This may sound like a dumb question to some, and I know a subject you don't see discussed to often, I was wondering what the best way is to determine the proper PCV valve for a engine? The best I understand, the PCV valve is supposed to be closed or very close to closed at idle when vacuum is higher and then open up as vacuum is reduced to evacuate any blow-by gases. I have seen some information talking about different flow rate and spring rate ratings in the PCV valves to better match the idle vacuum of the engine, but haven't found how to determine the rating of the PCV valve. I have also seen valves for the 300 listed as high flow and low flow and have seen adjustable valves for performance engines. I just don't want to pull too little vacuum and have a potential overpressure or, I don't want to pull too much that could cause a poor idle or economy and perhaps even pull oil over into the intake.


My engine is a 90 model 300,with the only changes being manifolds, Carter carb. and Dura spark from a 76 and Cloyes Timing Gear Set advanced 2 degrees. Trying to set up the engine for the best economy.
Any input would be appreciated....Thanks

 

Sorry I don't have any info on this, but I want to subscribe. I'd love to know this info too, as I'm having issues with oil out of my breather cap, and I'm pretty sure it's my PCV setup.

 

PCV calibration requires that blowby at various conditions be reconciled against the PCV flow rate at various vacuums, so the valve maintains a slight vacuum in the engine. Although I have a book of flow curves for the valves I have not found it very useful due to the fact that there are so many variables involved - cam profile, ring pack, honing process, etc. - in any particular engine build that influence the PCV requirements. I just try to use the highest flowing valve that does not introduce such a large vacuum leak that driveability is compromised.

Years ago when I was fiddling with three Holleys on a 3 x 2 trying to get it to idle lean enough I finally thought, "Wait a minute. These three carbs are designed to have 3 PCV valves hooked up to them and I have that port capped off." I added three air bleed restrictions where the PCV valves should have been and presto - good idle quality.

 

". . . the highest flowing valve . . ," but where do you find this info when you are in the auto parts store trying to find this valve among all the rest?

Flyer, I'm pleased that your description, above, is about what I had intuited. I think it should be the beginning of a discussion that ought to be right up front on all the hot-rodders and resto-rodders' sites. I've been trying to learn engine-tech and car-tech ever since Peterson Publishing (Hot Rod magazine and others) put out their series of softback books in the Sixties. But I have never seen an article that says, "Now that you've modified your engine, your factory-calibrated PCV system has to be RE-calibrated, by you, if you want the best results from all of your other work!," and then tells you HOW. Flyer, you surely have seen how all the emphasis is on selecting a cam, sizing your new exhaust system, single-plane vs. dual-plane, all the bolt-on stuff. At least the magazines were pretty good about telling us about re-jetting. But they weren't so good about saying, "Once you build an engine with all these non-factory components, you can't just take your blink-light and set timing by the factory spec and expect good results." Oh, if they were writing about yet another Chev 350 build, they could say, "Order the Mr. Gasket timing kit with the new weights and springs, etc, etc.." But if you were building anything unusual, say an old Dodge 318 poly, or an AMC 390, or a Ford Six, Mr. Gasket has nothing for you, and the magazines rarely would tell you how to figure out what advance curve YOUR engine wants, and how to find and modify springs and weights and so on.

Flyer, how many times have you read a post by some amateur (most of us) saying he tried this or that recommended component or modification on his engine and was disappointed that the result didn't meet expectations. My impression often is that the component or modification he used was a good one, but that he didn't recalibrate the timing curve (and often not even the jetting) to make the new configuration work.

Now think about the PCV system in this light; this subject is a real black hole! It's hardly ever mentioned in the magazines, and not even well-covered by the better hotrodding websites such as Speedtalk.

I wish somebody knowledgeable (like you) would do a long article or series on this subject. Start with a general description. Then say, "Okay, here's a project engine: '41 Lincoln V-12 flathead (i.e. NOT another SBC), bored and stroked, vintage Edmunds intake with two Strombergs, tube exhaust, modern cam profile, build for street use. If you go to (xxxx.com) they'll tell you how to recurve a dizzy. NOW, if you replace that old road-draft tube with a PCV system, or recalibrate the PCV system on your newer engine, your engine will run even better, stay clean inside, and work well for decades."

Next is a drawing of an adjustable PCV valve assembly that you or your buddy can make on a lathe. Then an explanation of how to set it up for your engine: here's how to size the valve slug, here's how to figure out how big a hole you make in it. Here's how big the hole in the end of the valve-body should be, here's how you figure out which spring to use and then how to adjust the pre-load screw you made while you were on the lathe. You might need to stick a roll-pin in the hose as a restriction; you might have to make up an adjustable bleed somewhere, etc. etc.. You need a vacuum guage and a Mityvac, and for road testing a tach and preferably a A/F meter. And so on. I'm just guessing with all of that.

Is this over-thinking something that's no real big deal? Then why are there dozens and dozens of PCV valves in individual see-through packaging, hanging on the shelf at the auto parts stores? The factories evidently feel that getting the PCV system precisely calibrated is too important to try a one-size-fits-all approach.

I wish we had had a PCV series like this long ago. Would have been a great project for one of the old tech writers like Jim McFarland or Bruce Crower, or more recently, David Vizard. What do you think about all this, Flyer?

 

I asked about this some time ago:


https://www.ford-trucks.com/forums/1...fferences.html

 

That's definitely an interesting read '86.

I had no idea that PCV valves were so calibrated. I thought that as long as one was on there, it was good.
So, I agree with Seattle, how does one go about picking one? Is it calibrated to the ENGINE? Or is it calibrated to all the modifications? What causes a need for a different PCV valve and how do you know what you need or what you're getting when you buy one?
Is the stock exhaust/1bbl going to need the same PCV as a 2.5" EFI manifold setup with a 4bbl?

This is a completely new topic to me, but has me wondering about a lot of things and wondering if my PCV may explain a lot of issues.

 

a guy COULD approach this PCV question considering that motors ran for many year w/o such a valve. Of course, the less blow-by the better. So, perhaps tthe healthier the motor, the less suction needed to keep it clean & breathing?...

Myself, I run a 60's specked motor, so I use very simple early specked pc Valve, or none at all.

 

I haven't read every post, I'll get to it, maybe. But I just want to say my thing, PCV "valves" are dumb. Ever take one apart they are just check valves and restrictors in a system that doesn't need a check valve. There is NO condition in which the flow could ever reverse, so why have a check valve. Secondly they are a restrictor, the only reason they need to restrict is that there is also a vent. There is no reason to have a vent, everything the vent does is bad. It of course causes the requirement of some controlled restriction to prevent to much vacuum leak, it lets in fresh air which contains oxygen and water which causes corrosion and degrades the oil, and it prevents the creation of a vacuum within the crankcase which would if allowed prevent oil leaks and help seal the rings.


I personally cap the vent and then take the PCV "valve" and gut it so it's just a fitting allowing full manifold vacuum to the crankcase. The engine runs better cause there is no longer a vacuum leak, the oil lasts longer, rings seal better, oil leaks are eliminated, etc. It's all win.

 

You're over thinking this, the PCV valve isn't that smart.


The PCV valve as setup stock actually functions opposite to what you have in mind here. It's open in all but wide open throttle(WOT), the flow through the PCV is proportional to manifold vacuum. Thus the only time there is no flow is when there is no vacuum at WOT. This is because there is also a vent, so gasses removed via the PCV are always being replaced, if not by blow-by then by the vent and thus NO vacuum is ever created.

As I said above, you to are making it more complicated then it is. And there is NEVER a significant vacuum in the engine due to the vent, because the the vent will always flow more then the PCV as it's unrestricted.



This makes no since.

 

Oh, man. Where to begin? I could spend an hour correcting grammatical errors alone.

I'm out.

 

The ideal Positive Crankcase Vacuum(PCV) system is very much supported by and in use by the aftermarket, mostly Moroso. It's a system that has many components that OEMs can't/won't bother with, in part due to EPA regulations.


Most important is exhaust evacuation, this is a component that uses exhaust flow to pull crankcase gasses into the exhaust stream. This in important to address the issue of WOT blow-by, of course blow-by is highest at WOT but also vacuum and thus the ability to remove the gasses is non-existent. However exhaust flow is at it's highest at WOT.


This is then connected to the PCV system via a one way(diverter) valves. So when exhaust flow is high that's where the gasses are pulled, when vacuum is high that's where the gasses go.

 

Ok grammar ****, can't refute my points thus insult and leave, I get it.

 

I understand how PCV valves work, under high vacuum they restrict the flow, under low vacuum they open up the flow, it's not just a matter of open and closed. Different valves flow different amounts, and I have not found the flow rates listed anywhere for the different valves, or how to select one for a non-stock motor, or as The Frenchtown Flyer was talking about all the different variables needed to figure the amount of flow needed at the different variables. And yes you can run without a PCV valve, but back in the days of the blow by tubes a motor was worn out in a 100,000 miles.

 

Thanks for your input.

 

Thanks a lot, Brute! Flyer is an automotive engineer, worked for Ford, races Fords, has more knowledge in his little finger than you have in your head. And he has enough respect for others to take a quick look at what he's written and edit the typos and such.

You have opinions. And an attitude. Flyer figured there's no telling you anything, and that it sure isn't worth a second of his time arguing with you. So, another promising internet thread gets abandoned to a blowhard. I'm gone, too.