Theory behind valve cover venting

[Eastbay851]

Totally get venting crankcase. No problem there. What I don’t get is valve cover venting. Every time a piston goes up (increasing pressure) on a given head, the other one on that side goes down (decreasing pressure). If you are venting the crankcase already, the neutral valve cover contents is doing nothing. I am venting through the old fuel pump location and don’t understand the engineering of venting valve covers beyond the crankcase venting. Can somebody sprain the engineering rational behind this, please?

[slalombuggy]

It's pointless. Crankcase pressure originates in the crankcase. The pushrod tubes are full of oil and pushrods, so very little pressure is able to get vented from the covers. All they are good at doing is allowing oil to get pumped out of the rocker box on the 1/2 side and out into the breather box. If you need to vent the crankcase and your valve covers to control oil leaks out the pulley hole, you gave piston ring problems and you're bandaiding the problem.

I only run lines from my oil fill and fuel pump blockoff to a 3qrt CSP breather box on my racecar and just run a CB breather tower on my street engines.

[Eastbay851]

Now, what if you are running a blower?

[vwracerdave]

I have a different perspective. Venting the valve covers does help vent the crankcase. It also allows oil to return to the sump much faster. On my 7000 RPM race engine I have no issues with oil being pushed out the VC vents. Some folks run stock fuel pumps and can not use that location for a vent.
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[BFB]

Nice to see a “new” discussion, this will be fun.
I agree with Dave on this one, if definitely allows the oil to drain better, also lets the engine breathe and helps keep the covers from leaking.
You dont think crank case pulsation affects the pushrod tubes?
Think the covers are neutral, then why do valve cover gaskets get sucked in?
Why do majority of engines have a breather on valve covers but it’s not necessary for a flat 4 vw? If anything it should be more necessary for us as our valve covers area fills with so much oil.
I know other guys will have really cool scientific explanations of the pulsation in the case and ive heard that theres actually a oil vapor “cloud” that forms in the case from it, but i dont know about all that, sounds plausible though. And i dont think a single 3/4” or 1/2” line is enough to help control that.

Ever since my first boosted vw ive vented the covers and the fill tube, typically to a multi port breather box of some sort, on almost every engine ive built.
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[slalombuggy]

[esde]

My experience is that with unvented valve covers, I suddenly was seeing more oil mist/ residue at the oil filler breather and rear pulley. No clue why exactly, just my experience, and I tried several things to see what helped or didn't.
Right now I have both valve cover vents connected to the lower back of the generator stand, and I have a pretty clean engine. My thought is that when one head/ rocker box is full of oil, the vent allows an air path to help the oil drain back faster. Without an air inlet, the head could be slower to drain.. in any even I'm not 100% sure why this works for me but it does without the need for extra breather boxes and miles of hose. And, FWIW, my pushrods are drilled to spray oil on the springs, and full hoover mods, so there is considerably more oil in the heads.
SD
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modok wrote:
Bent cranks are silent but gather no moss. I mean, ah, something like that.

[Vanapplebomb]

I’m not in favor of “venting” the valve covers. To me, it’s moving the gasses the wrong direction.

This is very evident when comparing used heads from PCV equipped engines (Type 3 & 4 with Djet), and those without. The PCV system had the fresh air inlets in the rocker covers. Clean filtered air enters the rockers, pulled through the pushrod tubes into the case, then out the top of the case into the intake. The PCV engines always have very clean rocker boxes with less buildup. Case internals are often cleaner too, with less pitting in the sump. The engines with the most crap in the heads are usually ones where people added vents to the heads. Just my observations.

In my opinion, it takes a very special engine to actually require more venting volume capacity than was available stock. Unless it is a very high rpm big bore engine, it shouldn’t need more than a 1/2” hose. All those sub 2.0 engines that people claim need the rocker boxes vented probably just have crap ring sealing and need to be rebuilt.
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[chrisflstf]

It seems logical that larger displacement and higher rpms create more turbulence. Also, do counterweights increase that as opposed to non counterweights?

[modok]

Why? why not. That's about it.
You can have a good breather system or a bad one, venting the valve covers or not venting them.

Strokers usually have an increase in pulsation from the lower rod ratio.

[raygreenwood]

[BFB]

[jpaull]

With less space in the case from stroker cranks via physical space, and via more stroke invading the area, the need for venting goes up. The worst time is when the rings have not seated and the case breather is working overtime.

Adding just a passenger side valve cover hose to the breather has been my "go to". I do 10AN for the most out of that one side.

If your able to survive without out adding a passenger side breather, thats great. However, i do not believe there is a downside, and letting the crank case breathe as good as possible could free up a horsepower or 2.

[Glenn]

Or it will piss past the oil slinger.
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[chrisflstf]

Hey Jeff - Did you tig that An10 on there? There isnt much room to get in there for welding. Or is it welded on the inside?

[jpaull]

Hey Chris, Tig would be nicer. I used a wire feed. Got around the slightly tight area between the bung and the gasket flange fine like that. Not ideal but gets it done. Steel 10AN welded to a steel valve cover is a decent way of doing it.

Much better then the Empi slip on bungs where people use breather hose aned a worm drive hose clamp that slips off and gives a james bond oil smoke down the street.

[Vanapplebomb]

[raygreenwood]

[oprn]

Venting the valve covers does help vent the crankcase. It also allows oil to return to the sump much faster.
That one defies logic! How does air flowing UP the pushrod tubes help the oil flow DOWN the pushrod tubes?
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[jim martin]

Why? I would think at some point someone copied the idea from the V8’S .
Then followed by a lot of monkey see monkey do .and here we are.
This topic is like asking what case sealer to use , everyone has either success
Or failure with the same product .
So why do some get away with more or less venting.It really comes down to ring seal , it’s already been mentioned. If you have it you will not have any issue and you can vent really where ever you want but if you have a crankcase pressure issue no matter what you do you will move oil .
Myself NO valve cover or fuel pump vent , just modified alt stand with a 1”id hose mounted to hi mounted moroso breathers.
If you have or think you have issues , measure it.
I use the fuel pump block off plate tapped with a barbed fitting attached to a vacuum/pressure sensor for you electronic data guys or s long piece of plastic vacuum hose mounted to a mechanical gauge up where you can see it .
Here is an old pic of what negative to positive crankcase pressure looks like when logged .again only an example . If you are N/A and using external breather setup you would hope for zero pressure across the complete run.
The line you’re looking at is the pink and in the boxes are the values .
Looks like I go from a negative to a positive value as soon as it hits 3rd gear.
You get the idea.

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