Catch Can VS Air/Oil Separator VS Breather Tank (Lions and Tigers and Bears!) I learned a lot in the last week, so in the absence of recommendations here, let me share what I found. First, people, even the vendors are using the term 'catch can' interchangeably to mean either a catch can, an air-oil separator and a breather tank. While they all do the same job, they function quite differently and are built differently. The plumbing may seem similar, but they are not the same. I will describe each-
Catch Can- a real catch can is what is referred to as a 'closed' system- meaning it does not vent to the atmosphere, but back through the intake. It has baffles inside to keep the oil and contaminants in the can before going to the intake with the use of vaccum pressure from the intake. I have seen some catch cans with an optional filter- don't be fooled by this as a filter will provide access to the atmosphere, and introduce un-metered air to the system-bad. There is no need or desire for a filter on a closed system. There is sometimes a metal screen internally, but you want no venting on this system. Qualities of a good catch can- a large tank may be desired to catch more fliuds and reduce service intervals (you have to dump it out). Also, make sure the hose size is pretty healthy- especially if you are turbocharged. The contaminates are caught in the can, and the air is returned to the intake cleaner. Cleaner, not clean. It does not perfectly remove everything from the air. It does however, take the gases and put them through the combustion cycle to be burned for better emissions.
Air Oil Separator (AOS). This system is almost identical to the Catch Can except that it recirculates everything back into the engine, and has no need of a can to be emptied. It is often referred to as a 'no maintenance' system. The gases are caught and sent to the intake, the oil (and other liquids) are sent back to the oil pan this is the way that it differs from the catch can. The fancier ones have coolant inputs for the purpose of keeping the oil at the same temperature as the engine so the oil coming back in doesn't foam from the temperature difference. Some characteristics of this system are that it has much smaller lines going in and out.
Breather Tank- This is the simplest of the systems, and has no connection to the intake. It vents the crankcase to the atmosphere through the filter on top, and catches the oil and contaminants to be drained later. Since there is no connection to the intake, no oil or contaminants will coat the intake, valves, etc. Simple. To utilize this system, you block off the line into the intake where the PCV usually goes. No need for a PCV anymore, and I am guessing the EGR is already gone if you got this far. Make sure your hose going from the crank case is large- let it breathe!
Make sure you know which is which and what system you are asking for before going and ordering anything, or you might get the wrong thing.
Now, here is my decision making matrix as to which kind to put on-
The catch can and the AOS are both closed systems, and lend themselves both best to newer cars where all the emissions have to be in place. It seems that the AOS will make the oil break down faster, so I would look at oil change intervals to keep the engine clean. If you don't mind emptying the can, then the catch can may be the best choice. I suppose it is a matter of what you want to do in maintenance. All the bad gases will be sent to the combustion for burning, and life will be good. If you are like me and live in a state that is still free and have a Ranger more than 20 years old, then emissions is not an issue. In that case, I removed the EGR and PCV, went to the breather tank. This gives me no contaminants in the intake at all, and I will empty the tank occasionally. As far as the gases vented to the atmosphere, I suppose we could debate about that. The one opinion I will give is that a better working engine will combust more completely, and therefore cleaner emissions. But let's not get a couple hundred thousand cars together in the same place and let them run for a couple hours at a time, that can't be good for anyone! Ok, this is my still novice knowledge on the subject. Experts, feel free to correct any errors-
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