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Engine - Breather System

The Basics

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:

  1. The oil/air mix will force its way out through any other convenient exit e.g. oil seals, dip stick, filler cap, etc.
  2. The efficiency of the oil control rings will be reduced creating increased oil consumption.
  3. Impurities such as water vapour and acids (by products of combustion) will build up and contaminate the oil causing sludging and increased engine wear.
  4. The adverse affect on the air/fuel mixture will result in starting problems and rough idling conditions.
  5. 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.

Positive Crankshaft Ventilation (PCV)

PCV Valve

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.

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.


Crankcase breather diagram

High Performance Engines

High performance breather diagram

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.

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.

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.

Rocker cover deflector plate

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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