If the crankcase is to be vented via the inlet manifold this should only be considered
where a mixing (plenum) chamber exists. Under no circumstances should any type of breather
union be connected to a manifold port dedicated to a single cylinder. Breather unions can
also be connected to an air box but this may exacerbate filter clogging and necessitate
regular cleaning of the filter(s). Small replacement ‘K&N’ type performance filters
(carburettor models) are not suitable for this type of conversion. For optimum efficiency
a PCV valve should be fitted.
To eliminate any charge contamination and subsequent power loss, most highly modified
engines should vent via an isolated catch tank, which will also act as a collector for
any oil lost. These tanks should have a minimum 1 litre capacity, 2 top inlet connections
(1 crankcase vent and 1 valve/cam cover vent), a sight gauge (to indicate the level of
any oil inside) and a bottom plug or tap to allow the oil to be drained off when necessary.
To avoid frequent inspection and draining of the oil level in the catch tank, an automatic
drain back into the sump can be improvised as shown. The vent outlet can be recirculated
through the intake system or left to vent to atmosphere via a suitable filter, the latter
being the more popular option. On dry sump systems, the
scavenging action of the pump should evacuate any excess blow by gases in the crankcase
and, in an ideal situation, maintain pressures at or below 2 inches of water. Depending
on the practicalities of individual engine types and installations, both ‘open’ and ‘closed’
systems can be adopted with some tuners preferring the closed system. Providing a closed
system (incorporating a PCV or similar check valve between the engine and oil tank) can be
seen to function efficiently, it can offer added benefits. The closed system allows the
scavenge pump to reduce crankcase pressures to a minimum, in some cases as low as zero or
even a slight vacuum. In such situations a small bhp gain is achieved by eliminating
combustion chamber contamination and reducing any residual oil drag (clinging to crank,
rods, etc.) to a bare minimum.
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