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Old 03-06-2011, 01:13 PM
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PCV frustration

I've been curious about whether to run a PCV valve on one valvecover or not. I did alot of reading on this and everything is all over the map. I understand it purges the contaminated air from the crankcase which helps to keep the sludge from building up and oil clean. I run my Hotrod seasonal and change the oil often. I don't want any air leaking into my carb at idle. I want a clean idle and I need to keep my throttle plates open as much as possible to keep proper transfer slot exposure. Leaking air into the carb at idle will raise the idle forcing me to have to close the plates a little thus closing off the slots almost to nothing. I pull about 8-9 inches at idle also my plates are drilled. When I ran a vacuum gauge on the car and took it for a spin, the vacuum would move around depending what type of driving I would do [usually a little higher than 10 inches but as approach WOT it would naturally fall. SO I thought, I'll select a PCV valve that is closed at idle and open at other times. FORGET it they don't make them that I know of. So I went to Auto Zone and started looking at an assortment of them. Believe it or not I started opening packages ( you can do it without damaging them) and started sucking on them. ( I know it sounds stupid) but this is what I found out. They are all over the map. Some stick, some leak, some open very quickly and some you need to really suck on them to get them to open. How can you possibly buy one to stay closed at say 7 inches and open at say 10-20 inches?
Right now I put two breathers on the valve covers. I adjusted my carb and have everything perfect ( transfer slots, mixture screws and rpms) so now what?
Should I just leave it the way it is, or does someone know of a really high performance PCV valve I can buy ( without going to the High dollar evacuation systems) that is more tailored to performance engines.
Is a PCV valve even suppose to be open at idle?
For all I know I could be completely wrong on how a PCV valve is suppose to work on a HotRod, but I'm willing to listen to all thoughts on this.
Lou

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Old 03-06-2011, 05:24 PM
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A PCV system with an oil catch can is a good idea on a street driven vehicle. Not running a PCV at all can lead to pressure building in the crankcase and causing leaks. And with the catch can you keep all the oil and sludge out of your intake. Running breathers is better than nothing, but I seem to remember reading a decent PCV valve hooked up to the throttle body/carb is a better system.

Doing a google search will net you ideas for some cheap and very functional systems.
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Old 03-06-2011, 05:45 PM
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Quote:
Originally Posted by Cobra720
I've been curious about whether to run a PCV valve on one valvecover or not. I did alot of reading on this and everything is all over the map. I understand it purges the contaminated air from the crankcase which helps to keep the sludge from building up and oil clean. I run my Hotrod seasonal and change the oil often. I don't want any air leaking into my carb at idle. I want a clean idle and I need to keep my throttle plates open as much as possible to keep proper transfer slot exposure. Leaking air into the carb at idle will raise the idle forcing me to have to close the plates a little thus closing off the slots almost to nothing. I pull about 8-9 inches at idle also my plates are drilled. When I ran a vacuum gauge on the car and took it for a spin, the vacuum would move around depending what type of driving I would do [usually a little higher than 10 inches but as approach WOT it would naturally fall. SO I thought, I'll select a PCV valve that is closed at idle and open at other times. FORGET it they don't make them that I know of. So I went to Auto Zone and started looking at an assortment of them. Believe it or not I started opening packages ( you can do it without damaging them) and started sucking on them. ( I know it sounds stupid) but this is what I found out. They are all over the map. Some stick, some leak, some open very quickly and some you need to really suck on them to get them to open. How can you possibly buy one to stay closed at say 7 inches and open at say 10-20 inches?
Right now I put two breathers on the valve covers. I adjusted my carb and have everything perfect ( transfer slots, mixture screws and rpms) so now what?
Should I just leave it the way it is, or does someone know of a really high performance PCV valve I can buy ( without going to the High dollar evacuation systems) that is more tailored to performance engines.
Is a PCV valve even suppose to be open at idle?
For all I know I could be completely wrong on how a PCV valve is suppose to work on a HotRod, but I'm willing to listen to all thoughts on this.
Lou
"The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve by manifold vacuum, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, vacuum is much reduced, down to between 1.5 and 3" Hg. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway."
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Old 03-06-2011, 05:49 PM
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I did more searching and found a pcv for a 1967 427 vette which is the same for a 70 LT1 camaro. It would be nice if we had a valve that we could adjust, like a vacuum canister on a distributor.
Lou
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Old 03-06-2011, 06:22 PM
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If you want to do it right get one of these from Summit or some other auto parts house.
I run their system on my car.

http://www.summitracing.com/parts/MIL-85700/
summits advertisement,

Crankcase Evacuation Systems
Milodon Vac-U-Pan systems are a highly effective method of achieving optimum power through significantly reducing oil pressures. The breather of the units attaches to the valve cover and the probe is inserted into the header. Exhaust scavenging pulls pressure and blowby out of the motor. The complete kits include breathers, check valves, and weld-in nipples.
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Old 03-06-2011, 07:09 PM
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I keep loseing the web site before I can send anything #fp1003 at Year one corvette Best reproduction low vacume cam pcv for 68-69 L88 and 70 Lt1
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Old 03-06-2011, 07:15 PM
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[QUOTE=1BAD80]If you want to do it right get one of these from Summit or some other auto parts house.
I run their system on my car.

http://www.summitracing.com/parts/MIL-85700/
summits advertisement,

I think you ment crank case pressure and blow by not oil pressure WE all say what we dont mean once and a while

Last edited by painted jester; 03-06-2011 at 07:23 PM.
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Old 03-06-2011, 09:28 PM
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Pcvs are good for 5 to 15 horse power over not running a pcv , tube header evacuation systems dont work well on the street! carbs are calibrated for pcvs so dont worry about the small amount of vacuume you think your loseing
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Old 03-07-2011, 05:58 AM
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Someone in an earlier Post was asking about breathers, PCV's, etc. I Posted what I did on my engine...a blown Hemi. I use a Hot Heads PCV that installs where my road draft tube went and run it to my rear carburetor...and don't use ANY breathers. I have no leaks/seeps what so ever on the engine, haven't "sucked" any gaskets in, and the idle has not been adversly affected. The vacuum from the engine (through the carburetor) appears to only pull out the excess crankcase pressure when it's there, and keeps enough vacuum to keep the crankcase fumes evacuated. I have a screw in oil fill on the front of one of my valve covers to allow me to add oil. When I unscrew it, and put my palm over it, at idle I can feel a very slight vacuum pull. When it's screwed on, it lets no air into the crankcase. The engine has been driven all last summer and the oil has remained clean, as have the spark plugs. It appears to be running just fine with the "PCV System" I have...
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Old 03-07-2011, 03:30 PM
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Two different worlds

Quote:
Originally Posted by 345 desoto
Someone in an earlier Post was asking about breathers, PCV's, etc. I Posted what I did on my engine...a blown Hemi. I use a Hot Heads PCV that installs where my road draft tube went and run it to my rear carburetor...and don't use ANY breathers. I have no leaks/seeps what so ever on the engine, haven't "sucked" any gaskets in, and the idle has not been adversly affected. The vacuum from the engine (through the carburetor) appears to only pull out the excess crankcase pressure when it's there, and keeps enough vacuum to keep the crankcase fumes evacuated. I have a screw in oil fill on the front of one of my valve covers to allow me to add oil. When I unscrew it, and put my palm over it, at idle I can feel a very slight vacuum pull. When it's screwed on, it lets no air into the crankcase. The engine has been driven all last summer and the oil has remained clean, as have the spark plugs. It appears to be running just fine with the "PCV System" I have...
Sorry but your running a closed crank case (its not breathing at all !) its called a breather becouse your pcv lets your crank case breath by drawing fresh air into the breather through the engine and then through the pcv (evaperating and drawing out contaminents)Like gas moisture acid oil fumes etc etc!!! Your acid build up must be hellacious!! if your running a blow through system (BOOST pushed through the carbs) your pcv will be pushed closed during boost ! If your carbs are on top of your blower its the blower creating the vacume by drawing through carbs , not the engine drawing through the carbs !!! Your pistons are not pulling the charge in Thuss no vacume At your cylinders . your charger is blowing your charge in(actualy blowing your pistons down thus creating gobs of power!!! ) Naturaly asperated and blown are two different worlds (I'ed rather be blown!!! )

Last edited by painted jester; 03-07-2011 at 03:36 PM.
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Old 03-07-2011, 03:35 PM
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Plug the holes that are drilled in the carb throttle plates (epoxy, weld, solder up, replace throttle blades) and use the PCV for the extra air flow at idle as the carb was origionally designed..
Drilled throttle plates are never nessessary on any thing less than a superstock/full race carb.
The other opposing valve cover must be vented.

Remove the carb, flip it over and reset the primary and sec throttles opening and idle transfer slot exposure correctly (about .020-.030" showing under the edge of the throttle at idle)
( edelbrocks and qjets only idle on the primary barrels. Holleys share the idle on all 4 barrels)
None need to be drilled.

Big duration , rough idle cams need more base initial spark advance at idle than a stock cam does.
Give it all the initial it needs. Recurve the distributor to shorten/limit the advance curve length to allow the increased idle timing and correct max advance at high rpm.

I suggest a short/limited 10 to 12deg advance curve. allows 24deg at idle and 34-36 at max advance.
Then when you get the base inital idle timing and mechanical advance redone right, dial in the vacuum advance for 10-12deg max at part throttle high cruise. (not all in at idle)

The carbs power circuit must stay lean at idle. (Power valve choice)
Big cam with low idle vacuum = lower rated power valve open point.

You don't need a special PCV valve. and you don;t need drilled throttle plates to get the throttles positioned correctly at idle with a big cam.
You need more base idle spark advance.

Last edited by F-BIRD'88; 03-07-2011 at 03:44 PM.
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Old 03-07-2011, 03:37 PM
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Crankcase Evac systems work just fine on the street. It will suck and stick to my palm at idle.
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Old 03-07-2011, 03:39 PM
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Listen to T bird he's right on the button!!!
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Old 03-07-2011, 03:40 PM
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Listen to T bird he's right on the button!!!
Who's T-Bird ?


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Old 03-07-2011, 03:47 PM
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Quote:
Originally Posted by BigLeoRocks
Crankcase Evac systems work just fine on the street. It will suck and stick to my palm at idle.
WOW YOU lost me for a minut on stick to my palm (I was wondering what you had stuck to your palm !!!!!!! My mind was going every where !!! YOU did mean the PCV I thought it was funny where my thoughts went when you said it sucks (I Thought you meant its bad) Changes the whole meaning of the sentance!!

Last edited by painted jester; 03-07-2011 at 04:05 PM.
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