PCV for turbo

Rt Jam

G-Body Guru
Mar 30, 2020
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For a 6L LS, single Precision 76 75. 12 psi. Holley Terminator Xmax.
One popular option is to vent to atmosphere. Great for engine seals as the crankcase will never see pressure.


How about for a street car that I want to have like a PCV system that evacuates the crankcase vapors?
If you have vacuum pulling dirty air through a catch can of one side with a check valve, then a inlet in the other side pulling from fresh, filtered and metered air in the air inlet tube before the throttle body. How to vent the crankcase during boost?
 

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81cutlass

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Feb 16, 2009
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Catch can.

I don't like venting to atmosphere straight because there is HP and oil cleanliness advantages to running a PCV system. You just need to tap a clean air source into the valve cover that leads to the air intake tube pre-filter.

These are fighting words, but a street car that vents to the atmosphere is wrong. The catch can is just a way to clean the oil vapors out of the crank case. I use steel wool packed in a cheap $25 fleabay catch can.



Catch-can.png
 
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Rt Jam

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Mar 30, 2020
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Thanks 81, that is something that I considered. I also want clean air evacuating my crankcase.

I guess the only question left is:
Is using the 3/8" line from the valve cover big enough to deal with pressure, I think so.
This must be filtered.
 

64nailhead

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Dec 1, 2014
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I’ve ran both open and PCV on multiple setups. Both have a place and both have issues. The best of both worlds is to have your line from the valve cover to the catch can having a vacuum pump pulling on another port on the catch can. But that is more components (more points of failure).

‘Fresh air evacuating my crankcase’ - impossible unless you have an air pump pushing air into the valve cover or valley cover.

PCV on a motor that is pushed hard consistently is a mistake - ask me how I know. But I agree 100% that street cars need them to stop the stench.
 

64nailhead

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Dec 1, 2014
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Why impossible?
Vacuum can evacuate.
Where is the vacuum/fresh air coming from? Keep in mind that on every compression stroke a percentage of the cylinder pressure escapes past the rings into the crankcase. What goes past the rings is at an extremely high pressure, until it gets past the bottom ring. Once the leaking cylinder pressure gets below the ring it expands (PV=nRT) causing a positive pressure on the crankcase - not the same pressure as is in the cylinder, but much higher than atmospheric pressure.

The only chance that you have of getting fresh air into the crankcase is at high vacuum conditions (idle) and you have to introduce the air into the crankcase via the block or valley cover - essentially, below the heads. If you were try that with a boosted application, then hold on for the amount of oil residue that will be pushed into the intake or into a catch can.


Like I mentioned previously, I've ran PCV successfully, but you have to tune for it. Oil ridden air has much less detonation resistance when you lean on the boost. I'm not saying that PCV won't work with a turbo, but be careful using it at boost levels over 14-18psi. With a normal LS PCV system, the air that is being evacuated from the crankcase is not fresh and the source of the air entering the crankcase is not fresh, but rather air mixed with exhaust that is passing by the rings.
 
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64nailhead

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Dec 1, 2014
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Catch can.

I don't like venting to atmosphere straight because there is HP and oil cleanliness advantages to running a PCV system. You just need to tap a clean air source into the valve cover that leads to the air intake tube pre-filter.

These are fighting words, but a street car that vents to the atmosphere is wrong. The catch can is just a way to clean the oil vapors out of the crank case. I use steel wool packed in a cheap $25 fleabay catch can.



Catch-can.png


Jake,
I used to run the set up your attachment identifies. You are aware that the stinky open line is still emitting stinky air when the you're in boost. But you usually don't have your nose anywhere near there when in boost haha. That line will have oil residue, either sticky or wet, whenever you check it.

Here is what I know that system created, when I used the port on top of the center of the intake for the PCV vacuum source, the rear cylinders' intake ports and head intake runners were covered with oil/oil residue; the rear plugs indicated oil in the combustion chamber. As mentioned in the previous post, that can be tolerated, but be careful leaning on it with that residue/oil entering the cylinder.


p.s. - no fighting words buddy, just friendly discussion of varying opinions (as it should be)
 
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Rt Jam

G-Body Guru
Mar 30, 2020
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What this discussion is missing is there are 2 scenarios like 81cutlass's diagrams.

In vacuum, the fresh air is entering the Stinky side like. This will be filtered and the OEM plumbs this into the tube between the throttle body and air filter, no stink.

In boost, there should be no pressure getting past the check valve, all the blow by will exit the Stinky side, granted it's big enough for the blow by which depends on boost and engine condition. I will be under 15psi.

I understand the potential for oil to enter the intake vacuum source but that's where I hope the catch can does it's job.

To further confuse the issue, I found this write up on LSTech.
https://speedtechperformance.com/wp-...entilation.pdf
 

64nailhead

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I don't buy what's in that article. Sucking air into the crankcase due to a pressure difference at the filtered air inlet vs. a port behind the throttle plate will lead you to an oily mess in your intake piping with a boosted application (except in one short period of time during engine warmup when the turbo's turbine is cold).

Here's the test that I ran to check on this setup - take the open line from the valve cover and run a piece of vacuum hose to it that is long enough to reach inside the car and hookup a vacuum gauge. Now go drive the car and see how much vacuum is coming from your crankcase and the amount of time that there is vacuum, and compare that to the amount of pressure read on that gauge and amount of time it is pressurized. GM and International/Ford tried this with TDI and TE444/7.3 Powerstroke respectively. These things have the oilyest turbo inlet you'll find in a turbo diesel application because they are using the system that this article describes minus a thottle plate.

You'll discover that the amount of time that you see vacuum will be so short and create so little vacuum that the effect won't move enough air to be worthwhile. But the constant pressure during normal driving will force the dirty air from the crankcase will be reintroduced into your motor (bad for fuel economy). And now check it when in boost, when you need the most uncontaminated air possible, and you'll be amazed. At least this is what my testing has found.

Respectfully, you can do whatever you choose to do and I'm not going to say that it is wrong for your setup.


In the article you attached, the discussion about ring gap and what occurs through that gap is hooey - to a point. The difference in.003-.008" is so minimal that it is not detectable via measurements of HP/TQ or blowby. Obviously if you increase you ring gap to .150", then you would have a measurable difference.
 
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