Air Oil Separators or Catch Cans? Which One Is Right for You?

Performance engines produce a lot of heat. In fact engines can produce so much heat that it vaporizes oil. Oil vapors are not good for the engine because these vapors can cause build up on the engine components like the valves and the vapors can lead to an increased oil consumption. Increased oil consumption it a serious problem because oil is basically the blood of the engine which keeps it running smoothly at a controlled temperature. So how does one avoid vaporizing their oil? Glad you asked! This where air oil separators or catch cans can help.

Some AOS systems are universal like the Morso 85474 which can be found on Amazon and will fit most engines.

What Is an Air Oil Separator?

An air oil separator or AOS as it is sometimes called, is a device which is intended to capture the oil vapors that are traveling through the PCV system, turn those vapors back into a liquid, and finally return the oil back into the oil pan.

During combustion there are gases and exhaust vapors that get around the piston rings and into the crankcase of the engine. These gases are called blow-by. Usually these gases also contain oil vapor. The AOS is located in the PCV system between the crankcase and the intake manifold. PCV system recirculates the gases in the crankcase into the intake system. These helps reduce emissions. The only problem is that without an AOS oil vapors could be getting sent to the intake and combustion chambers of the engine. The AOS is basically a filter to remove oil vapors from the crank case ventilation system.

Air oil separators should operate in temperatures above 212 degrees Fahrenheit which is the boiling point of water. If the air oil separator is below this temperature moisture can build up in the system causing rust or worse contaminating the oil.

The key to these setups is having the air oil separator cold enough so that the oil returns to the liquid state, yet still hot enough that the water remains a vapor. When the system is properly setup, the exhaust gases can be easily separated from the oil.

What Is a Catch Can?

Located in between the crankcase and the intake manifold, a catch can is very similar to the AOS. Its function is the same in that it filters the oil vapors out of the recirculating gases. However, there is one major difference. Unlike the AOS, the catch can does not return the oil to the oil pan. Instead, it only captures the oil and holds on to it.

It is very important to regularly check the catch can and empty its contents. If the catch can is allowed to fill up, it can plug the PCV system increasing backpressure in the crankcase. This increased pressure can cause excess oil vapors and blow-by. That being said, emptying a check can is a very simple process and its really just something to add to the routine maintenance.

Benefits of AOS and Catch Cans

AOS systems and catch cans have several benefits that promote a long engine life.

  • Reduce carbon build up on the intake valves
  • Eliminate recirculating contamination
  • Reduce Oil loss from combustion
  • Protect against detonation

AOS systems and catch cans will reduce carbon build up on intake valves. This is especially true of GDI (direct injection) engines. Direct injection engines don’t have the benefit of gasoline cleaning the intake valves. This causes recirculating oil vapors from the PCV system to cause carbon build up on the valves. However, an AOS or catch can with baffles and filtration can reduce carbon build up and capture contaminates that would otherwise recirculate.

An AOS system will reduce oil loss by returning oil back into the oil pan. A catch can does not have this benefit as it will just capture and hold the oil in the can.

Lastly if the engine has too much blow by and a lot of oil vapors are getting recirculated into the combustion chamber, it can cause premature ignition A.K.A. detonation. AOS systems and catch cans prevent this by causing those oil vapors to go through a phase change back to a liquid state, and the oil vapors never see the combustion chamber eliminating the possibility of detonation.

Is an AOS or Catch Can Necessary?

AOS and catch cans should be utilized on direct injection engines where the gasoline does not contact the intake valves. Gasoline is great at cleaning parts, but if it never sees the valves, it won’t be able to clean them. So how do you keep your valves from getting carbon build up? You prevent carbon build up by filtering the oil vapors from the intake air via an AOS system or catch can.

AOS and catch cans are also beneficial on high horsepower applications and even on stock motors. They can increase performance by removing oil vapors from the combustion chamber and it will also improve fuel economy too. These systems are a great modification for any motor enthusiast!