PCV system question

[neoviper]

Im in the process of designing a pcv system so ive been doing some research. I know that when the intake manifold sees vaccum the pcv valve opens and crankcase vapors are sucked out, all while fresh air is being pulled in from the pre-turbo intake pipe. under boost the vaccum created by the turbo sucks the vapors out of the valve cover, but theres no fresh air being supplied to the VC like when the IM saw vaccum.

My question is this, why does fresh air need to be pulled into the VC in the one instance but not the other?

 

[Calan]

http://www.dsmtuners.com/forums/articles-engine-fuel/366890-4g63t-pcv-system.html

 

[neoviper]

Thanks for the link, ive actually read that article several times before but i cant find the answer to my question in it. I quess what im trying to ask is why does the valve cover need to be "replenished" with fresh air under vaccum but not under boost? Would it be beneficial to add a line that allows it to draw fresh air in under boost as well? Why does the air need to be replensihed in the first place?

 

[90TSI717]

Thanks for the link, ive actually read that article several times before but i cant find the answer to my question in it. I quess what im trying to ask is why does the valve cover need to be "replenished" with fresh air under vaccum but not under boost? Would it be beneficial to add a line that allows it to draw fresh air in under boost as well? Why does the air need to be replensihed in the first place?

Everything is in that link. Just read a little harder.

 

[neoviper]

The article says that fresh air is drawn in to replace the air that was sucked out through the IM, thus the term ventilation. It doesnt say why there isnt a source of fresh air to replace the air being sucked out under boost. Im trying to find out what the reasoning is behind this, maybe it is in the article but ive read it through several times and cannot find it.

In the past i have used this article to design a pcv setup, and recently when i revisited the design i referred back to this article. Im trying to find out what difference (if anything) in the conditions produced under vaccum vs boost cause it to need a fresh source of air under vaccum. Under boost theres alot more vapors being drawn out than under vaccum, so would a fresh source of air to the valve cover be in order? Im just trying to get a little deeper understanding on the subject.

 

[90TSI717]

Yes, you need fresh air in the system. The air can contaminate the oil if not purged out and its used for emissions too. If the car is under boost, the turbo is used to help pull out excess crankcase vapors.

 

[neoviper]

I understand the point of not allowing crankcase vapors to pollute the engine oil, but under one condition (vaccum) fresh air from the intake is being used to replenish (ventilate) the air that was lost from vapors being pulled out. However under the second condition (boost) there is no such air being replaced (ventilated), rather the system solely helps to evacuate extra pressure. Using the reasoning that coditions under boost are much more extreme than under vaccum, can i assume that if its safe to not replenish the air removed during boost Than its safe to not replenish the air under vaccum? If thats the case than i can make a setup that only uses one catch can in the system. If this assumption is wrong than would i benefit by tapping an additional breather tube to allow the VC to replenish evacuated air under boost (ventilate)?

 

[tragdor]

I believe that the "fresh air" is really just additional vapors and gases passing through the system. The PCV valve really operates in only one direction. To vent the gases into the Intake Manifold. So the fresh air is only the metered air entering the system from the IM. How does it get back into the top of the head and valve cover? By drawing up just like the previously vented vapors. I would think that if you add an additional true fresh air port to the valve cover, that you would be neutralizing the vacuum that the engine and turbo are generating, reducing the engine's ability to pull these vapors out.
Hopefully that helps. At least it wasn't "read a little harder". Seriously, that's absolutely useless.

Let's hope that we can get Calan to chime in as he has an expert understanding of the system.

 

[90TSI717]

Sorry neoviper, didnt fully understand you question at first. Tragdor's post does make sence but we'll see.

 

[tragdor]

^^^Wasn't trying to bust your balls man, just more joking than anything.

 

[Calan]

The fresh air really is "fresh air"...it is being pulled from the intake before the turbo and circulated through the motor (at least the top part of it at the VC) as vapors are drawn out under vacuum into the IM. Keep in mind that this is a low amount of airflow, since there usually isn't much blowby when operating under vacuum. But as some air is drawn off, fresh air has to come in to replace it to maintain a balance.

When under boost, pressure in the crankcase can become excessive because of the increased cylinder pressures and gases escaping past the rings. This is bad for several reasons as explained in the article. So the point here is to just get rid of the pressure. You wouldn't want to replace the excessive pressure even if you could somehow.

Here's the best analogy I can come up with at the moment. When operating below boost, the PCV system works like a straw; as you suck on it, air has to enter the other end or else it will collapse (ehhh...your motor won't collapse...but you get the idea . When under boost, think of a balloon; if you don't have some way to relieve the pressure you are blowing into it, eventually it will pop (just like your dipstick).

 

[ulitka]

I built my PCV system where I am not using a PCV valve and do not connect any PCV hoses to intake manifold, instead I have two hoses from valve cover ports connected to an oil catch canister and from oil catch canister two more hoses are connected to an intake pipe. So, my PCV setup does not end up allowing any fresh air getting into valve cover and when turbo spools it sucks out like a vacuum cleaner all of the blow by gases there may be in the engine.

 

[Dragon34x]

I built my PCV system where I am not using a PCV valve and do not connect any PCV hoses to intake manifold, instead I have two hoses from valve cover ports connected to an oil catch canister and from oil catch canister two more hoses are connected to an intake pipe. So, my PCV setup does not end up allowing any fresh air getting into valve cover and when turbo spools it sucks out like a vacuum cleaner all of the blow by gases there may be in the engine.

That is how it is supposed to be
(add check valves if you don't have any)

Internally, an engine operates most efficiently under vacuum. At the same time, too much vacuum can be bad. Search online, you'll be surprised at how much information is out there on this subject

 

[neoviper]

Thanks calan! That the exact kind or information i was looking for. Very helpful. Can you forsee any issues if i tied the pcv and breather tube into one line going to a catch can and than ran two seperate lines from the catch can to the IM and turbo using check valves in each line to allow it to switch its source of suction when under boost or vaccum? Obviously this would eliminate the fresh air supply during vaccum.

 

[Calan]

Can you forsee any issues if i tied the pcv and breather tube into one line going to a catch can and than ran two seperate lines from the catch can to the IM and turbo using check valves in each line to allow it to switch its source of suction when under boost or vaccum?

It won't work that way. Read the article again and look at the diagrams.

If you aren't worried about oil contamination (i.e. you change it every 1000 miles or so or have it tested regularly, then you can remove the PCV, block the IM port, and just run the pressure evacuation side of the system. But there really is no good reason to do this on a street car that I can think of.

I still have a PCV valve (with an added check valve) plumbed into one of my small cans and then to the IM, and a single -6AN line going to one of my cans and then to the turbo inlet. I have ZERO oil in the IM, ZERO oil in the turbo inlet, and the most CC pressure I've seen is about .5 PSI at 26 PSI of boost or so. It usually sits at 0 psi or a little below. And my oil stays nice and clean.

I really don't understand why every month or so people try to re-invent the damn PCV system instead of just going with what is proven to work on various setups. (nothing personal OP ))

 

[GORBS88]

The fresh air really is "fresh air"...it is being pulled from the intake before the turbo and circulated through the motor (at least the top part of it at the VC) as vapors are drawn out under vacuum into the IM. Keep in mind that this is a low amount of airflow, since there usually isn't much blowby when operating under vacuum. But as some air is drawn off, fresh air has to come in to replace it to maintain a balance.

When under boost, pressure in the crankcase can become excessive because of the increased cylinder pressures and gases escaping past the rings. This is bad for several reasons as explained in the article. So the point here is to just get rid of the pressure. You wouldn't want to replace the excessive pressure even if you could somehow.

Here's the best analogy I can come up with at the moment. When operating below boost, the PCV system works like a straw; as you suck on it, air has to enter the other end or else it will collapse (ehhh...your motor won't collapse...but you get the idea . When under boost, think of a balloon; if you don't have some way to relieve the pressure you are blowing into it, eventually it will pop (just like your dipstick).

THAT IS by FAR the best way to explain it, makes the most sense for people to understand.

Like stated later in the thread,i also wonder why people try to or want to eliminate the PCV. I'm not saying you NEED it but by keeping it and just running some larger hoses to a catch can and then to the intake pipe works well. You can then benefit from BOTH, keeping your oil clean and stopping the stupid dipstick from hitting the hood!

 

[neoviper]

Thanks again calen, it was very helpful. The cars oil is RELIGOUSLY changed every thousand miles if not sooner (kinda over obsessive about it) so im not too concerned about oil contamination. Im just getting the car ready for bigger boost and trying to clean things up under the hood. Thanks again

 

[ulitka]

That is how it is supposed to be
(add check valves if you don't have any)

Internally, an engine operates most efficiently under vacuum. At the same time, too much vacuum can be bad. Search online, you'll be surprised at how much information is out there on this subject

Why would I want a check valve if I am not connecting to my (SMIM) intake manifold? Even with my old 2G intake manifold I had both vacuum hoses connected to an intake pipe and intake manifold port I blocked. Under boost you can evacuate more blow by pressure (fumes) through two hoses connected to an intake pipe VS. stock PCV setup with one hose connected to intake pipe and other hose to an intake manifold which stays closed under boost naturally because of the PCV check valve.

 

[prockish_90gsx]

take calan word on this his crankcase vent system is by far one of the most researched and tested theory around and i have a similar set-up on my fully built evo and works amazing!!!

 

[ulitka]

take calan word on this his crankcase vent system is by far one of the most researched and tested theory around and i have a similar set-up on my fully built evo and works amazing!!!

Calan has an awesome write up about PCV system. My PCV setup happens to be identical as described in "PCV Setup #3 – Maximum Pressure Relief" of Calan's write up.