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Crank Pressure

From my experience VQ35’s tend to have excess crank pressure. Think about it, how many 3.5’s do you hear about that are constantly knocking even with 93 octane fuel and nearly stock with little to no modifications. Part reason is because the inside of intake manifold is coated with oil that comes from the valve cover. Oil in the combustion reduces octane and causes buildup. Why are there so many 3.5’s that leak oil; there are even jokes and meme’s about it. Well, to understand let me take the opportunity and go over the PCV system and solutions.

PCV System

The PCV system is a closed system which circulates the crank case pressure into the intake manifold and back into the combustion; the crank case pressure being generated by the combustion.

Inside the engine, the pistons slide up and down on the cylinder. Oil is used to keep the piston from coming in direct contact with the cylinder wall; allowing it to glide and not rub. The pistons have rings around them which have a gap at one end that allows it contract and expand with the pistons. Part of the rings function is to push/squeegee down the oil that is on the walls to not allow it to enter the combustion. During the combustion some of the pressure escapes past the rings and into the crank case. The crank case is the area below the pistons where the crank rotates; the engine block.

The pressure from the crank case needs to escape, so it flows out of the crank case and into the timing cover area. Then it makes its way up to the heads and out of the valve cover via the PCV valve.

The PCV valve is connected to the intake manifold with a vacuum hose. The intake manifold is constantly in vacuum pulling the fumes from the crank case, into the intake, then into the combustion and out of the exhaust.


It sounds great to pass the wasted vapors from the crank case back into the combustion. The problem is that these vapors contain oil, so the intake manifold, cylinders, and valves all get coated with oil. This turns to gunk and build up which then can cause increase in compression and knocking. Oil in the combustion may also affect the octane level further causing knock; higher compression, less octane, its not a good mix.


Vent to Atmosphere

One option is to disconnect the vacuum line that goes to the intake manifold. Plug the intake manifold and leave the PCV open to atmosphere. Now the intake manifold will not be sucking in oil and the crank case still gets to vent. This is not a clean solution as you are venting that into the air we breathe; when it could have been processed through the combustion again.

One thing that you will notice immediately when you vent to the atmosphere is that it will smell bad. It gets in your clothes and in your pores.

Catch Can

The other option is to use a catch can. A catch can is a container which can be in any shape usually as a cylinder or a square aluminum box. When properly designed, this container is intended to remove the oil from the vapors that come out of the crank case.

You connect a hose from the PCV valve into this container. Then you connect a hose from the container into the intake manifold. As fumes are pulled from the crank case they enter the container and pass through turns and walls to help condensate it. The longer the vapors are out of the engine the cooler they get, then the oil particles become denser and stay in the container. Cleaner air exist the container and into the intake manifold.

DYI Catch Can

DIY PCV Vented Catch Can with a 1/2 Barb.

A home made catch can can be made using a PVC pipe that is closed in one end and with a screw cap on the other end. You can drill two holes then screw in two brass barbs. You fill the PVC with brillow pads. The purpose for the brillow pad is for the vapor to come in contact with the material and add resistance, condensing it. You screw the cap and you connect the PCV hose from the valve cover into one barb and the other barb is connected to the intake. It works better if the barb that takes in fumes has a hose inside that forces it to travel to the bottom of the container. This way is has to make its way up through the material inside and out of the container; otherwise it will enter and leave the container without barely touching the brillow material.

You can also use a catch can with PCV systems that are vented to atmosphere. A hose goes from the PCV valve to the catch can, then cleaner air exits out to atmosphere (it will smell less and keep the engine bay clean). Usually when you have the crank case vented you will notice your hood develop a film of oil and sometime even coat the whole side of the engine bay. Its not cool when you try to impress a girlfriend with a clean Maxima and smells like you drive a tractor.

Turbo and Superchargers

The PCV valve is a one way valve which allows air to flow out of the valve cover. This helps keep pressure from the intake manifold from going into the valve cover and further increasing the pressure of the crank case. When this occurs you will start to see oil leaks develop. The inner timing cover, the outer edges of the timing cover, the area between the inner timing cover and the valve cover, are all areas that are prone to leak because of excess crankcase pressure.

The problem is that these PCV valves do not seal well against pressure. If you have a turbo or supercharger its likely that the pressure from the intake is going to seep into the crank case. A solution is to put a real one way check valve on the hose. The problem now is, where does the crank pressure go if there is pressure in the intake manifold. Generally it will vent out of the front valve cover where a breather is usually placed on the outlet that often connects to the intake piping near the MAF. This is not enough to vent efficiently, the pressure builds up in the crank case till it finds a way out causing leaks and loss in power. If there is pressure in the crank case it takes more effort for the pistons to go down. You can fix this by putting the line that connects to the PCV valve to the suction side of the supercharger or turbo. This is after it has passed through a catch can; you don’t want your shiny compressor wheel coated in oil.

From my experience, when you start to boost the VQ, you need to also vent out more crank case pressure. You can drill out the PCV Valve and use an external one; you can also replace it with a larger one. If your system is vented to atmosphere then you can drill the valve cover, put a barb and run an additional vacuum hose to the catch can. Some catch cans have multiple inlets. You may also have a filter or breather on the front valve cover while you have the check valve in the PCV hose. When the engine goes into boost, it will close the PCV hose, then vent pressure out of the breather. This is not a good solution as you need more flow than just the breather’s port which usually gets routed to the intake.

One last solutions is to run the hoses from the catch can into the exhaust. The hose connects to a pipe that goes in to the exhaust in an angle in the direction with the exhaust stream. As the exhaust flows it pulls the air from the pipe and the hose sucking the crank case pressure out. The closer you have it to the exist of your exhaust the better because if its too close to the engine usually your exhaust system would have some back pressure. So although it would still work it wont be as effective.

There is another solution which is to use a vacuum pump to pull out the crank case pressure but I will not get into those details in this release.

You should now see if you didn’t already how important it is to deal with the PCV system when you modify your car. It directly affects the power of your engine, yet it tends to get forgotten.

Thank you for reading and following Fastmaximas.

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