Oil catch cans (also sometimes referred to air/oil separators) can look great in the engine bay, but are they really necessary, and if so, what do they do? If this is something you’ve been wondering about, then this article is for you.


An engine is a lot more complex than just suck, squeeze, bang, blow. There are a number of subsystems at play which all do specific things in order to keep your engine running properly. One of these subsystems is the vacuum system. The vacuum system does all kinds of things inside your car, from ventilating the engine internals, to providing assisted braking power.

One of the by-products created when running an engine is oil vapour. This vapour makes its way through the vacuum system, and back in to your air intake and throttle body, where it has a number of detrimental effects. This is known as oil blow-by. The amount of blow-by produces depends on a number of different factors, including the age of the engine, as well as what’s being demanded of it. On a typical road car, driven at relatively low RPM for most of its life, the amount of blow-by produced will in most cases be negligible, however in high performance engines, quite a lot can be produced, especially if they’re getting on in age, or being driven hard most of the time.

So what are some of the problems that can be caused by oil blow-by:

  • It leaves an oily residue throughout your vacuum and intake system which can over time clog valves, sensors, etc. This is particularly dangerous for boosted cars, as it can cause your blow-off/bypass valve or wastegate to stop functioning properly, or in some cases at all. This in turn can lead to over-boost which could destroy your engine. Other problems include clogged Mass Airflow and Manifold Absolute Pressure sensors, which can lead to incorrect values being sent to the ECU, which can in turn degrade performance significantly, as well as cause permanent damage to your engine.
  • It can pool in your intercooler and reduce cooling efficiency.
  • It makes its way through the throttle body and in to the combustion chamber, where it can cause a number of problems. By adding oil to your air/fuel mixture you lower the octane level, and the result is inefficient combustion which not only produces less power, but creates soot which clogs injectors, spark plugs, cylinders, pistons and your exhaust.
  • Increased Hydrocarbon emissions, which are bad for the environment.

There are typically two parts of an engine which release oil blow-by back in to the intake system. One is via the PCV valve, which is typically plumbed directly back to the throttle body and vents only when the engine is at idle, often releasing large amounds of blow-by in a single hit when you lift off the throttle after a hard pull. Oil which enters the system this way is most likely to end up inside the combustion chamber.

The other is via the crankcase breather, which is typically plumbed back in to the intake either right before the throttle butterfly valve, or on boosted cars, before the supercharger or turbo (and sometimes the intercooler). Oil which enters the system via the crankcase breather is typically going to clog and risk damage your Mass Airflow sensor, Wastegate, Blow-off/Bypass Valve and throttle butterfly valve/throttle position sensor.

This diagram by Radium Engineering illustrates a typical OEM (stock) ventilation system.

This diagram by Radium Engineering illustrates a typical OEM (stock) ventilation system.

A typical catch can setup consists of either two cans, or a single can with two inputs and outputs. The function of an Oil Catch Can is simple, it connects in line with the PCV and Crankcase ventilation systems, and passes the oil-saturated air through a series of baffles in order to separate the oil vapour from the air. The air is allowed to pass through the system and back in to the intake and throttle body, and the oil is trapped in the can where it can be drained, thus reducing or completely eradicating the problems associated with blow-by.


Not only that, but it’s a great way to keep tabs on how much oil vapour your engine is producing, which could be an early warning sign for a bigger problem.

There are also ventilated Catch Cans available, which vent the separated air to the atmosphere rather than back in to the intake to be burned. These can slightly increase performance as they produce less restriction on high boost applications, and completely eliminate the chance of blow-by entering the combustion chamber, but are typically illegal for street use. Not to worry though as a closed system catch can setup is perfectly adequate for the vast majority of street cars. On boosted applications, it’s important to choose a pressure sealed catch can setup to prevent boost leaks.

The easiest way to determine if you require a catch can is to simply check the inside of your intake system for oily residue. If it’s present in the inlet manifold only, there’s a good chance you would benefit from a PCV catch can setup. If it’s present in the entire intake system, then you’d also benefit from a crankcase breather kit as well.


Choosing an appropriate catch can is somewhat more complicated. Unfortunately there are a lot of cheap knock-off cans on the market that are no more than a can with two holes for barbs. With no internal structure, these do very little more than look cool in your engine bay. This is why a lot of manufacturers refer to their systems as “air/oil separators”. When you see this naming, its a good indication that some engineering has gone in to the product, and it’s more than just a can. Importantly, care must be taken when designing a catch can to ensure it does not add significant airflow restriction to the system.

A well designed system like this one will ensure the maximum amount of oil is captured, without significant restriction of airflow.

A well designed system like this one will ensure the maximum amount of oil is captured, without significant restriction of airflow. – Radium Engineering

Personally, I use and recommend Radium Engineering products, but there are plenty of other great brands available. As with any car mod. Do your research and speak to people with experience before parting with your hard earned cash.

I well designed catch can will have internal structures which separate oil and air.

A well designed catch can will have internal structures which separate oil and air.