I take it you agree with my position...
Question on PCV Setup
Thread Starter
Burning Brakes
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From: SoCal
Question on PCV Setup
I got into a discussion with an acquaintance of mine regarding the PCV setup on an SBC engine - 383 stroker, dual quads, Edelbrock Performer aluminum heads.
He setup the PCV as follows:
What say you experts out there?
He setup the PCV as follows:
On the driver's side valve cover he installed two breathers (an Edelbrock mounted to the side of the valve cover, and a normal billet aluminum breather in the oil filler hole of the valve cover).
On the passenger side valve cover he installed the PCV valve in the oil filler hole of the valve cover and an Edelbrock breather mounted to the side of the valve cover (just like on the driver's side valve cover). The PCV valve is connected to the vacuum port in the baseplate of the rear carb (again, this is a dual quad carb engine).
My feeling is that there should be no breather mounted on the passenger side valve cover (the side with the PCV valve) because you'll lose vacuum through the breather and the PCV won't work properly. I've always been told that the PCV valve (connected to a vacuum source) goes on one side and a breather goes on the other - and that's it. The other guy was insistent that a breather was needed on both valve covers.What say you experts out there?
Last edited by VCuomo; Jun 14, 2010 at 10:58 PM.
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The purpose of the PCV is to pull fresh air in on one side of the engine, have it purge through the crankcase (removing blowby gases and crap), and extract it out the other side for a complete "positive ventilation" system. Having the valve and the breather in the same valve cover completely eliminates the purpose of the system.
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From: Ontario
Just to show an example of what Lars has described. This is my 1974 and other years may differ somewhat but the basic principles are the same.
Fresh and filtered air is drawn into the crankcase here.
Fumes from the crankcase are drawn back into the carb for re-burn through the PCV. The rate is determined by the valve itself sensing operating vacuum at various RPMs and speeds. Having vented valve cover caps defeats the whole purpose of this closed loop system.
Fresh and filtered air is drawn into the crankcase here.
Fumes from the crankcase are drawn back into the carb for re-burn through the PCV. The rate is determined by the valve itself sensing operating vacuum at various RPMs and speeds. Having vented valve cover caps defeats the whole purpose of this closed loop system.
Last edited by Paul L; Jun 15, 2010 at 02:14 PM.
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Thread Starter
Burning Brakes
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From: SoCal
Thanks to you all for your comments! I thought it was strange having the PCV valve and a breather on the same valve cover...
Now, about the other side - in Paul 74's example, the breather on the non-PCV valve side goes directly to the carb's air filter. Does it matter, or does anything change, if you just have a regular breather cap installed on that side instead (i.e., not connected to the air filter)?
I thought it was strange having the PCV valve and a breather on the same valve cover...Now, about the other side - in Paul 74's example, the breather on the non-PCV valve side goes directly to the carb's air filter. Does it matter, or does anything change, if you just have a regular breather cap installed on that side instead (i.e., not connected to the air filter)?
Last edited by VCuomo; Jun 15, 2010 at 02:15 PM.
Team Owner
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From: Ontario
Thanks to you all for your comments! I thought it was strange having the PCV valve and a breather on the same valve cover...
Now, about the other side - in Paul 74's example, the breather on the non-PCV valve side goes directly to the carb's air filter. Does it matter, or does anything change, if you just have a regular breather cap installed instead (i.e., not connected to the air filter)?
I thought it was strange having the PCV valve and a breather on the same valve cover...Now, about the other side - in Paul 74's example, the breather on the non-PCV valve side goes directly to the carb's air filter. Does it matter, or does anything change, if you just have a regular breather cap installed instead (i.e., not connected to the air filter)?
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Now, about the other side - in Paul 74's example, the breather on the non-PCV valve side goes directly to the carb's air filter. Does it matter, or does anything change, if you just have a regular breather cap installed on that side instead (i.e., not connected to the air filter)?
Team Owner
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From: Ontario
Yes, I should have mentioned that the cap should have some form of filtering substrate inside. (My pic shows the factory white filter inside the air cleaner housing.) You can imagine what will happen when driving down a dusty country road without a filter of some type.
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From: At my Bar drinking and wrenching in Lafayette Colorado
For crankcase ventilation functionality, all you need is a filtered air inlet. This can be a simple breather in the valve cover with an element in it, or the filtered air from the air cleaner, as GM did it.
For emissions purposes, you do not want to use a valve cover breather. Under high cylinder pressures (wide open throttle) there is no manifold vacuum to pull air through the PCV system, yet cylinder blowby and crankcase pressurization are at their peak. Under this condition, the flow reverses on the breather side of the ventilation system, and crankcase vapors are pushed out the breather side of the system. On the OEM system, these vapors are pushed up into the air cleaner and ingested by the engine (this is the purpose of the tube up to the air cleaner). On a valve cover breather system, the vapors would vent into the engine compartment to atmosphere, something the EPA does not like...
For performance, you do not want to ingest the fumes through the carb, so the valve cover breather works well.
For emissions, you don't want to vent to atmosphere, so you need to run the breather into the air cleaner.
For emissions purposes, you do not want to use a valve cover breather. Under high cylinder pressures (wide open throttle) there is no manifold vacuum to pull air through the PCV system, yet cylinder blowby and crankcase pressurization are at their peak. Under this condition, the flow reverses on the breather side of the ventilation system, and crankcase vapors are pushed out the breather side of the system. On the OEM system, these vapors are pushed up into the air cleaner and ingested by the engine (this is the purpose of the tube up to the air cleaner). On a valve cover breather system, the vapors would vent into the engine compartment to atmosphere, something the EPA does not like...
For performance, you do not want to ingest the fumes through the carb, so the valve cover breather works well.
For emissions, you don't want to vent to atmosphere, so you need to run the breather into the air cleaner.
Last edited by lars; Jun 15, 2010 at 04:04 PM.
Thread Starter
Burning Brakes
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From: SoCal
Melting Slicks
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I'm with Paul! You learn something new everyday! I had never thought it really made a difference, and have been running my 383 with the factory GM setup. After reading this I'm going to a breather. If understand Lars' explanation correctly, the only time it changes emissions is at WOT. At WOT, who cares about emissions in a car like this...the WOT objective is Performance! 
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
If you run the valve cover breather and eliminate the air cleaner tube, keep in mind that the crankcase vapors emitted under high cylinder pressures will vent into the engine compartment, and you'll get some oil film contamination. This is not bad if your ring sealing is good, and gets worse with older engines running higher blowby. The air cleaner tube does a good job of keeping the engine compartment clean.
Lars
Lars
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Resurrecting a very old thread, but I have a question.
I read enough that I understand getting the airflow through the crankcase. Pull from one valve cover through the engine and from the other vented (maybe filtered) valve cover.
What about high HP motors that don't make much vacuum? I have to rev the motor a little on mine to build enough vacuum to move the lights. I don't think there is a leak in the canister, but maybe there is. Typically one light moves and then after a little rev, the other light moves. If I'm not really putting much pull on the PCV valve most of the time, is it really pulling air properly? Does the PCV system need much vacuum to work? If it doesn't I could see the PCV valve starving the vacuum down most of the time and even an engine without much vacuum would function OK. But I don't know how much vacuum it really needs.
I read enough that I understand getting the airflow through the crankcase. Pull from one valve cover through the engine and from the other vented (maybe filtered) valve cover.
What about high HP motors that don't make much vacuum? I have to rev the motor a little on mine to build enough vacuum to move the lights. I don't think there is a leak in the canister, but maybe there is. Typically one light moves and then after a little rev, the other light moves. If I'm not really putting much pull on the PCV valve most of the time, is it really pulling air properly? Does the PCV system need much vacuum to work? If it doesn't I could see the PCV valve starving the vacuum down most of the time and even an engine without much vacuum would function OK. But I don't know how much vacuum it really needs.