Crankcase breather question

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71DodgeDemon340

71DodgeDemon340

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I just want some clarification, im running stock valve covers on my 340. On the drivers valve cover i have my pcv valve and the hose routed to the carb. On the pass side valve cover i have a edelbrock breather with a hose going to the air cleaner. Now in theory the pcv valve opens with a vacuum from the carb and by doing that then draws clean air through the pass side breather? I thought the pcv valve vented the crankcase and the other breather vented into the engine? How does the pcv valve allow air to flow through the other breather into the engine with air going into the carb at the same time?
 
PCV uses vacuum to vent the crankcase, where the breather is a passive system, older ones just vented to the atmosphere, emissions regs changed that requiring them to feed into the air cleaner to be reburned.
 
how is air going through the pass side breather thats connected to the air cleaner? I figured the carb would be using most of the air entering the air cleaner and that breather wouldnt be receiving much of any
 
There is some pressure created by the running engine, as well as a minor amount of draw through the air cleaner as the engine runs.
 
So the pressure created by the engine helps draw air into the pass side breather when the pcv valve is open and vacuum is applied to the pcv on the drivers side from the carb?
 
Just like the video above. I do better with pictures. lol

PCV SYSTEM.jpg
 
Thanks for the video link, exactly how i was thinking it worked, how does higher rpm affect the way this system works? Also doesnt certain cams affect the system as some high performance cams dont produce as much vacuum as others? Or is there always vacuum present at the carb regardless of cam?
 
And don't forget; when the engine is reving and working hard, it will create far more blow-by gasses than the PCV can get rid of. They have to somewhere or the pressure will make it it's way out; usually starting with blowing the dipstick out.
The factory routes this extra blow by out the closed system breather, up the hose and into the air cleaner box, where the carb will now suck it in from the top; simply ingenious.
Along comes Joe HotRod and installs a; pretty,chromed up, open faced,air cleaner, and throws the hose away. Now when he gets on it, the factory CC breather pukes it's vapors all over everything. So he goes and gets a shiny Mr.Gasket breather, and limits the mess to on the valve cover, and then gravity and the cooling fan blow it onto the sparkplugs.
What's the solution? IDK.
What I did was; I installed file-fit Plasma Moly rings and limited the ring leakage to almost immeasurable. Then I just re-routed the output from the factory breather thru a hose to the passenger-side shock-tower where it will I suppose, one day,puke onto the tire. So far, at 60,000 miles it hasn't yet started to.
But I have a back-up plan; I left the dipstick a lil loose-fitting,lol.
 
71DodgeDemon340 said:
Thanks for the video link, exactly how i was thinking it worked, how does higher rpm affect the way this system works? Also doesnt certain cams affect the system as some high performance cams dont produce as much vacuum as others? Or is there always vacuum present at the carb regardless of cam?
Click to expand...

Vacuum drops out on acceleration.
 
71DodgeDemon340 said:
Thanks for the video link, exactly how i was thinking it worked, how does higher rpm affect the way this system works? Also doesnt certain cams affect the system as some high performance cams dont produce as much vacuum as others? Or is there always vacuum present at the carb regardless of cam?
Click to expand...

it does! Good questions. Balance is hard, the correct PCV helps based on such. Do not think there is a hard set high science to apply here and I have always experimented. Those high price adjustable units seem smart but, catch can assemblies and remote PCV help some overcome oil pickup. I have always been able to find a PCV that works for me and helps keep the engine clean and no leakage, blow out. Good Baffles in the covers Like the originals and cross flow are instrumental. JMO
 
AJ/FormS said:
And don't forget; when the engine is reving and working hard, it will create far more blow-by gasses than the PCV can get rid of. They have to somewhere or the pressure will make it it's way out; usually starting with blowing the dipstick out.
The factory routes this extra blow by out the closed system breather, up the hose and into the air cleaner box, where the carb will now suck it in from the top; simply ingenious.
Along comes Joe HotRod and installs a; pretty,chromed up, open faced,air cleaner, and throws the hose away. Now when he gets on it, the factory CC breather pukes it's vapors all over everything. So he goes and gets a shiny Mr.Gasket breather, and limits the mess to on the valve cover, and then gravity and the cooling fan blow it onto the sparkplugs.
What's the solution? IDK.
What I did was; I installed file-fit Plasma Moly rings and limited the ring leakage to almost immeasurable. Then I just re-routed the output from the factory breather thru a hose to the passenger-side shock-tower where it will I suppose, one day,puke onto the tire. So far, at 60,000 miles it hasn't yet started to.
But I have a back-up plan; I left the dipstick a lil loose-fitting,lol.
Click to expand...


Ahhh so its a two way system then, at idle and light acceleration the pcv valve opens which pulls a vacuum and the gasses go out the pcv valve which in turn draws fresh air through the pass side breather. On hard acceleration the pcv valve is closed due to low vacuum and the gasses exit the pass side breather and go into the carb.
 
71DodgeDemon340 said:
Thanks for the video link, exactly how i was thinking it worked, how does higher rpm affect the way this system works? Also doesnt certain cams affect the system as some high performance cams dont produce as much vacuum as others? Or is there always vacuum present at the carb regardless of cam?
Click to expand...
If the engine is running, the pistons are pulling in air. How hard they are pulling, can be measured with a vacuum gauge. High vacuum pulls the PCV plunger up to restrict the metering into the manifold to a minimum amount. At very-low vacuum the plunger drops down, and more or less closes the valve. At inbetween settings, the valve will be somewhere in between and could be jumping around.( You'll hear it when it does that,lol) With HotRod engines idling at 10 to 13 inches, this plays real nice with the PCV, as it can be used as your principal idle air bypass.
Depending on your idle-rpm, a 292/292/108 cam can idle between 8 to 10 inches on the vacuum gauge. A 270 might be 12-14. A 260 about 16/17, I'm guessing. The usual range is 8 to 18 @~1000 ft elevation, but a sharp hi-compression, factory cammed teener might pull 20 or a tiny bit more.
The bigger the cam (later Ica), the lower the idle vacuum will be.
 
71DodgeDemon340 said:
Ahhh so its a two way system then, at idle and light acceleration the pcv valve opens which pulls a vacuum and the gasses go out the pcv valve which in turn draws fresh air through the pass side breather. On hard acceleration the pcv valve is closed due to low vacuum and the gasses exit the pass side breather and go into the carb.
Click to expand...
You sorta got it. But you got your words a lil mixed up here;at idle and light acceleration the pcv valve opens which pulls a vacuum. It should read something like;
When the engine pulls a vacuum ( as in the doggone thing is running,lol), like at idle and light acceleration, the pcv valve opens.
Otherwise you aced it.
 
AJ/FormS said:
You sorta got it. But you got your words a lil mixed up here;at idle and light acceleration the pcv valve opens which pulls a vacuum. It should read something like;
When the engine pulls a vacuum ( as in the doggone thing is running,lol), like at idle and light acceleration, the pcv valve opens.
Otherwise you aced it.
Click to expand...

lol i meant the vacuum pulling the pcv valve open allows a vacuum to be pulled on the crankcase
 
Thank you, so is there any downside to the cheap pcv valves? Any need for a adjustable pcv valve?
 
Ok here is the PCV question of the century...

Why do 67 dart (maybe others) PCV valves have a stamped steel nut on them?

s-l400 (1).jpg
 
Lol i knew that was coming, reason im asking is because there seems to be so many pcv valves out there how do you know which one to use? I have a generic one now from the parts store. Only seems to be either closed or open when i sucked air through it using my mouth.I am running stock valve covers with baffles and i noticed some oil residue on the pcv valve nipple. How do you know if your getting too much oil through the pcv valve? Naturally theres going to be a light oil residue in the hose right?
 
There are different PCV valves for performance motors vs standard.
nm9stheham has some knowledge on the subject maybe he will chip in ?
 
Well, I just know a few. They variations are:
  • The vacuum level at which the PCV valve transitions from low suction (idle) mode to high suction mode
  • The size of any restriction inside the PCV.
The pricey ones allow you to adjust both in the same unit. But you can find ones that have varying transition vacuum levels. That is the most important thing for most of the engines here.

The stock Mopar PCV transitions from low to high suction mode at around 14" of vacuum. But a lot of cams are idling with less vacuum than that. So the stock Mopar PCV will be wide open at idle with a moderate cam and letting too much air into the intake and making idle and transition tuning difficult.

So the objective is to find a PCV that transitions from low to high flow at a vacuum level below your engine's idle vacuum. MP makes one (can't recall the number) and we used a Fram FV191 IIRC, which is a GM one for performance V8's in the 70's-80's. (But you have to change your valve cover grommet size for that one.) There are a few PCV valve threads here that, if you search for them will give more info.

If you want to limit the PCV flow then a simple restrictor in the hose works. But I have not seen the need for that so far. I mess with 70's Opel and the 'factory PCV' was simply a small restriction in a hose nipple.

And BTW, at WOT there is no 'shut-off' of the PCV that I am aware of.... it just stops flowing much simply because it does not have the vacuum drop in the intake to be effective.
 
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While expensive, this is where a vacuum pump starts to make a lot of sense.

I'd also like know, if anyone can answer it, why you can run an evac system if you run mufflers. Also, do glasspacks count as mufflers?
 
roccodart440 said:
While expensive, this is where a vacuum pump starts to make a lot of sense.

I'd also like know, if anyone can answer it, why you can run an evac system if you run mufflers. Also, do glasspacks count as mufflers?
Click to expand...

I have been playing w/ the pcv system some. I think the breather to carb hose deal will pull more out of the engine than we realize.
All street hemi`s came w/ a pcv on the drivers side and a breather hose to air cleaner on the pass. side . "Good enough for me !"
I just recently rigged up a oil mist catch can , because I had the junk laying around, it has a site tube to check oil level . Altho I haven't had any problems in that area , or any oil consumption , or blow by on the valve covers with just a breather on one side , will probly remove it after a while.
What I do wonder is , what effect my 6 pack scoop will have on the pass. side vent/breather to air cleaner deal. Any thots on that ??
 
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