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Old 05-22-2020, 04:37 PM
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351w pcv

Appears to be a lot of debate on if a sbf needs some form of PCV and, if so, what that should be. This is a naturally aspirated engine with carburetor. Currently it has a breather in each valve cover. Should I be looking to add a PCV valve? Should it be from a valve cover into the filtered side of the air cleaner? Or, valve cover to port in intake?

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Old 05-22-2020, 04:57 PM
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PCV in one valvecover to a full vacuum source, either large port on carb or a fitting in the manifold.
Breather in cover on the opposite side, so it draws across the crankcase from one side to the other.

Keeps the oil cleaner, draws out condensation and blow-by moisture and acid by-products, and keeps the engine exterior cleaner.

Got to have a good baffle under the PCV valve in the valvecover, or a PCV valve/separator canister combination or you'll have a high oil consumption issue through the PCV.
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Old 05-22-2020, 04:57 PM
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YES
Valve cover to base of carb or intake port.

you dont want to smell that ( fumes from the crankcase)
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Old 05-22-2020, 05:46 PM
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Ford typically used a PVC with no breather. One thing that does is to keep a constant vacuum on the crankcase, assuming the blowby isn't so great as to feed as much into the valve as it can take out. Certainly one thing this configuration does is to maintain a tighter fit of compression rings to cylinder wall. Sounds like a racer's trick, doesn't it?

GM typically uses a PCV valve at one end and a vent connection to the aircleaner at the other, so obviously there is a fresh air flow that the vacuum through the PCV valve is pulling through the crankcase. So no assistance to ring seal is offered while cruddy blowby gasses are diluted. One thing this system offers is a high blow by engine putting more pressure into the crankcase has somewhere to blow the crud as evidenced by a constantly oily dirty air filter.

Given both systems work for long periods of time it's reasonable to consider on the street it's 6 of one to a half dozen of the other. My personal Chebby power plant uses a PCV valve off one rocker cover and a pair of K&N washable, mesh filter vents on the other. I don't have head space under the hood for a GM air cleaner so it runs an aftermarket oblong 2 inch tall air filter with a filter cover all of cleanable mesh. Otherwise I'd have to put a bubble in the hood which would detract from the sleeper look.

Bogie
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Old 05-23-2020, 07:16 AM
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Is the volume of air circulated through a PCV system so little that there no concern about the effect on air/fuel ratio when using a port in the manifold?
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Old 05-23-2020, 09:45 AM
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Quote:
Originally Posted by Molon Labe View Post
Is the volume of air circulated through a PCV system so little that there no concern about the effect on air/fuel ratio when using a port in the manifold?
Actually, in normal driving, the PCV valve is closed and the intake is just sucking up a minor amount of blowby...
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Old 05-24-2020, 07:29 AM
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What's the take on these breathers with a PCV valve in them? It doesn't sound effective to have the vacuum and the make up air in the same valve cover.
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Old 05-24-2020, 09:21 AM
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Quote:
Originally Posted by Molon Labe View Post
What's the take on these breathers with a PCV valve in them? It doesn't sound effective to have the vacuum and the make up air in the same valve cover.
That's just a different shaped PCV... at WOT the PCV opens and excess blowby is routed to the air filter and taken into the top of the carburetor...
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Old 05-25-2020, 07:15 PM
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So, PCV opens at low intake vacuum (WOT) and is closed during high vacuum (idle)?
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Old 05-26-2020, 12:27 AM
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Quote:
Originally Posted by Molon Labe View Post
So, PCV opens at low intake vacuum (WOT) and is closed during high vacuum (idle)?
PCV stays closed at any vacuum in the crankcase... opens if WOT blowby makes pressure in crankcase that isn't being sucked up...
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