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It certainly is easy to get confused about crankcase ventilation, not only from the present discussion, but also an interesting (and now since removed) exchange of views on FB recently.
Basically there are four types of crankcase ventilation systems for Mini. A description of each follows, but we must first remember what the ventilation system is for. As we all know, the seal of the piston rings is not perfect, and during compression and combustion strokes, there is blowby from the combustion chamber into the crankcase. It is thought that some 80% of this blowby is unburnt fuel, the remainder being products of combustion – a large part of which is water. If these gases and water vapour were to remain in the crankcase, they would form sludge with the oil which is of course undesirable, not to mention the presence of corrosive compounds. The engine ventilation system is supposed to circulate a continuous stream of fresh air through the crankcase so as to remove the blowby gases before they have time to do any damage.
Why I say all this is because I often see people just dumping their ventilation pipes to a catch can. Perhaps in a race car it might be OK so as to maximise power output, but for a road going car which probably doesn’t even get to operating temperature half the time, this practice will eventually lead to many problems.
The first method of crankcase ventilation (used by many cars of the early 60s) was to have a vented oil filler cap and what is called a “draft tube” which was connected to the crankcase and pointed downwards into the under-car air stream so that passage of the car would create a suction effect thus drawing fresh air though the cap, through the crankcase, and out the tube. Effective, but hardly environmentally friendly.
The next method (such as that found on an 850 or early deluxe and Cooper S) is similar in operation but the air goes the other way. The oil cap is sealed. A pipe joins the rocker cover to the air cleaner intake, and there is a tube in the engine side plate. This tube has all the appearance of being crimped off, but in fact, there is a very small gap in the end of the crimp. Air is drawn through the crimped end, through the crankcase, and out the rocker cover into the air cleaner. This also works quite well, but is unregulated – so that much the same amount of ventilation occurs at idle as compared to full throttle. In this system, removing the oil filter cap shouldn’t make any different to the idle rpm. The oil cap is plastic, (steel on very early cars) short neck, and non-vented.
The third type of system is one which involves the PCV diaphragm valve which can be found on a MKII Cooper S (not sure if any MKI’s had it). In this system, there is a vented filler cap and a breather separator in the timing gear cover. Air travels into the rocker cover via the vented cap, through the crankcase, through the oil separator in the breather, up to the control valve, and then into the inlet manifold. The control valve is needed because (unlike the air cleaner depression), the vacuum level can be quite high. What is unusual about this valve compared to just an ordinary PCV valve is that it is controlled by crankcase pressure, or more correctly, crankcase vacuum. At idle, there is a certain amount of crankcase vacuum present due to the action of the PVC system as a whole, and the valve is designed to limit the amount of ventilation applied. A sort of a feedback loop. At higher engine loads, the blowby increases and so the vacuum falls, and the valve opens more to increase the ventilation effect to accommodate the extra blowby that needs to be handled. In this system, removing the oil filter cap should result in a slight increase in idle rpm. It should not stall. The filer cap has a long neck, and is vented – either through two holes in the top surface, or slots under the rim. Inside, there is a wire mesh filter. The valve can be dismantled to some extent and it is possible to get a new diaphragm.
The fourth type worthy of mentioning is found on MKII Cooper S, Clubman/Leyland Mini engines and involves a connection to a port on the carburetter. The ventilated gases from the crankcase are fed into a white plastic pipe connector at the carburetter body. This connector is positioned up stream of the throttle disc and downstream of the carburetter piston. Since the velocity of the air flow through the carburettor past this port depends on the throttle opening, then the resulting depression (similar in action to the original draft tube) serves to control the amount of crankcase ventilation depending on engine load. Air is drawn in at the rocker cover.
My apologies for the long lecture, but it is an interesting topic and these parts (rocker cover, cap, breather, etc) are often mixed up which is why things often go wrong.
Last edited by eightfifty on Sat Oct 14, 2023 4:18 am, edited 4 times in total.
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