Im in the process of completing a chevy 350, mostly stock, for a model A coupe. The motor is pretty low key and will make around 250hp. I have a 3x2 intake set up with 3 Rochesters and an oil fill on the manifold. My valve covers have no provisions for breathers or PCVs. My original plan was to add a breather on the side of each valve cover and in conjunction with the oil fill tube I figured I would have enough ventilation to avoid an oily mess out of the breathers. After reading numerous posts on here I am questioning my logic. I understand the theories of PCVs and I am willing to run one if absolutely necessary. However the car Im building will more than likely see less than a 1000 miles a year and Im not terribly concerned about reducing the life of the engine. So my questions in the end are: If I run the oil fill tube in conjunction with a breather on each valve cover am I still asking for a mess? Or is that adequate ventilation for my application? Im new here but have spent some time on this site these past few months. Great site and great info. Thank in advance. Chris
I would highly recommend using a PCV in your motor. Cars that didnt have a PCV had a road draft tube that drew the oil fog created inside the crank case out. Just using breathers will not do that. I put a gm crate 350 (brand new) into my dads roadster with just breathers on it no PCV. With in 1000 miles the oily fog was all over the engine and firewall. Put the PCV valve on it no problems. You need a PCV or a road draft tube to keep it clean. And there is no provision for a road draft tube on a 350
The biggest mistake people make when doing valve cover mounted breather add-ons is failing to baffle direct oil spray from the entrance to breather and pcv ports. Most OEM valve covers have splash baffles on the inside. More examples https://www.google.com/search?q=spl...iYHOAhVBwGMKHdDKDBYQ_AUICSgC&biw=1280&bih=752
On my model a with 327 I had a pcv and that gave me oil spray on the windshield. Switched to the valve cover breathers and no more oily mess. Pcv doesn't work in low vacuum situations. I had too much air flow over engine and I think that changed the pressure in the engine or vacuum maybe. My point is it's not so cut and dry. Sent from my iPhone using The H.A.M.B. mobile app
I think there was an oil fill tube (late 60"s?) with a PCV and a sealed cap. Someone else might know better. Not that attractive. Road draft tube provisions were eliminated on later blocks. X2 for the baffles.
Thanks for the replies. So after reading more than I care to admit, I am probably going to run a PCV. So with that said, if I run a PCV off one of the valve covers and I have a vented oil fill tube, do I close off the opposite valve cover? Can I have a breather on the opposite valve cover along with the vented oil fill tube? or will that not work? I was under the impression that you want a breather on one valve cover and a PCV on the opposite cover. Im just not sure how the oil fill tube on the front of the manifold plays into this. Thanks.
It's easy, the PCV system needs two holes, one for air to be drawn in and one that gets sucked into the air intake via the PCV valve. The hole can be anywhere, but across the from the PCV valve is best. Also should be filtered so dirty air isn't drawn into the engine. The PCV can go anywhere you can fit it as well, tit just needs to be connector to a vacuum source on the carb or intake. The problem most people run into with the later blocks and no hole valve covers is they only have one hole, usually in a sweet-ass vintage intake. This is a good place for the oil fill and breather but leaves the question of where to put the PCV valve? I've seen dudes drill and tap intakes into the oil galley/ lifter chamber and run the 66/67 screw in PCV valve.
I made my own system, with a conventional type breather cap on one valve cover, and a hose set up on the other. It goes to manifold vacuum and has a piece of steel with a 1/16" hole drill in it to restrict sucking too much. It cured my oil drips and no adverse effects. I guess it is just a PCV type setup done another way.
I cant see any reason you can't run the suction hose direct to a fitting on the oil filler neck, and not run anything on the valve covers to let air in. It means you'll have a slight pressure drop inside your motor, but that is not a bad thing.
At of mixed advise here, the first thing is, do you have a Carb in you multi carb set up that has a provision for a PCV system. If you connect the PCV hose to the manifold below Carb it will run very lean at low speeds, just like an air leak. You cant just put it into any vacuum port on the manifold as that will make the cylinders that run off it very lean too. Having a closed system will not work the best If you have a poorly baffled breather oil will be pulled up and burnt. Remember that PCV stands for Positive Crankcase Ventilation so if it is a sealed system its not positive. There have been many good threads on PCV systems on here in the last couple years. JW
The short answer is, if you have a lot of crankcase pressure, as in poorly sealing, or worn rings, you will have oil come out the breathers. PCV just directs it back into the engine to burn. If the engine is mechanically sound, you'll have a minimum of oil escaping.
What the last 2 guys said^^^ Think air in, air out where out is actually going to manifold vac location. I would think a simple hose fitting from both valve covers would do. One would carry the vacuum and PCV valve,the other can go into the base of your air cleaner. The signal at the PCV will be much stronger than the air cleaner signal (almost nothing there) but kept inside the filter element the air in will be filtered. I like simple, I'm kinda lazy like that.
Ok thanks. I understand the air in air out concepts. I am going to run a pcv on one valve cover. What I am trying to understand is does it matter if I have more than one vent, as in a front oil fill and the other valve cover? I know it needs a fresh air draw I am wondering if it matters if I have 2? If so, can I cap the opposite valve cover and use the oil fill tube as a fresh air supply? Thanks for the patience.
Some of this is confusing but how can 26T be correct?? Some Chevy 350s have the PCV connection in the base of the carb and some are about 4" away in full manifold vaccum
Your understanding in post #16 is correct. You can have the PCV in one cover and have it in either the opposite valve cover (which is generally the way stock engines are), use the oil filler in the intake with a breather, or use both. Either should work.
Any other vacuum port other than in the base of the cab is for stationary vacuum, Dist advance, Auto modulator, Brake booster etc. Any other point other than the Carb base is not correct. JW
I totally just realized my last post made little sense. Yes, you can have two breathers for air inlets in addition to the PCV.
Since you said you were running 3 X 2 manifold I found these pictures in my photo file on somones solution to this issue. Cannot tell you it work but it looked sanitary plus there is even P/N's for the PCV
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