Catch cans and oil separators are simple devices designed to fight the potentially harmful effects of engine blow-by and sludge. Blow-by is the process of air, moisture and fuel being forced past the piston rings into the crankcase during the compression and power strokes of an internal combustion engine.<br>
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The higher pressure within the combustion chamber during operation naturally forces some air past the piston rings and into the crankcase. Older engines and those that have not been properly maintained are particularly susceptible to the effects of blow-by because their piston rings and cylinder walls are worn, and have lost some of their sealing ability. Engines with higher compression ratios, such as those with forced induction, are also more vulnerable to blow-by gases. Blow-by increases the pressure within the crankcase, causing gaskets to fail and leaks to develop. In addition, the hot gases contain unburned fuel and moisture that can mix with the engine oil and cause the formation of sludge. This sludge can interfere with the proper operation of the engine and can cause damage. In extreme cases, sludge can even cause an engine to seize.<br>
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The standard solution to sludge formation and crankcase pressure is venting the crankcase through the use of a Positive Crankcase Ventilation (PCV) valve. The PCV valve typically consists of a vacuum-operated valve that routes the air in the crankcase back into the air intake, allowing the unburned fuel contained in the air to be burned in another combustion cycle. A breather, usually located at the opposite side of the engine, allows for the introduction of fresh air that helps move blow-by air from the crankcase into the PCV valve. However, the system has a drawback: The recycled air also contains minute amounts of oil from the crankcase.<br>
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Oil is a lubricant, not a fuel. In fact, oil reduces the octane of whatever fuel it is mixed with. Since octane is a measure of a fuel's resistance to detonation, so it goes without mention that the more oil is in your fuel/air mixture, the more likely it is that your engine will suffer from damaging detonation. This is in addition to the damage caused by oil deposits coating the fuel injectors, spark plugs and even your intercooler, if your engine is equipped with one. The carbon deposits left behind by oil can contain flammable molecules of unburned fuel that could detonate as well. Because supercharged and turbocharged engines are extremely susceptible to damage from detonation, these engines stand to benefit much more from catch cans and oil separators than stock engines. <br>
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Both catch cans and oil separators serve basically the same function, but a catch can simply removes oil from the air, so it will need to be emptied periodically. An oil separator, on the other hand, returns the captured oil to the crankcase after it is isolated from the intake air system.<br>
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There are 2 types of catch cans available, sealed and breather-style. High-performance vehicles, such as those equipped with forced induction, generate higher cylinder pressures. This means more blow-by gases and higher air pressure within the crankcase. As previously noted, the increased pressure can cause leaks, but it also effectively resists the movement of the pistons. This resistance generates unwanted heat and requires force to overcome, force that never makes it to the street. Thus, by venting the crankcase pressure directly into the atmosphere, a breather-style catch can will probably lead to an increase in performance over a sealed system while still catching the harmful sludge.<br>
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But because a breather-style system vents hydrocarbons into the atmosphere rather than re-burning them, it harms the environment as well as your fuel economy rating. As a result, breather-style cans are illegal for street use in many states. On the other hand, a sealed-type catch can returns the air to the intake air stream, keeping both your wallet and the EPA happy.<br>
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Oil separators differ from catch cans in that they don't 'catch' much oil at all. After separating the two, the oil is returned to the crankcase and the air is returned to the intake air supply. *<br>
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*Note, there are DIY solutions which air compressor air/water or air/oil separators. DIY installers typically call these underhood setups "oil separators" though technically they are a sealed catch can solution, unless they have an automatic oil drain back mechanism. Some DIY setups are fashioned to return oil via the oil dipstick tube.<br>
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By Vanessa Wilson