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10-01-2002, 06:58 PM
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Senior Club Cobra Member
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Join Date: Mar 2001
Location: Southern Connecticut,
CT
Cobra Make, Engine: SPF - 351W, 944 non-turbo
Posts: 2,105
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Not Ranked
Which PCV valve - 351W
I've got a 351W crate engine, dual plane, Holley 750 CFM. I've decided to install a PCV valve. When I go to the auto parts store there are 50 of them hanging on the hooks. Which one should I be using? I imagine that different valves flow different amounts of air. I would like to flow a lesser amont of crankcase air.
Any suggestions (brand, models)?
Thanks, Bob
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10-01-2002, 10:24 PM
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CC Member
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Join Date: Dec 2001
Location: San Diego,
CA
Cobra Make, Engine:
Posts: 2,979
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Not Ranked
Why run one at all?
Is there a specific reason you decided to use one?
Why not just vent it?
__________________
Remember, It's never too early to start beefing up your obituary.
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10-02-2002, 09:21 AM
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CC Member
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Join Date: Mar 2002
Location: so cal,
Cal
Cobra Make, Engine: I used to fix them for a living
Posts: 2,563
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Not Ranked
Billit specialties sells a breather cap with a replaceable PCV valve in it. Looks good too.
__________________
In a fit of 16 year old genius, I looked down through the carb while cranking it to see if fuel was flowing, and it was. Flowing straight up in a vapor cloud, around my head, on fire.
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10-02-2002, 02:38 PM
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CC Member
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Join Date: Oct 2000
Location: Crystal Lake,
IL
Cobra Make, Engine: Everett-Morrison, 434 cid
Posts: 977
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Not Ranked
Curious... why one that would limit venting?
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10-02-2002, 03:56 PM
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CC Member
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Join Date: Oct 2001
Location: Greensboro, NC,
Posts: 17
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Not Ranked
Let me take a stab at answering this.
1) I went to our local NAPA store and the counterman sold me a PCV valve for a 351W. He said it was for just about EVERY 351W. If you need, I can pop it out, read its number, and report back.
2) I too got a breather cap that the PCV valve fits in.
3) When I got my car, I was told that it would not require a PCV valve. But, within a few months, there was a fine layer of oil on top of both valve covers under the breathers. The PCV valve cures this and now the valve covers remain clean.
I hope this helps,
Steve
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10-02-2002, 04:06 PM
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CC Member
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Join Date: Mar 2002
Location: so cal,
Cal
Cobra Make, Engine: I used to fix them for a living
Posts: 2,563
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Not Ranked
If your breathers are dumping oil on the valve covers, a PCV is NOT the answer. All that will do for you is let your intake suck up that oil and have your motor burn it. You need better baffles in the valve covers to separate the oil mist from the "air" before letting it out. If yoy can't baffle the valve covers any better, there are breather stands available which have good baffling.
__________________
In a fit of 16 year old genius, I looked down through the carb while cranking it to see if fuel was flowing, and it was. Flowing straight up in a vapor cloud, around my head, on fire.
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10-02-2002, 04:36 PM
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Senior Club Cobra Member
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Join Date: Mar 2001
Location: Southern Connecticut,
CT
Cobra Make, Engine: SPF - 351W, 944 non-turbo
Posts: 2,105
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Not Ranked
Right now it's just vented, with no problems. I feel the oil would be better off if there was some circulation of new air in the crankcase.
Bob
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10-02-2002, 04:42 PM
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CC Member
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Join Date: Mar 2002
Location: so cal,
Cal
Cobra Make, Engine: I used to fix them for a living
Posts: 2,563
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Not Ranked
Bob, I would advise you to leave things as they are. One good breather on each valve cover is all you ought to need.
__________________
In a fit of 16 year old genius, I looked down through the carb while cranking it to see if fuel was flowing, and it was. Flowing straight up in a vapor cloud, around my head, on fire.
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10-02-2002, 05:26 PM
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CC Member
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Join Date: Aug 1999
Location: Canada's beautiful West Coast,
Posts: 723
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Not Ranked
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10-02-2002, 07:50 PM
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Senior Club Cobra Member
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Join Date: Jul 2000
Location: Las Vegas,
NV
Cobra Make, Engine: ERA 427 SO
Posts: 1,126
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Not Ranked
Good info, whaler...I would strongly recommend using a PCV valve for the engine protection it will provide...emissions are a plus. Some here say our motors never see that much "daily" type driving, so a PCV valve is unnecessary...I think if the jetting accounts for it, you're better off with one than without...just my dos centavos.
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Ken
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10-03-2002, 06:51 AM
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CC Member
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Join Date: Feb 2000
Location: Long Island New York,
NY
Cobra Make, Engine: SPF 974
Posts: 737
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Not Ranked
Interesting article ..but I do not see where the PCV ensures engine protection over a properly vented crankcase. It helps with emmisions which is usually not an issue with our cars.
A clogged or dirty PCV, according to the article, can "cause" engine damage. ..oh..oh. I never noticed oil residue on my covers that could not be wiped clean with my regular maintenance. A vented crankcase is a must, whether it is vented with breathers on the covers, tubed down below or with a PCV. I prefer open breathers and nothing but a COOL/CLEAN air charge flowing down the carb.
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10-03-2002, 07:22 AM
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CC Member
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Join Date: Oct 2000
Location: Crystal Lake,
IL
Cobra Make, Engine: Everett-Morrison, 434 cid
Posts: 977
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Not Ranked
Because piston rings don't seal 100% you have to have a place to vent the blow-bye. A PCV valve's size is determined by someone's best guess as to how much venting your engine needs to maintain normal pressure. Engine size and condition will change an engine's venting requirements, too much venting and you will get moisture in the oil, too little and you will get oil leaks and overloaded oil rings. PCV valves were developed because the government cares more about emissions than about the inside of your engine.
Scott
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10-03-2002, 05:56 PM
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CC Member
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Join Date: Jun 2001
Location: Washington DC Metro (Virginia),
VA
Cobra Make, Engine: Classic Roadsters, Tweaked 351W, T-5Z, CRII Tech Support Team.
Posts: 1,895
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Not Ranked
Bob in Ct.
If you have a race engine in a race car, you don't need a PCV. If you have a street engine in a street car, you need a PCV.
PCV (positive crankcase ventilation) valves have been installed on virtually all engines since about 1963. It puts the inside of your engine under a slight vacuum, removes the oil vapors generated in the crankcase from your engine, injects this under the carb base into the intake where the vapors are burned. And it only does this when the engine is under high vacuum or low load so it doesn't mess up your power enrichment.
There should be an air inlet breather on the drivers side valve cover. With the engine running, if you put your thumb over this hole with the breather out, you can feel the vacuum. The air then circulates through the engine to the passenger side valve cover where the PCV pulls the air and oil vapors out through a hose to the intake.
Go to the Fram or Purolator book alongside the rack of PCV valves, find the part number for a early - mid-70's PCV for a Ford 351W. If there are a couple to choose from, the one you want is the one with the little plastic 90 deg elbow on top with a 3/8" hose fitting.
Remember to jet your carb primaries about 2 steps richer to make up for the extra air the PCV lets in. Secondaries shouldn't be affected.
Can you get by without one? Sure. But your engine, and engine compartment will get dirtier (oilier) faster.
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10-03-2002, 06:47 PM
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CC Member
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Join Date: Jul 2001
Location: Southwest,
WI
Cobra Make, Engine: Shell Valley, Mopar thingy (small block of course)
Posts: 2,215
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Not Ranked
I would ask your engine builder how much you can drop the octane. Oil vapor sucked in and burned can make your octane rating drop from 92 to 88-89. I vent my engine, properly baffled there is very little oil to wipe off and here in the desert I don't worry about condensation much. PCVs are better for emmisions.
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Brent Dolphin
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