crank case pressure question

Discuss topics related to the CA18DE and CA18DET series engines.
Margus
Posts: 16
Joined: Tue Jul 21, 2009 12:38 am
Car: RS13

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Hi.
You have all got it a bit wrong.

Venting air through the PCV valve to your intake, only circulates fresh air in your valve covers. Fresh air goes in to the ex.side, and gets sucked out through the intake side, to the intake manifold. The crankcase gets absolutely no fresh air circulation of what so ever. Clean air will never enter there, because there is no vacuum. Only pressure, that gets reliefed through the metal 90degree pipe.

If this pipe is blocked, under boost blowby will force air out through the internal oil return lines. Causing no oil return to the sump, and we all know what happens next. First, oil pump gets dry (common bearing failure), then oil gets pushed out through the valve cover vent holes. Enters your intake piping, next it enters intake manifold and now you have a nice detonation that kills your pistons (common failure).

To prevent it.

Crankcase is not vented, because there is only one (exit) venting point (90degree metal pipe). To vent it, you have to make another venting point, and leave it open in the atmosphere connected with catch can and filter. Then connect original venting point straight to the intake using PCV valve. (yes you can use catch can there, but not necessary).

But under boost all changes. PCV closes, and if blowby occurs, it gets vented out through the seccondary venting point into the catch can and then in atmosph. All goooood but...
Too complicated? yes.

The best and the far easyest way to vent your motor, is to replace the 90degree narrow metal pipe, with bigger one, and connect it to the catch can, and thats it!
There is no need to vent your valve covers into intake with PCV (but you can). Just put filters on both side.
If crankcase venting pipe is big enough, there is no oil return problems from head.

Hope that helps.

Margus


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float_6969
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LOL, you must not be understanding something if you're saying we've got it a bit wrong. I think everyone is aware that the 90° pipe behind the oil filter the is the crankcase vent and that if restricted, causes issues with the dipstick blowing out, front, rear, and cam seals to leak, and prevents oil from draining out of the head.

The ACTUAL problem is that most motors are worn and have too much blowby. The CA's PCV system is marginal in stock form, but functions properly given a normal amount of blowby. You can rebuild the engine to stock specs and the issue goes away. If you rebuild with looser specs, as many engine builders do, then the problem can get worse.

The BEST way to deal with crankcase pressure is to place a vacuum on the crankcase. This can be expensive and complicated. Venting to the atmosphere is the least complicated way to do this. From my experience, you want the LARGEST diameter hose coming off of the 90°fitting you can fit. You also want it to run to a high point in the engine bay somewhere. At the end of the hose you need some way to separate the remaining oil mixed with the crankcase gasses. The SR AOS works, but the hose diameters on it are small, and so restrictive. Ideally the connections on the AOS/catch can would the same large diameter as the hose coming off of the 90° fitting. The outlet of the AOS/catch can can be vented, but will cause the car to smell bad and is technically illegal as that is a source of pollution. It SHOULD feed back into the intake of the engine, but this causes charge dilution, and can lead to detonation and fouling of the intake tract. Another potential source of vacuum is via a venturi in the exhaust path. If placed close enough to the engine, would allow the gasses to be burned as they would running through the intake and prevents any offensive odors. I don't have experience with this method yet, but I plan on experimenting with it on this build.

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Cams
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I have some questions regarding this (please humor my crappy Paint skills)

ImageCA18DET Vent options by CamsX200, on Flickr

Top left is standard JDM config. Top right is my current config, where I'm venting exhaust side to atmo.

Bottom is the setup I'm considering, connecting both intake and exhaust valve sides (this is how it was on my EDM config) and venting them to Atmo through a catch can, but blocking PCV and Intake ports.

I understand that this defeats the purpose of the mild vacuum effect the PCV generates on the Crankcase and Intake side ventilation. But is it really necessary?

How is the Exhaust Valve side connected to the Crankcase?? Internally? Having both valve covers+crankcase venting to atmo would allow enough pressure release under boost to prevent other negative issues, right?? (regardless of the environmental concerns)

My dipstick is popping up after hard driving (I think it is too loose even on engine off), but almost no oil is coming out of it. Engine has less than 2000 kms since rebuild.

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float_6969
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The two valve covers are connected together inside the head at the front and back. That MAY help blowing out the dipstick. Underboost, the PCV valve closes so the only outlet for the crankcase gasses is through the exhaust side hose. Conecting the intake side to the atmosphere may allow enough flow to stop the dipsick from blowing out.

From experience, it probably won't work. The main problem with the CA's system is the tube from on top of the oil filter mount to the intake side valve cover. It's too small and convoluted for the crankcase gasses to make it up to the head quickly enough to get vented. This pressurizes the crankcase causing oil to back up in the head, blow out the dipstick, and leak from the main seals.

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Cams
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Joined: Sat Jan 27, 2007 3:42 am
Car: 1993 Nissan 200sx CA18(DET) RS13
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2016 Infiniti QX50
Location: Panama, Central America

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Cool, thanks for clarifying. I will give it a go and monitor.


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