Pcv Tech

There seems to be some confusion on how a PCV valve works. To engineer a custom system for our engines, we need to understand how it works.



The first ventilation system was the road draft system. A tube connected from the crankcase extends down below the engine into the air steam. As the car travels down the road, the air passing over the end of this tube (road draft) creates a vacuum in the tube pulling the gases that “blow by” the rings of the engine along with the moisture that builds up in an engine that causes sludge. For this to work there has to be a breather somewhere else on the engine to let fresh air in. Usually it is the vented oil fill cap/breather. That is why you see vapors coming from the breather cap on a road draft car when it’s idling…the car is not moving to create the air flow through the engine.



For you 8BA flathead guys, it looks like the road draft tube and the oil fill tube go the same place in the intake…they don’t. What you can’t see is the tube in the valley that aligns with the road draft tube to extend it down into the crankcase below the valley.



The PCV system is just an improvement on that system primarily for lower emissions. Instead of dumping the gases into the atmosphere under the car, the gases are sucked into the intake manifold through a PCV valve to be burned in the cylinders.



Do you absolutely need a PCV system? No. If you are tired of the oily film on the engine and firewall or you don’t want sludge build up inside the engine, then one is recommended.



Is it necessary to replace an existing road draft system on an old car with a PCV system? No, if the original is not clogged and complete. A good cleaning is recommended. If you are concerned about oil film on your chrome drive shaft or air pollution, then you might want a PCV system.





This is a 76 350 Chevy. The PCV valve is in the drivers side valve cover with a hose to the intake right below the carb. base.





The breather element can be seen inside the air cleaner with a pipe that runs down to the passenger side valve cover. Normally the air enters the engine through this element. It passes down through the pipe into the engine carrying the vapors and moisture with it. The mixture flows through the engine up into the driver’s side valve cover, through the PCV valve into the intake manifold to be burned in the cylinders. If under extreme load the PCV valve can’t handle the extra blow by, the vapors will back up into the air cleaner and get sucked into the carb through the air filter instead of dumped into the atmosphere through an external breather. Putting the breather in the air cleaner, called a closed system, is a back up pollution control that is not necessary for our cars.





How does a PCV valve work? You would think that the PCV valve would flow more at high vacuum situations like at idle and less at lower vacuum situations like pulling a hill. It does the complete opposite. If you look at the above picture, you can see that the plunger is tapered on the right (vacuum) side that fits into the bore of the body. At idle or cruising when the vacuum is high the plunger is pulled farther into the body. The taper reduces the opening and the amount of air flow through the valve is reduced. Under acceleration or heavy load, such as pulling a hill, the vacuum drops. There is more blow by under a heavy load producing more vapors. In this case when the vacuum drops the spring pushes the plunger to the left opening the port up for more air flow when it’s needed the most to remove the extra gasses. Many of the early PCVs were designed for vertical operation using gravity on a heavy plunger instead of a spring to modulate the valve opening. To work right, they need to be mounted vertically.




In the case of a backfire that could send flames into the volatile fumes inside the crank case, the backfire pressure pushes the plunger to the left where the valve seat on the left (crank case side) of the plunger closes off the opening eliminating a possible crankcase explosion.



A custom PCV system needs a PCV valve connected to the intake preferably near the carb base and a place for the fresh air to enter the engine. The fresh air intake needs to be as far away as possible from the PCV valve so that the whole engine gets ventilated. If you put a breather right next to the PCV the PCV will just suck fresh air into the intake and not remove any blow by or moisture.



There are more ways than one to skin a cat. Armed with how it works and what is needed for it to work, you can come up with some creative ways to accomplish your goal…a clean engine inside and out. You've all seen my hot rod breather idea. I've seen PCV's drilled into intakes both on the outside and underneath in the valley. I've heard of people plumbing them into the fuel pump block off plate on Chevys.




It’s my non-engineer opinion, that whichever PCV valve you select is not THAT critical. It certainly won’t affect the engine performance. If you still get an oil film then you may need to get a PCV designed for a bigger engine. Even Chevrolet with their closed system planned for the rare occasion when the valve couldn’t keep up.


If you add a PCV system to your hot rod you'll need to adjust the idle mixture screws to compensate for the extra air entering the intake below the carb. No biggie.

 

Great post !!!!! Now I need to change my PCV valve to one that doesnt have to be vertical !!!!

 

Good work Tommy.
After sorting this all out with you last week I was discussing it with a mate at the weekend. At the risk of confusing the issue this was his version. (He's a mechanic at a Ford dealership) Pretty much everything you said except (surprisingly) No breather. He said the new cars have a sealed system where the only "inlet" to the crankcase is the blowby. I asked him about that creating a vacuum in the crankcase but he said it evens out. In reading your description above i.e. when the engine is under load, more blowby, PCV opens more I think he could be right.
Sorry Tommy, I hope this doesn't start another one of those bullshit debates.

Pete

 

Well done Tommy.

This is the best write-up I've seen on how PCV's work.

Upcoming tech articles will be hard pressed to keep up with this one.

 

Wasn't really worried about oil film before installing a PCV on my flathead. The only thing I didn't like was the stink and the smoke seeping out of the hood louvers when sitting at stop lites.

 

A good PCV valve for a non vertical install would be for a California 65,66 Caddy 429 The valve is mounted under the intake manifold and rests on the valley pan and vents from where the draft tube originally was mounted.

Dawg

 

Pete, could it be that newer engines create less blowby (better rings?) and the vacuum created takes care of what little there is created?

One function of earlier PCV systems was to "sweep" moisture (short drive; engine not brought up to temp) out of the crankcase. Do the new engines (by way of the EMS) come up to temp faster?

I took a look at my Dodge truck, (sadly, NO Hemi! ) and it appears there is no breather. (inlet) Unless I missed something.

Just curious.

 

Just my opinion but I think either way will work. Just still a bit concerned that with a breather it might create a constant vacuum leak (compensated for by mixture setting as Tommy said). I'm gunna try both and see which works for me. It's only a matter of having a sealed cap and a breather cap to interchange.

Still great tech Tommy

Pete

 

"....You've all seen my hot rod breather idea....."



This part of "you've" hasn't seen it. Do you have a pic??

Nice post, by the way. -Abone.

 

I limited my research to engines that a hot rodder would use. There were other engine diagrams with sophisticated systems that even had arrows denoting the flow through the engine but they weren't V8s with carbs so I was afraid they would just complicate the situation. Most of the cars built since about 1962 have this constant vacuum leak engineered into them.

Here is a link to my PCV hot rod breather idea.

 

Awesome post....

Danny

 

Here's a pic of the old road draft system. I remember Los Angeles in the fifties. The air gave new meaning to the term "Burning Eyeballs"!!!!

pigpen

 

i've seen some pcv's that screw in, has anyone drilled and tapped an intake for one. i was thinking a sbc with 3x2 between the dizzy and rear carb, then running hard line to the center carb (into the vacuum booster hole). if those aren't big enough for a V-8 then how about putting a 90 deg fitting then the pcv. don't want to cut into my covers and don't want oil everywhere also.

 

Good stuff.

I didn't mean to move the subject to NEW engines; I was just curious if they worked the same as the system you described. (very well I might add!)

There was a flathead PCV setup that "tapped" into the plenum UNDER the intake manifold. (the idea was to hide the PCV valve)

If it was assumed that the plenum was common to all vac. signal (multi carb) and (for example) a 2 x 2 manifold was being used; would the opening of the PCV valve cause a major change (lean) in air/fuel mixture?

Then, when the PCV "closed" (low vac. signal) would the air/fuel become too rich? Because of two different conditions; would you have to adjust carb mix somewhere in the middle?

In your other post you show a breather (non operational) that sits on the fuel pump tower. If I remember right that style of fuel pump tower had a "sleeve" that connected to the crankcase. (would allow air to "sweep" into the crankcase)

If you returned the breather (filter) to operational status and relocated the PCV to another location; would that still allow a "loop?" (that filter looks like it would be a good intake source) BTW - What's the filter from?

Thanks for an excellent piece!

 

The PCV valve is open when ever the engine is running it justs modulates back and forth depending on the vacuum signal. At Idle when the mixture is more critical the valve reduces the flow so the idle mixture screws don't need a great deal of adjustment to make up for the extra air if you are adding a PCV to an existing engine.

Everytime you changed carbs on a PCV valve engine you compensated for the PCV when you set the idle mixture screws with out knowing it.

You have it backwards and that IS the confusing part. The valve opens up during low vaccum (pedal to the metal) conditions. It closes down on high vacuum conditions. (idle and cruising) The 94 carb's power valve would kick in and enrich the mixture at the same low vacuum condition. The 97 valve opens if you floor the pedal. I'm no carb guy but I believe all carbs have some built in system to enrich the mixture for heavy load (low vacuum) conditions.

It's not a pure vacuum leak because it is a mixture of very volatile vapors, moisture and air and if it's tied into the central plemum it will get homogenized. They have been running them on street engines for over 40 years. We've all probably street raced with one with no ill affects.

I remember when they came out. All us younguns said I don't want one gumming up my intake and valves. What we didn't know is that with out it we were sludging up the crankcase.

high vacuum = low PCV air flow
Low vacuum = higher PCV air flow.

 

A bump fpr tech week.

 

Howdy, All! First post to the HAMB.

Just added a PCV system as part of my return to a 2-barrel, (Rochester 2GC on a merc iron intake) on my 8BA.

I found a nice little all-metal PCV valve at the parts store which is threadded on the sucker-end and has a 5/16" hose fitting on the other end (FRAM FV266). I popped a freeze plug into the road draft tube hole, drilled and tapped it, then screwed-in the PCV valve.

I have a fuel-pump/breather from an early flathead on the back of the manifold for the crankcase vapor inlet. Looks pretty tidy and my clothes don't stink after a half-hour drive now (ahh, but what a stink it was!).

I initially ran the hose from the PCV valve onto a fitting at the base of the carb, but switched it to come in at the top of the carb a few days ago. The Rochester 2GC has provisions for both built-in. My uneducated guess behind doing this is that the vacuum will still draw vapors from the crankcase, yet not affect the air/fuel ratio entering the intake manifold.

From reading the earlier posts here, I am led to believe that running the hose in below the carb is the established way to go. Can I really gunk-up the venturis and metering system by drawing the crankcase vapors in upwind of the venturis? Anyone ever try it this way? Also, I am still using non-detergent oil, so I don't know how that might weigh-in on this issue.

Open for suggestions,
chrx

 

That's how I see it.

 

This is a great explanation of how this works.

Does anyone have pictures of a pcv they have installed on a 235 with dual carbs. I would like to see how you plumb it.

Thanks, Bill

 

Nice post! Now it raises a question on my ride. The engine....... 1972 Cad. 472, breather on one valve cover, PCV on the other. Works great at low rpms. Car is geared way to deep so 60mph is 3 grand. As soon as I hit 3 grand for over.... say a few mins. oil starts leaking out the dipstick tube. Would hooking the breather up to the air cleaner help this problem? And yes I know that my 472 shouldnt be running that many rounds any way, but thats the way it is for now!

 

Great post, Tommy, very informative. Thanks !

 

OK so here's a question:

So what if you want to run a PCV but want to run those "cool v.covers" with no holes in them and you aren't really too fond of drilling a hole in you super rare intake???

Anybody ever thought about drilling the back side of the intake valley on the block (basically where the dizzy is) kinda the same idea as the early SBC's???? or is that just a dumb idea??? I think it would be fine for SBC - since they are so available that you aren't really "ruining" the motor. But for say a early HEMI - I don't think I would do it.

Bad Idea? Good Idea? Other Ideas? Just Drill the F'in Valve Covers?

 

I didn't want holes in the fins but I like the add on hot rod breathers that were popular in the early 60s. The top of this breather was blocked off with 5 min epoxy so the PCV valve can only get air from the side of the valve cover below the epoxy. It's mate on the other side was left alone to let air into the engine and you can see the oil fill tube has the usual breather as another air inlet. This system gives me good cross flow ventilation of the crank case with that old timey look...at least I think so.



There have been a few posts where people have drilled their intake for PCV valves. For most old engines like a Hemi, a flathead or a Buick, just fabricate a way to plug in the PCV where the road draft tube comes out of the block. Most of the old engines used the oil fill as the air intake for the road draft system. It will still work that way with a PCV. It's just the newer Chevy blocks that have eliminated the road draft hole in the block that presents the problem.

 

Tommy, what is the part # on that horizontal PCV valve?

 

Tommy--awesome post. There's been a ton of PCV posts on here, and this is the best one yet. Thanks.

striper--I don't know of any newer cars without make-up air/breather. Of course, I'm not a mechanic, so I don't know every car out there. It is true that the systems are "closed" though--they aren't allowed to vent to atmosphere, because they would fail the evaporative emissions spec.

53chieftan--Sounds like you need a higher flow PCV valve AND a better flowing breather. You're building up pressure in the crankcase because there is more gas flowing by the rings than the PCV valve AND the breather can flow. The pressure in the crankcase is pushing the oil up the dipstick tube. Short of a new bore and rings, try a different PCV valve and cleaning out the breather. Hooking the breather to a vacuum source will help it draw more gasses (and make less of a mess) but you've got bigger problems than that.

 

Just get a PCV grommet and it should go right in the road draft tube hole in the valley cover. stick in a PCV and run it to port on the back of the carb.

It was a little more difficult for my Buick straight 8. The road draft tube comes out of the push rod cover on the side of the motor and the intake is on the other side. I drilled a hole in the top of the draft tube connection and inserted a grommet and pcv. I connected these to the vacuum line going from the fuel pump, up to the manifold, next to the carb. I had to try a couple of different PCV until I got one that was light enough to give me good line vacuum at idle.

 

Here's how I did mine for my flathead intake. Good enough for me. I bought the PCV valve and grommet from Hot Rod Custom Supply out of Florida, but I could have rolled my own too.

Mike

 

tommy - First; THANKS for bringing this subject BACK!

When we last discussed PCV's, I had gotten the "action" of a PCV backwards. (which you straightened out for me)

But since then (2005!) something came up the "muddies the water."

I'm posting where we left off, then I'm throwing a "Curve" into the discussion.

THE "CURVE!"

To this point we have been discusing NORMALLY ASPERATED engines.
What happens when the engine is BLOWN??

If we have a blown engine that has say 1 lb. boost at an idle and as revs increase, rises to 7 lbs.; did we just "kill" any vacuum at the PCV??

EDIT: It's funny; you sit at the keyboard with a "thought", type a question; post it, re-read what you wrote, THEN BINGO! you realise that if the PCV valve is attached to the base of the carb (on top of the blower);
NOTHING CHANGES!!

 

Great post, thanx! This is why I love this place!

 

You know they sell 2x2 manifolds right?