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PCV system theory and Oil catch cans.

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Old May 12, 2006, 06:10 PM   #1
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PCV system theory and Oil catch cans.

Link to cool catch can

I did some research so I could could post on this topic better since it has always bothered me seeing jacked up oil catch can setups. In case people did not know, both the connections that are on the head (PCV and vent) are actally connected too the crank case, not to the air volume in the head itself.

Here is a simple diagram of the stock configuration.

In the stock configuration fresh filtered air enters the crank case through the vent connection and exits through the PCV valve into the intake manifold. This maintains a negative pressure in the crank case and carrys out moisture and any blowby gases thus preventing corrosion and sludge buildup in the crank case.

The PCV valve itself is a check valve (one way valve) to prevent back flow from the intake manifold into the crank case. In a naturally asperated carborated car of the past this check valve prevented any fuel/air mix from entering the crank case, in a modern car it prevents exhaust gases from the EGR system from entering the crank case, and in turbo applications like ours prevents the crank case from being pressurized due to the positive pressure in our intake manifold.

Next post shows how a proper single catch can system needs to be configured.

Keith
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PCV system theory and Oil catch cans.-stock-pcv-configuration.jpg  

Last edited by Fourdoor; Feb 28, 2007 at 06:02 AM.
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Old May 12, 2006, 06:11 PM   #2
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Link to a cool catch can

With a proper catch can setup, the crank case vent is still connected to the air filter box, then you replace the PCV valve on the head with a simple nipple connected to your catch van via a hose. The PCV valve should be moved to the outlet of the catch can to prevent pressurizing the catch can when under boost. From the catch can a hose goes to the intake manifold like normal.

Since most PCV valves leak on turbo cars we end up with a slight positive pressure in the crank case under boost and this back flows out the vent connection into the air inlet hose leading to the turbo crapping it up with oil, water vapor, and blowby gases. My next diagram is of a dual catch can setup to prevent this.

Keith
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PCV system theory and Oil catch cans.-catch-can-setup.jpg  

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Old May 12, 2006, 06:12 PM   #3
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link to a cool catch can

OK, here is a diagram of a properly configured dual catch can setup. Each can should have some form of filter media in it to help condence out the nasty crap you are trying to capture.... an open can works, but not nearly as well.

Keith
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PCV system theory and Oil catch cans.-dual-catch-can-setup.jpg  

Last edited by Fourdoor; May 18, 2006 at 06:06 PM.
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Old May 12, 2006, 06:13 PM   #4
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link to a cool dual catch can

My current setup only has catch can #1 of the dual catch can setup because I have a sheet metal intake manifold that does not have a nipple on it for the PCV valve to connect to. Simply put I do not have positive crank case ventilation on my setup and as a result I do oil analysis (blackstone labs oil test system) to determin when to change my oil since I will get more blowby gasses, water vapor and whatnot in my oil than a car that has a correctly operating PCV system. My car is a weekend toy, but if I was going to use it as a daily driver I would go to the trouble of getting another nipple installed on my sheet metal intake and add the second catch can.

Keith

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Old May 12, 2006, 06:22 PM   #5
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I'm running a dual, separate catch can setup exactly as you have pictured (2 Helix cans).

Just in case people were wondering, the amount of substance collected in catch can 2 >>>> the amount of substance collected in catch can 1. In fact, I don't think I've even emptied catch can 1. The only thing I've seen collected in it was just a fine film of oil. However, in catch can 2, I empty that thing every oil change or so due to a visible collection of fluid.
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Old May 12, 2006, 06:39 PM   #6
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I have a single catch can. I take both ends from the engine and vent to atmosphere. Also, intake is plugged. I discovered that you can 't completely stop the oil going into the intake even with the catch can. It seems to work fine without the negative pressure.
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Old May 12, 2006, 06:49 PM   #7
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The problem with venting the entire system to the atmosphere is that, while any positive pressure is relieved, you lose the vacuuming effect from the intake manifold (under vacuum) and possible the aspirating effect from the intake nipple under boost.

Also, keep in mind that most street turboed cars spend most of their lives under vacuum--having things vented to the atmosphere isn't going to catch any of the sludge that seeps into your intake manifold during this time.
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Old May 12, 2006, 07:15 PM   #8
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Quote:
Originally Posted by denial
The problem with venting the entire system to the atmosphere is that, while any positive pressure is relieved, you lose the vacuuming effect from the intake manifold (under vacuum) and possible the aspirating effect from the intake nipple under boost.

Also, keep in mind that most street turboed cars spend most of their lives under vacuum--having things vented to the atmosphere isn't going to catch any of the sludge that seeps into your intake manifold during this time.
Hey ok! I get what you were saying in my project thread.
These pictures help alot!
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Old May 13, 2006, 06:10 PM   #9
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You must also consider.
For a crankcase to be properly vented, you need to have manifold vacuum for low throttle conditions and venturi vacuum for high throttle conditions.
If you only have the PCV from the intake manifold, the only time you maintain negative pressure is under light loads (when you need it the least). Under high boost when the PCV valve is closed, you need to have a vacuum source in the intake tract before the turbo where there is a negative pressure, even though slight.
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Old May 13, 2006, 06:20 PM   #10
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Wow! You guys are coming up with some good suggestions
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Old May 13, 2006, 06:38 PM   #11
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Quote:
Originally Posted by BMan
If you only have the PCV from the intake manifold, the only time you maintain negative pressure is under light loads (when you need it the least).
It's also important to keep a temporal aspect on things: It's true that the placement of a catch can in this location would benefit the least in an instantaneous setting (ie, high-load settings), but because most street cars spend most of their time under light loads, a can in this position (PCV/intake manifold) will see more "use" than a can in the other position (intake/valve cover). I know for a fact that mine does (from the amount of oil collected).

Now I'm not 100% sure of the benefits of preventing oil from being dragged into the intake manifold during non-high load applications has, but I'm sure it can't be a bad thing in the long run. And that's why I run 2 just to make sure I've covered my bases. Great explanation btw.
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Old May 13, 2006, 08:18 PM   #12
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Quote:
Originally Posted by BMan
You must also consider.
For a crankcase to be properly vented, you need to have manifold vacuum for low throttle conditions and venturi vacuum for high throttle conditions.
If you only have the PCV from the intake manifold, the only time you maintain negative pressure is under light loads (when you need it the least). Under high boost when the PCV valve is closed, you need to have a vacuum source in the intake tract before the turbo where there is a negative pressure, even though slight.
So, are you saying what is being suggested above shouuld not be done? I guess I don't understand what is being implied. Thanks!
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Old May 13, 2006, 08:43 PM   #13
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Quote:
Originally Posted by EvoIXMR
So, are you saying what is being suggested above shouuld not be done? I guess I don't understand what is being implied. Thanks!
Are you asking me or the BMan? If it's me then here we go...

BMan has an excellent point in that the most important setting where you would want to prevent oil from entering the intake tract is when the engine is under high load (and thus high boost). So you would put a can in b/t the intake tract and valve cover. The benefit here is that you keep oil out of the entire intake tract and intercooler optimizing effeciency and possibly preventing detonation if it reaches the combustion chamber (as oil has a very low ocatne rating).

But what I'm saying is that a can placed b/t the PCV and the intake manifold gets more use, literally, than a can placed in the aforementioned position. This is simply because you mostly cruise around town in a low load (and thus negative pressure) fashion. The benefit here, isn't so clear--all you're doing is preventing oil from entering the intake manifold and combustion chamber in a low load setting where detonation really isn't a problem. However, perhaps it would prevent gunk buildup etc. in the long run (at least I think it would).

I don't have an answer to which one is better, or more neccesary. Ideally it wouldn't be one or the other, but both. Maybe Fourdoor or BMan could share their opinions. FYI between two I have, I seem to only get a significant amount of oil in the PCV can. The other seems to remain pretty empty.

I apologize if I'm repeating a lot of what I said earlier--I'm not too good at typing out things.
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Old May 14, 2006, 12:48 AM   #14
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I listed them in the order of worst to best. Stock < single can < dual can. If my intake manifold had a fitting to connect to the PCV valve I would be running a dual can set up no questions asked.

Keith
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Old May 14, 2006, 12:55 AM   #15
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hey thanks for the input... i plan on at least one oil catch can in the near future
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