PCV system questions

[onemancrew]

But you can add a catch can "inline" with th stock system .... u will need to extend hoses

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Cool, Thanks, Seems pretty simple.

 

[Cyrus]

read through the thread... i think i understand how the PCV system works and based on that understanding, this is how i would install a catch can if i had an aftermarket intake manifold without the specially sized/designed "orifice" on the stock 7m intake manifold:




the pcv valve would allow flow FROM the catch TO the throttle body, blocking reverse traffic.

Catch can -> TB = YES!
TB -> Catch Can = NO!

edit: This setup can also incorporate another valve between the catch can and the turbo inlet allowing flow FROM the catch can TO the inlet.
Catch can -> inlet = YES!
inlet -> catch can = NO!

 

[MarkIII4Me]

That's basically how I ran mine, except I have a ffim. I still don't understand how the TB isn't just pulling air from the accordian hose when under vacuum rather than the valve covers. You'd think a one-way check valve should be used on the accordian pipe as well to prevent this. I was told not to use one there so I didn't, but I still don't get it.

 

[onemancrew]

read through the thread... i think i understand how the PCV system works and based on that understanding, this is how i would install a catch can:

View attachment 57702

and everything else stays the way it was. does that seem correct?

I reckon thats right,, nice detail on the pic,, its the best ms paint diagram ive seen yet.

 

[Cyrus]

That's basically how I ran mine, except I have a ffim. I still don't understand how the TB isn't just pulling air from the accordian hose when under vacuum rather than the valve covers. You'd think a one-way check valve should be used on the accordian pipe as well to prevent this. I was told not to use one there so I didn't, but I still don't get it.

I think you are right, the valve could (and should) go between the catch can and the accordion pipe, allowing air flow FROM the can TO the accordion pipe and thus TO the turbo inlet. It seems like overkill, as I can't imagine a situation where the accordion pipe would be under positive pressure, but really, placing a valve in this line is the ultimate and correct way to protect and facilitate the functions of the PCV system. this allows positive pressure from the TB to exit into the accordion pipe and when the TB is at negative pressure, it will maximize scavenging from the valve covers and prevent air from being sucked into the Can/TB from the accordion hose.



i'm pretty sure this is what jdub described in post #42 of this thread. so yea, thanks jdub!

great thread, very informative.


new, updated diagram:



edit: this is what it should look like when adding a pcv valve to a system using the stock intake manifold.

 

[MarkIII4Me]

You must have misunderstood. I was saying that it would make sense to add a check valve to the accordian hose IN ADDITION to the check valve to the throttlebody/intake manifold. Your latest diagram would have you pressurizing the catchcan under boost, or at the very least, have a massive boost leak through the accordian hose.

 

[Cyrus]

i dont see how the valve is necessary between the throttle body and the can/accordion hose. the stock system allows the TB to vent positive pressure into the accordion hose, right? my understanding is that it is a "metered" amount based on size of the orifice. maybe at much higher boost applications, a valve between the TB and can would help keep the released pressure proportional to the amount the orifice would release at stock boost levels? i think someone else mentioned that earlier, hopefully they can chime in again to clarify.

my diagram is intended as a schematic for incorporating a catch can into a more or less stock 7m. the stock system doesnt use a valve, so to me, the only reason to incorporate a valve is to encourage suction from the catch can and thus the valve covers by the throttle body. with that valve in place between the accordion hose and the catch can, it wont ever be easier (or possible) for the TB to pull air from the accordion hose instead of the valve covers, and the occasional positive pressure release from the TB can still escape to the accordion hose instead of the valve covers, as was the original plan by whoever engineered this system.

that is how i see it anyway. what say you?

 

[MarkIII4Me]

Like I said, I run a ffim. Not using stock tb or turbo or boost levels for that matter. So pretty sure a check valve is needed run from the ffim to the catchcan. IJ's setup uses one there.

 

[Cyrus]

ok so here is my theory with your setup. a valve between the accordion and can isnt necessary because it's fine for you to pull air from the accordion straight to the throttle body. that air is already accounted for by the MAF/AFM, it just hasnt gone through the more typical route of compressor -> intercooler -> TB. also, i seem to remember reading somewhere that in moments of negative pressure at the TB, the movement of air from the accordion to the TB past the valve cover outlets actually helps the circulation of the vapors coming out of the crankcase. the flow of air from the accordion to the TB is just movement of air from a higher to lower pressure area, and if it is drawing from the accordion, it will also draw from the crankcase. in fact, crankcase vapors would be the first to be sucked in by the TB considering that the pressure inside the valve covers will be higher than the pressure in the accordion hose, thus forcing the crankcase vapors into the TB inlet hose before or with any air pulled from the unpressurized accordion hose.

so the only valve you need is the TB -> Catch Can, to protect against positive pressure from the TB going back through the system, lowering boost pressure and possibly blowing up the catch can if the air flow exceeds the limits of the hose connecting the can to the accordion hose.

that being said, i dont see any reason why it would be BAD to have a valve preventing inward airflow from the accordion hose.

hopefully one of these knowledgeable gurus will step in here at some point and produce a real answer. ill shuddup now since im pretty much just guessing...

 

[MarkIII4Me]

^
That's pretty much where I am with all of this. Unmetered air isn't an issue for me since I'm now on standalone and pressure readings are taken from behind the tb butterfly. Thinking of adding a check valve to the accordian hose to provide maximum vacuum on the valve covers at idle, although like you said, it may not be needed. Wondering if it will have negative effects on the system tho. Is it possible to pull too much vacuum from the valve covers? I'm pulling 21in/hg at idle.

 

[FuzzayD]

I've had my can vented to atmosphere for awhile and it just reaks of fuel and gives me that horrid wet looking black oil. Also has alot of condensation develop. Stock system is best. maybe just a can T'd into the suction of the air inlet. /wrists
But in your pics there's no where that the suction is drawing a source of air through, one of your valve cover ports should just go to before/after the afm. That way fresh filtered air is drawn in to replace the crank fumes.