Why does a PCV need a breather at all?

[davepl]

Why don't we just run full manifold vacuum to the crankcase -without- a PCV, or even without a breather, so that it builds up a full 20 inches of vacuum in the crankcase?

Is the fresh air system an emissions thing? What does that accomplish?

As I have my PCV on my LS motor right now it's capped off, so I get massive vacuum in the crankcase. That's not "correct", I need to open one bleed port to the atmosphere.... but why?

My best guess is that even under vacuum, without a fresh air path, you've still got blowby gasses that lead to sludge and acidity, just at a lower level because it's under vacuum. With a fresh air intake, however, that's always replaced by fresh air. But that's totally a random guess.

[dmjlambert]

I think your random guess is pretty good. Get those fumes out of there and burn them. I read an article about PCV being good for the environment and increases mileage and engine life. Plus the breather makes you engine look cool. So it seems there is really no reason not to do it. It's a win win win win situation.

[trac209]

Because at full vacuum you would pull the seals in and that's no good

[hamjet]

in the late 50s and early 60s they just used a road tube out the back of the intake down to the ground. back then the roads had a black oil slick on the road from all the vehicles. the engine has to have some kind of escape for the + and - pressures caused by vacuum and pistons going up an down. as pistons go up they suck air upward from the bottom, on the way down the cylinder it fills the oil pan area, with out some place for pressures to escape could be bad. a pcv basically pulls the gasses from the engine and sends it back to reburn oil vapors and such..

[garyd1961]

Originally Posted by hamjet

in the late 50s and early 60s they just used a road tube out the back of the intake down to the ground. back then the roads had a black oil slick on the road from all the vehicles. the engine has to have some kind of escape for the + and - pressures caused by vacuum and pistons going up an down. as pistons go up they suck air upward from the bottom, on the way down the cylinder it fills the oil pan area, with out some place for pressures to escape could be bad. a pcv basically pulls the gasses from the engine and sends it back to reburn oil vapors and such..

Wouldn't the pistons just cancel each other out. I mean as one piston goes down another goes up.

[Gimme Jimmy]

On any engine there is a certain amount of blow-by exhaust gas that escapes past the piston rings. On older (pre '60s) vehicles this was vented it to the atmosphere via the filtered breather cap and the "road Draft" tube.
Since then the EPA has outlawed this setup. The PCV (Positive crancase ventilation) valve and attaching hose draw up the crankcase gasses routes the fumes into the intake where it is drawn into the engine and burned. To complete the PCV circuit there is usually another hose from the crankcase to the inside of the air cleaner to catch any fumes that may escape. This setup is much cleaner than the old breather cap and there is no "smoke" emanating from the engine compartment at idle. Your engine also appears cleaner with a PCV system as the oily fumes from the crankcase do not condense in the engine compartment.

[davepl]

Originally Posted by garyd1961

Wouldn't the pistons just cancel each other out. I mean as one piston goes down another goes up.

Yes, I agree. It's stuff that gets past the rings (blowby) the fills up and contaminates the crankcase air. You can suck most of it out with vacuum, but without a fresh air source, you can't "flush" it all through.

At least I think that's what we've determined here!

[garyd1961]

Originally Posted by davepl

Yes, I agree. It's stuff that gets past the rings (blowby) the fills up and contaminates the crankcase air. You can suck most of it out with vacuum, but without a fresh air source, you can't "flush" it all through.

At least I think that's what we've determined here!

Yes, I know about blow-by but I was referring to the statement by hamjet that the pistons moving up and down created pressure that had to go somewhere.
I said wouldn't they cancel each other out as one piston was going down another would be going up.

[RichardJ]

>>Why don't we just run full manifold vacuum to the crankcase -without- a PCV, or even without a breather, so that it builds up a full 20 inches of vacuum <<

The short answer is that you don't always have 20 in of vacuum. You have 20 in at idle, but at idle, blow-by is the lowest at that time. Full manifold vacuum to the crankcase would just be an unregulated VACUUM LEAK.

The PCV valve regulates the flow through itself, depending mainly on engine load and operating condition.

At WOT and Heavy Load, there is minimal intake vacuum and at the same time, maximum crankcase blow-by. Even though the PCV Valve is open, there is often more blow-by than can pass through the PCV Valve.
During this condition, crankcase blow-by will flow backward, through the hose to the air cleaner at the top of the carb. During this condition, air flow through the carb is high and this will pull blow-by out of the crankcase, through the carb and into the engine.

This is called a CLOSED PCV System.

Placing a breather on a valve cover, turns your PCV System into an OPEN System and will admit crankcase fumes into the engine compartment.

[davepl]

Originally Posted by garyd1961

Yes, I know about blow-by but I was referring to the statement by hamjet that the pistons moving up and down created pressure that had to go somewhere.
I said wouldn't they cancel each other out as one piston was going down another would be going up.

And I said "Yes, I agree". The volume of the crankcase doesn't change, but it sure churns around!

[hamjet]

garyd1961 , I agree on piston pressures do cancel each other out, (after deep thoughts,lol) but even with a fresh or new engine there will be blow by, thats why you have to vent the bottom end some way..

[davepl]

A whole 'nother problem is that the PCV used to be a carefully calibrated part of the induction system, tuned to the carb. Now we just grab a generic one off a rack somewhere and call it a day.

There are aftermarket billet fancy ones that you can adjust the bleed and flow rates, etc, that would probably be a good idea... if you could set it up accurately.

http://mewagner.com/?page_id=444