Oil Catch Cans

By SRT-Tom · Dec 27, 2018 ·
  1. SRT-Tom
    There has been much discussion if an oil catch can is necessary on a high performance engine. It is my opinion that the relatively modest price (usually about $100) is cheap insurance for your engine.

    First, let's go over what is currently happening in your engine without a catch can installed. All internal combustion engines that run off gasoline are 4-stroke engines. This means that the piston has to go up and down a total of 4 times to complete a cycle. The piston first goes down with the intake valves open creating a vacuum. This draws in the cool dense air for combustion. At the same time, fuel is injected into the cylinder. The intake valves close and then the piston rises up towards the top of the cylinder. This compression creates an immense build-up of pressure in the cylinder. The only things containing this high pressure are the cylinder itself, the piston and the piston rings that seat against the walls of the cylinder. The intake and exhaust valves are obviously closed as well. The pressure is so high that a very small amount of the air escapes around the piston and piston rings into the crankcase. This is called blow-by. The amount of blow-by increases as the engine RPMs rise. Also, an engine with 8 cylinders will have more blow-by than one with only 4 or 6. Obviously, not all of the air escapes or else combustion wouldn't take place.

    Inside the crankcase, you have the crank which is turning in the oil pan which is full of oil. This keeps it properly lubricated. Positive crankcase ventilation (PCV) is necessary to ensure there isn't a build-up of pressure in the crankcase. This would cause the crankcase to possibly crack under the pressure and create a huge mess of oil on the street. So the PCV system removes the pressure from the crankcase and reverts it back through the intake tract via crank case vents. This pressure isn't made up of 100% air. It will also contain a very small amount of oil. This air and oil mixture is then entered somewhere after the intake system, passes through the intercooler (if your car is turbo or supercharged) and then re-enters the combustion chamber (cylinder) through your intake valves to be re-burned. The blow-by oil will actually coat everything on its way back to the combustion chamber- the intercooler, boost hoses, intake manifold, throttle body and intake valves. Just on the other side of these valves is where the combustion is taking place under extreme temperatures. This is what actually causes the oil to solidify on the valves. For cars with intercoolers, it can actually coat the cooling fins which will hinder the intercooler's ability to cool the air therefore lowering the efficiency.

    This is mainly due to the fact that the air that comes through your intake system and goes into the combustion chamber won't just be air. It will contain some oil particles which cause the combustion process to be slightly less efficient.

    Having oil caked onto your intake valves can cause the following symptoms:
    • Knocking
    • Pre-ignition
    • Loss in power
    • Loss in fuel economy
    Here are two images of carbon build-up on valves and the throttle body.



    You might ask why do car manufacturers revert this crankcase pressure back into the intake tract? Well, for starters there isn't anywhere safe to revert it to and you can no longer expel it into the atmosphere, like was done on cars built before the mid-60s, with oil vent caps, because it's not environmentally friendly and is not street legal (see photo, below).


    Another common question that arises is why isn't a catch can included from the factory? Well it's pretty simple. Since most people don't change their oil on time, it is pretty safe to say that they wouldn’t empty their catch can regularly. This would cause a big problem. So, it is more convenient to just burn the blow-by. Another reason is cost savings.

    An oil catch does a good job of catching or preventing oil from re-entering the intake tract. A catch can is placed right after the PCV and before the intercooler, if so equipped. This means that a more pure (sometimes 100% pure) air mixture will go through the intercooler and intake valves. A more pure air mixture entering the intake valves means no caking and none of the symptoms listed above.

    An oil catch can typically uses steel wool or a baffle system that removes the oil from the air-oil mixture. Higher quality cans use the baffle systems since it is less messy and better at separating the mixture. The oil falls to the bottom of the can where it is stored until the can is emptied. This is typically done at every oil change.

    The less restrictive types have a steel wool medium at the top of the can. The many strands of the steel wool provide a surface for the air/oil vapor to adhere to, and once they stick to it, they are no longer in the air flow. This “scrubbing action” is how most of the larger oil droplets are removed. The “torturous path” the air/oil vapor takes, as it passes thru the separator, removes the rest.

    By torturous path, I mean the air flow in through the inlet hose into the separator head area. The vapor must make a 90 degree turn into the bowl area, and then another 180 degree turn back up into the head area again, then a final 90 degree turn to go out the outlet hose.

    All these quick changes in the direction of flow will strip oil droplet molecules out as they weigh more than the air molecules, and can't make the redirecting turns as easily. As a result, the oil will end up on the walls of the separator bowl or on the steel wool instead and will drip down into the bowl for collection. The air will then continue to flow right on out.

    In the first photo below, is a 6.1 Hemi engine with an installed Billet Oil Catch Can (to the right of the fuse box).


    The second photo, below, depicts a catch can installed on a 392 Hemi.


    The third photo shows how much blow-by accumulates in the catch can between oil changes and is prevented from being burned by the engine. (Note- Always discard the blow-by and never pour it back into your engine. It is a nasty brew of oil, water, gasoline and acid).


    An oil catch can doesn't add any power or make any cool noises, so it is often overlooked when modifying vehicles. What is does, however, is prevent a loss of power, as miles pile up, due to your PCM retarding the timing to prevent pre-ignition. Simply stated, a catch can will ensure you are always running the most power possible by having a cleaner intake tract free of oil.

    For more information on this topic, see:

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