To PCV or not to PCV

[Hoodabest]

If I finish the pun, I'll be hit by lightning (rightfully so). Anyway, it was suggested by another long time member to start a new thread on this subject to keep it as a specific topic of interest. I guess I'm asking what are the advantages to these examples:

- a "standard" PCV setup like a oh let's just say stock '66 mustang 289. PCV valve goes out 1 valve cover to plate under carb, opposite valve cover has vented filler cap with a hose into air cleaner.

- 289 cobra. Vented caps on front of both valve covers, hoses to puke tank on rear.

- FE with cast valve covers. A vented cap per side no PCV at all, and would that be it?

I'm guessing there's more configurations than this but, for the moment, my most pressing question is relating to the 408w being built in my engine thread. Below are pics of the valve covers I intend to use on this engine. They came with the caps and pcv shown.

The intake is an Edelbrock RPM air gap dual quad with matching carbs. I believe there is a provision for a pcv line on this setup. Is it as simple for this application as hooking up the pcv, or am I missing something. (Do I need a puke tank, etc.)

[Tommy]

When I built my Cheetah tribute I wanted to use C2 Corvette finned valve covers and they have no openings for anything. After much research I made my own PCV system for the usual reasons: releasing crankcase pressure to preclude oil leaks at seals, drawing clean air into the crankcase to reduce contamination of engine oil, and reduce harmful emissions into the air. I mounted a fitting into one valve cover with a baffle inside to prevent liquid oil being drawn out. I drilled a hole in the intake manifold just below the carb for a PCV valve, and connected it to the valve cover fitting with a hose. Lastly, I attached a breather to the other valve cover that was large enough for adding oil, and had a filter element to clean air being drawn through it. The key with a custom setup is to get a PCV valve suited to the engine. So far, so good.

[Hoodabest]

You should put one speck of dust somewhere on your engine and post a photo offering a prize to the first one to find it. I truly can't recall ever seeing an engine so clean. Even more impressive is the workmanship. To me it's these details that lend credibility to our choice of recreation. OK, that being said,

I was under the impression that the "P" in PCV meant positive displacement, or air going out. But where I get confused is why one side will have the one-way action of a "valve", the other side is merely vented to atmospheric equality.

My only point of reference on this is building Harley-Davidson pan and shovel head engines. They have a breather gear that is actually a hollow tube with a window cut in it that is timed with the crank position to open and release pressure at the right interval in the cycle.
However, there is an additional vent out the back of the crankcase housing that is typically left open, and only pcv'd as a "performance mod" by an obscure few.

See what I mean?

Why pcv, only to vent too? Buuuut, that seems to be extremely common, and also works well. Why?

[Tommy]

I speak as one who has done the research but claims no expertise. Here's how I understand the system. At idle the engine produces relatively high vacuum that opens the PCV valve to suck out crankcase vapors. The filtered vent on one valve cover allows outside air to replace that being sucked out. At high RPM crankcase pressure increases and the PCV valve allows that pressure to be released into the intake for the vapors to be burned. If the pressure exceeds the flow capacity of the PCV valve, excess pressure is vented through the filtered valve cover vent. . . . So there are times when air in entering the engine through the valve cover vent, and times when vapor is exiting. ... Lastly, the PCV valve is designed to allow flow only one way to prevent a backfire in the intake from traveling back to the combustible vapors in the crankcase.

The Cobra I used to have was built with a racing setup that included catch cans for fluids. The engine vent system consisted of baffled fittings on each valve cover that were plumbed to a remote catch can with a filter vented top. It served the purpose of allowing the crankcase to vent at high RPM, but did nothing to provide clean air at lower speeds. It would accumulate 2-3 ounces of fluid after a weekend of track time. That system seemed right for a race car that would have the oil changed frequently, but not a street car.

[Gaz64]

I run a PCV valve on my engines, BUT I run a large (-16AN ) hose from the other cover to a Moroso Air/Oil Separator (Stef's is similar), and then uphill to the aircleaner base.
The PCV valve engine intake manifold vacuum draws fresh air into the crankcase at part throttle through the large hose. Under load, the airflow reverses, since the crankcase blowby increases, and PCV flow decreases due to lower manifold vacuum.
At wide open throttle, PCV flow is near zero, crankcase blowby is higher than part throttle, and flows out via the large hose towards the aircleaner. I don't get any oil mist on the valve covers, and no leaks from the rearmain seal etc.

[Grubby]

I have always run a PCV.

For this build my engine builder is strongly against PCV. I trust his opinion and plan to go without.

My 496 FE has breathers on both valve covers.

Feedback will follow once I start putting miles on the engine.

John

[Gaz64]

For a street registered car that spends 90% of it's time at part throttle, you will get more benefit out of having a well designed system like I quoted above, versus a non-positive crankcase system, which simply allows blowby to vent to atmosphere.
A non PCV system with 2 vented filters at part throttle, instead of consuming the blowby gas, is just topping up your oil with blowby acid.

A car meant for off street racing is a completely different ballgame.

[scootter]

I would run one. I didn't run one for a short period of time. Had a lot of blow by coming out of the hood at stop lights. I installed the Wagner adjustable valve. Well worth the money.One off the shelf more then likely won't work correctly.