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  #1  
Old 06-23-2011, 03:42 PM
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Valve cover breather VS. PCV valve? Why is there a difference?

I was wondering what happens to a vehicle when you change your original OEM valve covers (that have grommets, a PCV valve that goes to the carb and a line from the other valve cover to the air cleaner) to "performance" valve covers?

I have a set of valve covers from Edelbrock, but both have a breather on them, and no PCV grommet or opening.

I'm wondering what happens when you replace the original valve covers with ones like I've got, and what you need to do with the holes in the air cleaner and carb.

Do the holes in the air cleaner and carb need to be plugged? Do I need to take the breathers off the Edelbrocks and put grommets on?

How does this affect performance? What's the difference?

Just a few (lol) questions I've got, cause I want to start setting up my new parts on my 351W.
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Old 06-23-2011, 03:47 PM
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You should get a grommet for the new valve covers and hook up the PCV just as it was with the old valve covers.

Breathers without PCV will get you oil residue puked on the valve covers, and no real evacuation of fumes from the engine. That could lead to sludge build up.
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Old 06-23-2011, 03:56 PM
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You should get a grommet for the new valve covers and hook up the PCV just as it was with the old valve covers.

Breathers without PCV will get you oil residue puked on the valve covers, and no real evacuation of fumes from the engine. That could lead to sludge build up.
Why the heck were breathers even produced on valve covers then? What's the difference?

I don't see why ANYONE in their right mind would run breathers if the engine, in a sense, can't "breathe"!
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Old 06-23-2011, 04:45 PM
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Correct me if I'm wrong but the breather let's air in and the Pcv sucks air out. Breather on one side Pcv on the other. In essence evacuating the air inside the valve train and oil galley.
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Old 06-23-2011, 04:56 PM
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Correct me if I'm wrong but the breather let's air in and the Pcv sucks air out. Breather on one side Pcv on the other. In essence evacuating the air inside the valve train and oil galley.
That's the way I understand it too. Because when my 302 had stock valve covers on it the PCV valve was in the drivers side connected to the intake manifold under the carb and the passenger side hooked to a nipple on the air horn of the carb so it was getting filtered air.

Now I have it the opposite the passenger side is hooked to a nipple on the bottom of my Holley carb and I have a breather cap on the drivers side.
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Old 06-23-2011, 05:00 PM
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Even a new engine with tight cylinders will have some blowby and create crankcase pressure, right?

Why not use a PCV valve on both valve covers?

Wouldn't it be better to have a slight negative presure on the crankcase?
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Old 06-23-2011, 05:08 PM
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Originally Posted by jsutton View Post
That's the way I understand it too. Because when my 302 had stock valve covers on it the PCV valve was in the drivers side connected to the intake manifold under the carb and the passenger side hooked to a nipple on the air horn of the carb so it was getting filtered air.

Now I have it the opposite the passenger side is hooked to a nipple on the bottom of my Holley carb and I have a breather cap on the drivers side.
Same here. PCV on the passenger side and breather on the drivers side.
You need the PCV hooked up or you'll likely run rich without the extra bit of air it provides. A breather on the opposite valve cover is usually used when you go to a open style air cleaner. In stock form, the hose connects under the air cleaner behind the filter to give it clean filtered air. With an open air cleaner there is often no provision for this so you give it it's own little air filter.
So you can either keep using the one breather and go to an open air cleaner (adds a few hp), or just ditch the breathers and hook it up as normal.

@1FastGambler it needs to be a circulating system. Clean air goes in one side and gets pulled out the other.
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Old 06-23-2011, 07:50 PM
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This is where the terms "open emissions system" and "closed emissions system" come from.

Open systems have a breather cap on one valve cover and the other has a PCV (Positive Crankcase Ventilation) valve. It's considered positive because crankcase air expands when heated creating positive pressure. Conversely, a negative pressure creates a vacuum.

The PCV valve is a check valve (take it out should rattle when it is shaken) that allows crankcase gases to be vacuumed out and mixed with the air-fuel mixture and burned during combustion. The breather cap is the "intake" side of the crankcase.

On a closed system, the breather cap inhales on the clean side (filtered) of the air cleaner.

If the rig is subject to emissions testing or visuals, then ya gotta replicate what it came with. Otherwise, feel free to do what ya want but as others have posted its' a good idea to evacuate the stale air and airborne particulates in the crankcase to keep the oil that much cleaner.

Oh, a clogged PCV will cause your engine to run rough because it is not receiving the volume of air vacuumed out of the crankcase... therefore, it will run rich.
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Old 06-24-2011, 12:58 AM
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Quote:
Originally Posted by Sleepy445FE View Post
Same here. PCV on the passenger side and breather on the drivers side.
You need the PCV hooked up or you'll likely run rich without the extra bit of air it provides. A breather on the opposite valve cover is usually used when you go to a open style air cleaner. In stock form, the hose connects under the air cleaner behind the filter to give it clean filtered air. With an open air cleaner there is often no provision for this so you give it it's own little air filter.
So you can either keep using the one breather and go to an open air cleaner (adds a few hp), or just ditch the breathers and hook it up as normal.

@1FastGambler it needs to be a circulating system. Clean air goes in one side and gets pulled out the other.
Very interesting, but how do you determine which side does what? Drivers side is positive pressure, or is it passenger side? Just something I thought of haha.

And if I wanted to use an open air cleaner, I could use BOTH breathers, correct?

I'm not really a big fan of all the stuff that's attached to the air cleaner/carb because it makes it difficult to take the carb of and re-assemble (sort-to-say) when I go to start the vehicle back up.

And why would an open air cleaner allow me to run BOTH breathers? I'm not sure I understand the difference and how it creates BOTH vacuums on the valve covers instead of one vacuum and one positive pressure (crankcase gasses that need to be pushed out).

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Originally Posted by HIO Silver View Post
This is where the terms "open emissions system" and "closed emissions system" come from.

Open systems have a breather cap on one valve cover and the other has a PCV (Positive Crankcase Ventilation) valve. It's considered positive because crankcase air expands when heated creating positive pressure. Conversely, a negative pressure creates a vacuum.

The PCV valve is a check valve (take it out should rattle when it is shaken) that allows crankcase gases to be vacuumed out and mixed with the air-fuel mixture and burned during combustion. The breather cap is the "intake" side of the crankcase.

On a closed system, the breather cap inhales on the clean side (filtered) of the air cleaner.

If the rig is subject to emissions testing or visuals, then ya gotta replicate what it came with. Otherwise, feel free to do what ya want but as others have posted its' a good idea to evacuate the stale air and airborne particulates in the crankcase to keep the oil that much cleaner.

Oh, a clogged PCV will cause your engine to run rough because it is not receiving the volume of air vacuumed out of the crankcase... therefore, it will run rich.

Great info on the PCV valve there HIO.

Can you help out to answer the questions I gave sleepy?

I'd be interested in going with an open air cleaner (and both breathers as sleepy was suggesting), but if it's going to clog the S**T out of my engine and not allow it to get rid of those harmful gasses, then I'll stick with one breather and one PCV.

One last thing... If I went with a 1 PCV valve & 1 breather set-up, the PCV valve goes on which side (how do you determine)? AND, where does the PCV valve get hooked up to? Air cleaner or directly to the carb?

There are no emissions BS to deal with here in Saskatchewan, and even if there were, they're so loose about it that it wouldn't matter because vehicles are never checked over before they're registered and licensed.
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Old 06-24-2011, 04:12 AM
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Originally Posted by Aaron-71 View Post
Very interesting, but how do you determine which side does what? Drivers side is positive pressure, or is it passenger side? Just something I thought of haha.

And if I wanted to use an open air cleaner, I could use BOTH breathers, correct?

I'm not really a big fan of all the stuff that's attached to the air cleaner/carb because it makes it difficult to take the carb of and re-assemble (sort-to-say) when I go to start the vehicle back up.

And why would an open air cleaner allow me to run BOTH breathers? I'm not sure I understand the difference and how it creates BOTH vacuums on the valve covers instead of one vacuum and one positive pressure (crankcase gasses that need to be pushed out).




Great info on the PCV valve there HIO.

Can you help out to answer the questions I gave sleepy?

I'd be interested in going with an open air cleaner (and both breathers as sleepy was suggesting), but if it's going to clog the S**T out of my engine and not allow it to get rid of those harmful gasses, then I'll stick with one breather and one PCV.

One last thing... If I went with a 1 PCV valve & 1 breather set-up, the PCV valve goes on which side (how do you determine)? AND, where does the PCV valve get hooked up to? Air cleaner or directly to the carb?

There are no emissions BS to deal with here in Saskatchewan, and even if there were, they're so loose about it that it wouldn't matter because vehicles are never checked over before they're registered and licensed.
Reread my earlier post. I said you can either run 1 breather and a PCV valve with an open air cleaner, or you can keep the stock air cleaner and hook it up as normal. Either way you have a PCV on one side and a source of filtered air coming through the other. It doesn't matter what side either is on, just as long as they're on opposite sides.

It's not good to run two breathers and no PCV. You won't get any circulation this way. A breather is for incoming air not outgoing. If it's venting smoke then it will quickly become oil covered and plug your system up.

The whole reason for the breather is because there is no place to hook up the hose on most open air cleaners. You just need filtered air coming in that's all.
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Old 06-24-2011, 07:09 AM
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As far as which valve cover to put the PCV in it makes no diff, but is normally put in the passenger side with breather/oil fill in the drivers side for easy access adding oil.
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Old 06-24-2011, 08:28 AM
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As far as which valve cover to put the PCV in it makes no diff, but is normally put in the passenger side with breather/oil fill in the drivers side for easy access adding oil.
The oil fill is especially true for a factory A/C truck.

Over the years, there have been a few various setups used. Some have the PCV valve in a grommet on the passenger rear and a breather cap that has a hose going to the air cleaner on the driver front.
Others have a breather tube going from the air cleaner to the passenger rear, and a PCV valve in the oil fill cap on the driver front.
Either system works basically the same, filtered air in one side, dirty air sucked out the other.
I've seen guys with older high mile engines try using 2 PCV valves to overcome the excessive blowby, but it never really works right.
Tyipcally, their main issue is the PCV hose and/or port on the carb or intake is partly/totally clogged with sludge. It's amazing what cleaning that crud out can do for a PCV system......

As mentioned, the lack of PCV valve and breather will result in an oily film building up on the covers, even on a fresh engine. This isn't too much of an issue for the racers that only go 1/4 mile at a time...... Any other use, and the dual breather caps should be avoided, and not just because the EPA idiots said so.
Plus, if the breathers get clogged, oil leaks can start due to excessive pressure building up in the engine causing seals and/or gaskets to fail.
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Old 06-24-2011, 08:47 AM
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Originally Posted by Sleepy445FE View Post
Reread my earlier post. I said you can either run 1 breather and a PCV valve with an open air cleaner, or you can keep the stock air cleaner and hook it up as normal. Either way you have a PCV on one side and a source of filtered air coming through the other. It doesn't matter what side either is on, just as long as they're on opposite sides.

It's not good to run two breathers and no PCV. You won't get any circulation this way. A breather is for incoming air not outgoing. If it's venting smoke then it will quickly become oil covered and plug your system up.

The whole reason for the breather is because there is no place to hook up the hose on most open air cleaners. You just need filtered air coming in that's all.
Good to know. One breather, one PCV. Got it!

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Originally Posted by Rogue_Wulff View Post
The oil fill is especially true for a factory A/C truck.

Over the years, there have been a few various setups used. Some have the PCV valve in a grommet on the passenger rear and a breather cap that has a hose going to the air cleaner on the driver front.
Others have a breather tube going from the air cleaner to the passenger rear, and a PCV valve in the oil fill cap on the driver front.
Either system works basically the same, filtered air in one side, dirty air sucked out the other.
I've seen guys with older high mile engines try using 2 PCV valves to overcome the excessive blowby, but it never really works right.
Tyipcally, their main issue is the PCV hose and/or port on the carb or intake is partly/totally clogged with sludge. It's amazing what cleaning that crud out can do for a PCV system......

As mentioned, the lack of PCV valve and breather will result in an oily film building up on the covers, even on a fresh engine. This isn't too much of an issue for the racers that only go 1/4 mile at a time...... Any other use, and the dual breather caps should be avoided, and not just because the EPA idiots said so.
Plus, if the breathers get clogged, oil leaks can start due to excessive pressure building up in the engine causing seals and/or gaskets to fail.
Great info. Definitely won't use 2 breathers, cause it just sounds stoopud to use two, and I don't really need my old engine getting any older.

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As far as which valve cover to put the PCV in it makes no diff, but is normally put in the passenger side with breather/oil fill in the drivers side for easy access adding oil.
Adding oil? HUH? Adding oil to what? The breather hole?

Never heard of this, and I'm not quite sure I follow. I never knew oil must be added to the breather... bit more explanation please?
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Old 06-24-2011, 08:51 AM
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He was refering to the engine oil fill cap that also doubles as the breather cap. You gotta add oil somewhere......
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Old 06-24-2011, 09:37 AM
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He was refering to the engine oil fill cap that also doubles as the breather cap. You gotta add oil somewhere......
Lol, ahhh I see.
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Old 06-24-2011, 09:37 AM
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