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The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!
The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!
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Old 07-22-09, 09:10 PM   #41
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

I've read through this thread and, at least to me, it seems there are some things that would be not so clear to the average person reading it.

Blow-by is combustion pressure that escapes past the piston rings and this does not necessarily always happen under high boost conditions. The level of blow-by is usually dependent on the conditions of the ring seal to the cylinder wall or piston. Damaged pistons such as ones that have cracked ring lands will leak significant pressure.

Blow-by causes pressure to build up in the crankcase. Measuring crankcase pressure is one way to determine the condition of the ring seal.

The PCV system is designed to recirculate these blow-by vapors so it can be burned in the engine and avoid releasing it the environment. The PCV valve is screwed into the intake manifold (someone posted a good picture). The idea is that when there is a specific pressure differential between the crankcase and the intake manifold then vapors are allowed to pass into the intake manifold. The valve will be closed when there is either significant positive or negative pressure in the intake. The system is not designed to generate a large negative pressure in the crankcase.

There are oil separators in the valve covers to limit the oil vapors that enter the PCV hoses attached to the valve covers. On AVCS-equipped cars there is also a second set of hoses connected to the valve covers that are not specifically part of the PCV system but are there to improve the oil return from the cylinder heads back to the block. It should be noted that oil vapor or mist is created by oil thrown off and whipped up by the crankshaft and the PCV system needs to allow this oil mist to separate out from the blow-by gasses.

The only difference between a catch can and an AOS is that an AOS allows return of the liquid back to the block whereas a catch can simply stores it until you empty it.

As someone mentioned....oil vapor in the PCV system contributes to detonation because it essentially lowers the octane level when added to the intake charge. It also builds up carbon on pistons and valves which further contributes to poor performance and the possibility of engine-damaging detonation. Race engines are set up without a PCV valve to eliminate as much as possible the oil vapors being drawn into the cylinders.

A really high end race system will use a dry sump system to substantially reduce oil vapor and ideally will create a small negative pressure in the crankcase. This negative pressure can actually improve the piston ring seal.
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Old 07-22-09, 09:55 PM   #42
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Is anyone out there running their daily driver without the PCV? Any ill effects? I can see that in theory you'd get less blow-by extracted from the engine due to the lower vacuum in the turbo inlet (as compared to the intake manifold) but does that have any real drawbacks in practice?

On the one hand, it seems like any pressure differential at all should be enough to have circulation. Just slow circulation. But if there's little pressure differential, there's little blow-by, so no problem... right?

On the other hand, I'm guessing Subaru felt the PCV valve was useful, else they wouldn't have bothered.

Thing is, I kinda want to switch to blow-through in the not-too-distant future to see if it helps with a rich dip on rapid throttle openings (even with tip-in completely disabled). With blow-though MAF, the PCV becomes a post-MAF leak, so it has to be plugged.

If anyone has experience with disabling the PCV of a daily driver I'd love to hear about it.

Thanks!
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Old 07-23-09, 06:07 AM   #43
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Quote:
Originally Posted by RalliSpec_Dave View Post
The PCV valve is screwed into the intake manifold (someone posted a good picture).
I think that's the european model. US model is on the block.



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Old 07-23-09, 06:32 AM   #44
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

very nice!
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Old 07-23-09, 06:40 AM   #45
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

We have hundreds and hundreds of customers that are running their DD equipped with our AOS, without a PCV, with zero ill effects.

The only time you have vacuum in your inlet manifold is in a off boost situation. When you are not in boost, your crankcase pressure is little to none.
On the other hand, when you are making boost you are also making crankcase pressure. At this time your PVC is closed and not evacuating the crankcase. This is when our AOS shines as there is plenty of vacuum in the turbo inlet while making boost.

Team Crawford



Quote:
Originally Posted by NSFW View Post
Is anyone out there running their daily driver without the PCV? Any ill effects? I can see that in theory you'd get less blow-by extracted from the engine due to the lower vacuum in the turbo inlet (as compared to the intake manifold) but does that have any real drawbacks in practice?

On the one hand, it seems like any pressure differential at all should be enough to have circulation. Just slow circulation. But if there's little pressure differential, there's little blow-by, so no problem... right?

On the other hand, I'm guessing Subaru felt the PCV valve was useful, else they wouldn't have bothered.

Thing is, I kinda want to switch to blow-through in the not-too-distant future to see if it helps with a rich dip on rapid throttle openings (even with tip-in completely disabled). With blow-though MAF, the PCV becomes a post-MAF leak, so it has to be plugged.

If anyone has experience with disabling the PCV of a daily driver I'd love to hear about it.

Thanks!
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Old 07-23-09, 08:05 AM   #46
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

If you have done any UOA with and without a correctly functioning PCV system you would see a huge impact to oil performance and engine wear. Blowby must be evacuated from the crankcase.

Team Crawford,
Why do you combine both the clean and dirty side of the PCV system? These are two mutually exclusive parts of the PCV system that should NOT be combined.
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Old 07-23-09, 08:32 AM   #47
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Quote:
Originally Posted by NSFW View Post
Is anyone out there running their daily driver without the PCV? Any ill effects? I can see that in theory you'd get less blow-by extracted from the engine due to the lower vacuum in the turbo inlet (as compared to the intake manifold) but does that have any real drawbacks in practice?

On the one hand, it seems like any pressure differential at all should be enough to have circulation. Just slow circulation. But if there's little pressure differential, there's little blow-by, so no problem... right?

On the other hand, I'm guessing Subaru felt the PCV valve was useful, else they wouldn't have bothered.

Thing is, I kinda want to switch to blow-through in the not-too-distant future to see if it helps with a rich dip on rapid throttle openings (even with tip-in completely disabled). With blow-though MAF, the PCV becomes a post-MAF leak, so it has to be plugged.

If anyone has experience with disabling the PCV of a daily driver I'd love to hear about it.

Thanks!
A closed PCV system is for emissions compliance only. The PCV function, open or closed, is a vital component to consistant engine performance and longevity. If designed correctly, a closed PCV system is more effective at removing blowby contamination from the crankcase.

There are many ways to design an effective PCV system. From my experience a properly designed catch can is required to reduce carry over oil contamination within the intake manifold and turbo inlet.

This is a very hot topic in the Chevrolet LSx world with lots of discussion. Here is a link to a great thread on PCV systems: My catch can routing ok? - LS1TECH
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Old 07-23-09, 09:31 AM   #48
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Quote:
Originally Posted by 405HP_Z06 View Post
If you have done any UOA with and without a correctly functioning PCV system you would see a huge impact to oil performance and engine wear. Blowby must be evacuated from the crankcase.

Team Crawford,
Why do you combine both the clean and dirty side of the PCV system? These are two mutually exclusive parts of the PCV system that should NOT be combined.
I am feeling really ignorant here… could you please explain your question so I can understand what you are asking / stating?

Thanks,
Team Crawford
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Old 07-23-09, 09:52 AM   #49
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Quote:
Originally Posted by CrawfordPerformance.com View Post
I am feeling really ignorant here… could you please explain your question so I can understand what you are asking / stating?

Thanks,
Team Crawford
A closed PCV system has two separate parts.
  1. Clean side
  2. Dirty side

The clean side brings fresh metered air into the crankcase. The dirty side evacuates combustion by products out of the crankcase.

Your design joins the two sides at the AOS. The issue with this is reversion and system flow. Most stock PCV systems barely have the required capacity to properly evacuate the crankcase and keep pressurization at bay. Joining the two sides reduces the number of paths and causes issues due to reversion on the clean side. Maintaing both sides as separate parts is very important to system efficiency.
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Old 07-23-09, 10:28 AM   #50
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Default Re: The Official STi engine Venting, A/O Separators, and Catch Cans Thread!!

Quote:
Originally Posted by 405HP_Z06 View Post
The clean side brings fresh metered air into the crankcase.
Can you explain where this clean side is on the STI? where does the clean air enter the crankcase?
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