PVC valve
Melting Slicks
Joined: Feb 2006
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From: Park City to SoCal - according to the map it's all down hill. No bad days in Indian Wells, California
Don't mean to be so clinical.... but read this and then make an informed decision:
As an engine runs, the crankcase (containing the crankshaft and other parts) begins to collect combustion chamber gases which leak past the rings surrounding pistons and sealing them to the cylinder walls. These combustion gases are sometimes referred to as "blow by" because the combustion pressure "blows" them "by" the pistons. These gases contain compounds harmful to an engine, particularly hydrocarbons, which are just unburned fuel, as well as carbon dioxide. It also contains a significant amount of water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds begin to condense out of the air within the crankcase and form corrosive acids and sludge on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.
Hope this helps
Rich
As an engine runs, the crankcase (containing the crankshaft and other parts) begins to collect combustion chamber gases which leak past the rings surrounding pistons and sealing them to the cylinder walls. These combustion gases are sometimes referred to as "blow by" because the combustion pressure "blows" them "by" the pistons. These gases contain compounds harmful to an engine, particularly hydrocarbons, which are just unburned fuel, as well as carbon dioxide. It also contains a significant amount of water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds begin to condense out of the air within the crankcase and form corrosive acids and sludge on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.
Hope this helps
Rich
Team Owner
Joined: Oct 2000
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From: Washington Michigan
Trivia: PVC is polyvinyl chloride (plastic pipe); PCV is Positive Crankcase Ventilation.
Melting Slicks
Joined: Feb 2006
Posts: 3,105
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From: Sunrise FL
Also remember that the carb idle circuit is expecting to see more air (air+fumes). The PCV valve is a calibrated vacuum leak and one way check valve so a backfire does not ignite the fumes. You will need to re-adjust the idle mixture screws and speed to compensate.
PCV systems do good for the engine by keeping the oil cleaner as Rich sez
PCV systems do good for the engine by keeping the oil cleaner as Rich sez
Last edited by mikem350; Apr 4, 2007 at 04:00 PM.
Team Owner
Joined: Mar 2003
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From: Greenville, Indiana
In theory, the old road draft tube provided crankcase ventilation as long as the vehicle was in motion. That's where the name came from. Airflow passing by the end of the angled cut on the rear facing end of the tube would draw a vacuum in the crankcase. To make the system work, a vented oil fill cap, usually with a steel mesh packing allowed fresh air in to displace what the draft tube drew out. The incoming air was filtered by the mesh. This is why oil change instructions at the time included cleaning and oiling the cap in order for it to function somewhat like an oil bath air cleaner for the carburetor.
It didn't take long for engines of the day to develop sufficient blowby to overcome this system though and start blowing oil out the breather cap when the engine was run hard. It was also not effective at all when the vehicle wasn't in motion. I'm thinking the early dual quad engines were retrofitted with solid oil fill caps to quell customer complaints of oil on the engine. This effectively prevented any crankcase scavenging from happening though.
It didn't take long for engines of the day to develop sufficient blowby to overcome this system though and start blowing oil out the breather cap when the engine was run hard. It was also not effective at all when the vehicle wasn't in motion. I'm thinking the early dual quad engines were retrofitted with solid oil fill caps to quell customer complaints of oil on the engine. This effectively prevented any crankcase scavenging from happening though.
Race Director
Joined: Nov 2000
Posts: 10,340
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From: Beverly Hills (Pine Ridge) Florida
If you have ever torn apart an early (early 60's and back) street engine that did not have a PCV on it, you would have been surprised at the amount of sludge, etc. that was on the inside. Of course newer detergent oils have helped as well (and a good oil and filter maintenance schedule). But engines in the "early" days died early due to sludge, acids, and "garbage". PCV systems have really helped increase the longivity of engines.
Common failure modes that you rarely hear these days are: Noisey lifters (was especially a problem with hydraulic lifters), stuck rings, and blocked oil passages.
Plasticman
Common failure modes that you rarely hear these days are: Noisey lifters (was especially a problem with hydraulic lifters), stuck rings, and blocked oil passages.
Plasticman
Last edited by Plasticman; Apr 4, 2007 at 07:26 PM.
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Team Owner
Joined: Mar 2003
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From: Greenville, Indiana
If you have ever torn apart an early (early 60's and back) street engine that did not have a PCV on it, you would have been surprised at the amount of sludge, etc. that was on the inside. Of course newer detergent oils have help as well (and a good oil and filter maintenance schedule). But engines in the "early" days died early due to sludge, acids, and "garbage". PCV systems have really helped increase the longivity of engines.
Common failure modes that you rarely hear these days are: Noisey lifters (was especially a problem with hydraulic lifters), stuck rings, and blocked oil passages.
Plasticman
Common failure modes that you rarely hear these days are: Noisey lifters (was especially a problem with hydraulic lifters), stuck rings, and blocked oil passages.
Plasticman
The PCV system is one of the best things that ever happened to the automotive internal combustion engine. I'm really suprised it took so long to adopt it. I've torn down many of those older engines that had the valve covers packed almost full of sludge. Flathead V-8's had the lifter valley and oil pan full of the stuff too. You rarely see those problems like that anymore.
Drifting
Joined: May 2005
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St. Jude Donor '09
Bad PCV valves and vacuum leaks have always been my friend
Been fortunate to get some decent cars that no one else wanted because they ran bad. Fear was more serious engine issues. Only to find out that vacuum leaks and PCV maintenence was the majority of the problems.
As MikeM said, "The PCV system is one of the best things that ever happened.." To me
John
Been fortunate to get some decent cars that no one else wanted because they ran bad. Fear was more serious engine issues. Only to find out that vacuum leaks and PCV maintenence was the majority of the problems.
As MikeM said, "The PCV system is one of the best things that ever happened.." To me
John
Team Owner
Joined: Oct 2000
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From: Washington Michigan
It didn't take long for engines of the day to develop sufficient blowby to overcome this system though and start blowing oil out the breather cap when the engine was run hard. It was also not effective at all when the vehicle wasn't in motion. I'm thinking the early dual quad engines were retrofitted with solid oil fill caps to quell customer complaints of oil on the engine. This effectively prevented any crankcase scavenging from happening though.
Race Director
Joined: Feb 2004
Posts: 14,984
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From: CT
Not quite. The old "road draft tube" system with a vented or non-vented oil fill cap (or just using breathers on the valve covers) doesn't provide any crankcase ventilation - only pressure relief. A properly-configured PCV system draws outside air (from the clean side of the air cleaner) into the crankcase, through it, and those vapors are metered by the PCV valve into the intake (drawn by manifold vacuum), where they're burned with the intake charge. All "breathers" do is provide pressure relief, and they blow oil vapors out on top of the engine.
Trivia: PVC is polyvinyl chloride (plastic pipe); PCV is Positive Crankcase Ventilation.
Trivia: PVC is polyvinyl chloride (plastic pipe); PCV is Positive Crankcase Ventilation.
Thread Starter
Instructor
Joined: Nov 2005
Posts: 124
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From: Pittsburgh Pa.
also if you ran without PCV, Would it affect Manifold pressure,and cause carb to foul plugs and load up on a low rpm , under load, say going up steep grade?
Team Owner
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From: Washington Michigan