I have 4 bolt on style breathers on my sbc valve covers and I want to put in a pcv valve ( drill and tap into intake ) But my worry is can I draw to much air into the system by having all 4 breathers "open" or do I need to "cap" 3 out of 4. If I leave all 4 open will this be like a giant vac leak or am I way wrong?
A PCV doesn't create any measurable vacuum in the crankcase. The PCV valve itself is a vacuum leak but its a metered vacuum. The number of valve cover breathers will have no effect on the PCV.
Old wolf is right. You need to have a breather for air to come in. In this case size doesn't matter. Sent from my iPhone using H.A.M.B.
Thanks for clearing that up ! it's one of those things the more you think about it the more you just get confused !
Take into consideration where you drill the hole. The cylinders close to the hole may run lean. Sent from my iPhone using The H.A.M.B. mobile app
Although you got your answer, it's worth mentioning that it is the PCV valve that meters the "controlled vacuum leak", and the leak comes from the intake runners, not from the crankcase. Crankcases are generally pressure neutral (excepting some blow-by), which is why draft tubes and pcv systems were invented in the first place. The one problem with multiple vents is that they may prevent true circulation, meaning that the air will mainly flow along the path of least resistance, and there may be places in the crankcase that don't catch the breeze.
mike 51merc has a very good point about the path of least resistance , may be it is just time for some new valve covers with holes for a breather and pcv valve .
This isn't very "scientific", but with the 283 in my roadster, I added a PCV to the Cal Custom valve covers, between the two carbs. These had cool breathers on them, they should have been swapped side to side, but the breathers wouldn't clear the carb linkage on the dual quads. I epoxied the three 1/4" holes behind each breather up, so they're no functional, and oil can't slosh out. The early Offy intake has the front oil fill, so I put a chrome fill tube in with a vented cap. It seems to work fine, the PCV is routed to the primary (rear) carb, it runs great, doesn't puke oil out of the breathers nor the fill tube cap, so it seems to work. This engine doesn't have the road draft tube, it's a '65 block.
Eckler' s sells a pvc "kit" listed for $31.99 for early chevys so we don't have to ruin our early valve covers and intakes. The other option I've seen is to run a line from the oil full tube in the front to the carb. I think chevy eventually went to this set up in the 60s as the smog requirements started to kick in. Sent from my SM-G550T using The H.A.M.B. mobile app
If you're running the PCV hose to the fitting on the oil filler tube, I'd recommend that you use a sealed oil fill cap on the tube. Then place some sort of breather or vented fill cap toward the rear of the valve cover if possible. If you use a vented fill cap on the same fill tube where the PCV hose is connected, it's not going to do a very good job of actually venting the whole crankcase.
I'm not an expert and as always, I'm subject to correction, however, 10 thru 20 years ago, I miss-spent a great deal of time and money messing around with radically turbocharged vehicles. PCV systems were something we had to learn the ins and outs about just to cope. Anyway, road draft tubes and valve cover breathers are anachronisms from the good ol'/bad ol' days before pollution awareness. If you have a breather, you may as well skip the PCV because it will only be 10% effective for any anti-pollution benefit, drawing 90% fresh air..... 1% effective if you have multiple breathers. The crankcase vapors will seek the path of least resistance and exit the breathers anyway. If you have a blend of breathers and PCV, it won't make your car run better or worse so, no worries there; the PCV just becomes redundant, purely cosmetic. Blow-by increases with load/RPM/HP/etc., and will become positive crankcase pressure if it's not purged either with breathers or a sealed valve cover w/PCV system. You don't want positive pressure. It works against the piston ring seal and against piston stroke, and it leaks into places you don't want it to leak. But it's very possible in high output set-up for the PCV to become a bottleneck and not keep pace with the positive pressure. This is why you may see the oil dipstick pushed up, and in some cases, oil actually sloshing out the tube. There are some complicated fixes but breathers are an easy and effective solution. (But not without pollution consequences so it's not a free lunch exactly.)
Its not because of the so called pollution that I retrofit a PCV. Its the elemanation of water vapor and combustion gases that dirty the oil and cause sludge to form. The oil is a natural thing it comes out of the ground. Back when I was a teen my shop was a very large white oak tree on the creek bank. I had a chain hoist hung from a stout limb. And I cleaned parts and scraped grease and had the ground looking like asphalt. And no fifty odd years later that tree is still alive and the grass is taller and greener than the surrounding area. You cannot make something into nothing. If you burn or evaporate a gallon of gas you are still going to introduce the same amount of so called pollution into the planet. It don't matter if you run it thru a cat conveter. or have a PCV or EGR. The best thing is to tune and drive to get maximum fuel economy.
Bob Maybe this will help here are 2 links that explain how things work. Sounds like you have a basic engine so this should apply. In my opinion what the Shift Wizard is saying is correct. it is further explained in the links below. hopefully I attached them correctly. You should have 2 things going on in your valve covers. 1- PCV valve hooked from valve cover to manifold constant vacuum either on carb base or manifold somewhere with constant vacuum. 2- opposing valve cover should have hose connected from a CLOSED breather or 90 degree tube directly to bottom of air cleaner. basically thats a filtered breather above manifold vacuum. Thats it nothing more. this allows vapors to circulate from one side of the engine to the other as needed. so they can be dealt with by essentially burning them up. This works for stock engines and performance engines. It can get more complicated, People do all different sorts of things for there race applications. but the basic concept is the same. evacuate Crank Case Pressure There are many reasons why. like Old wolf pointed out. It protects the oil makes it last longer and perform better. It also can keep oil mist off your engine. If it is excessive you can smell it and feel it burn your eyes. Its bad for you to breath. Its better for engine performance if its done correctly. the list goes on. Your initial question about having 2 breathers and a PCV wouldn't work properly. Thanks, Michael https://en.wikipedia.org/wiki/Crankcase_ventilation_system http://www.aa1car.com/library/pcv.htm
I think the OP is referring to drilling the manifold to vent the crankcase [not for the source of vacuum ] You can reverse flow the direction of air/vacuum to the PCV valve and have the vacuum drawing from the fill tube at the highest point......... Then use the draft tube as the vent. This method is best for removing condensation. Attached is a photo showing the PCV attached to the oil fill tube [but it can be mounted anywhere between that and the carb base]
Maybe what you could do, is mount the pcv valve in one of your existing breathers, plug the other one on that side and draw in thru the breathers on the other cover. If you are clever you make it and the plumbing not too obvious/obnoxious looking; but be sure to baffle it so you draw vapors not oil mist.
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