Which PCV Valve?

[Orion 762]

I've got a '71 Cheyenne, 350 SB, but with:

Headers,
Edelbrock intake,
Holley 650 cfm double pumper (Model 4165, List 6210-3)
HEI

I added a PCV valve recommended by Autozone, based on their "lookup" of the original engine. Tonight I noticed "at idle" that the PCV valve was rattling a LOT, giving a very rough idle. Seems that the Holley carb. is really pulling hard on the valve at idle . . . the valve is pulled "all the way up" and I can still get a strong suction on the bottom of the valve using my thumb.

When the valve bottom is plugged (with my thumb), the engine smooths out. Remove thumb, and I get rough idle.

I suspect either a BAD PCV valve OR maybe Autozone sold me the WRONG ONE. Seems to me that the "correct" valve should be based on the vacuum level from the carb.

Can anyone recommend the "right" PCV valve to use on this engine setup? Which ones work good with a 650 cfm Holley??

Regards . . . Orion 762

[dinnut]

why not just add a breather and an oil cap with a breather also

[Orion 762]

"dinnut" . . . Will simply having two "breathers" (one on each valve cover) provide enough ventilation. I was under the impression that "positive" ventilation (by sucking through the carb) was needed to get adequate flow.

Right now I have a breather-cap on my oil-filler hole, and the PCV valve on the other valve cover. Should I just replace the PCV valve with another 'breather cap"??

Orion 762

[Rooster's 67]

PCV stands for Positive Crankcase Ventilation.
It will remove vapors from the crankcase and pass them into the motor to burn.
two breathers will vent just fine. you will just be polluting the air more than with a PROPERLY setup PCV system

[Rooster's 67]

how rough is the idle, if it is dig difference I would look at the mixture setting of your carb, when you block it off you are eliminating the amount of ventilation it is suposed to do at idle(designed vac leak)so adjust the carb with the PCV in the system

[dinnut]

it was just a suggestion.........

[Tx Firefighter]

A PVC valve is a good thing. The gasses that escape past your rings into the crankcase are nasty and corrosive. A PVC valve will get them out of the engine. Those same gasses are the main cause of nasty sludge buildup in the engine.

Ever notice how the breather is on one valve cover and the pvc on the other? That's so fresh air is brawn in through the breather and pulled across the engine by the PVC valve. That picks up the maximum amount of nasty vapors. The PVC sucks them out and they go through the carb and are burned inside the engine.

Don't believe Dinnut. Find the right PVC valve.

FWIW, mine always rattle or buzz continuously when the engine is running too.

I guarantee you Rooster knows this because I see the L1 in his signature. That means he is a drivability/ emmissions specialist.

[Rooster's 67]

Tx Firefighter is very correct on the corosive and detrimental effects of the blow by gasses. PCV will make noise at idle, but definantly keep the PCV.
And my shop is a Texas State Certified Emissions Repair Station.

Tx Firefighter you tend to know your stuff too.

[Orion 762]

I definitely will keep the PCV valve. Right now it is hooked up through the 3/8" vac. port on the front of the carb. There is another similar 3/8" port on the back of the carb.

Does it matter which one it's hooked up to?? Now the one at the back of the carb. is simply plugged closed.

My main question earlier was "does it matter WHICH PCV valve" I buy to put on the valve cover? I thought that the valve needs to be "matched" to the typical vacuum level from the carb. Since I've switched from the original stock carb. to this 650 cfm Holley, won't the vacuum levels be different . . . hence needing a "different from stock" PCV valve?

At Autozone, they only "lookup" the valve recommended for the original engine/carb.

On the roughness issue (Rooster 67), it's not a LOT different . . . but noticeable when I plug/unplug the bottom of the PCV valve. At idle, there's enough vacuum to pull the PCV valve's "shaky thing" (not sure what it's called) all the way "up". But even in this "up" position, there's still considerable vacuum pulling through the bottom of the valve.

If I understand the operation of the PCV valve right, it's supposed to be "closed" at idle (low vacuum) and closed again when you really accelerate (high vacuum). Then at intermediate RPM (say hiway cruising), it's supposed to open to allow maximum circulation. In other words, the little "shaky thingy" inside the valve is "closed" when it is down (not enough vacuum to pull against the spring), and it should be "closed" when pulled all the way up by high vacuum. Then at intemediate vacuum, the "thingy" sits on the middle of the valve, letting it open.

If this is the case, then the strength of the spring and the weight of the "thingy in the valve" need to be matched to the vacuum levels from the carb. This would explain why we have so many "different PCV valves" for "different engines).

Which PCV valve do ya'll use?? Do you use the one "Autozone recommends" for the original stock engine/carb??

I appreciate yall's help on this . . . Orion 762

[Longhorn Man]

Have you read this page? http://67-72chevytrucks.com/vboard/s...threadid=81616

...If I understand the operation of the PCV valve right, it's supposed to be "closed" at idle (low vacuum) and closed again when you really accelerate (high vacuum).

Problem is here, you have high vacume at idle...als little to none when accelerating...and then plenty again at steady throttle.

[Orion 762]

I guess I'm confused about "manifold vacuum", but I'm definitely willing to learn!

Longhorn Man . . . I read the article you linked to . . . . .

Why does the manifold have "high vacuum" at idle, then "low vacuum" at high rpm?? Seems backwards of how the "spark advance works", and they both pull from the manifold vacuum level.

It would seem to me that at higher RPM, the engine should be drawing a "lot more air". To get this "higher draw of air" for combustion, won't it take a "higher vacuum level" to get this?

What is it about the design of the carb/manifold that reverses this "basic physics" issue of vacuum? Higher flow rates (of air needed for combustion) should require "higher vacuum", not lower vacuum (if I understand flow physics right anyway).

Any info to "clear this up" would be helpful . . . . Thanks.

Orion 762

[dinnut]

mine is still hooked up. i didnt know pcv valves did that. i learned something new today

[tom hand]

It is simple...when the carb is wide open the motor is pulling lots of air...but the carb is letting it have all it wants. At idle the pistons are trying to pull more cfm's of air than the closed butterflies on the carb will allow. Like sucking on a garden hose...you can suck as hard as you want...now put your finger over the end of the hose and try

[Rooster's 67]

the camshaft will determine the amount of vac produced at idle. the more overlap(the amount that both valves are open at TDC((exhaust ending Intake starting))the less vac produced at idle and rougher the idle.

vac adv. can be run off of manifold(below the throttle plate) or off of ported vacuum.(middle of the venturi) Boyle's law says that a liquid that is at one pressure and speed, when sped up(by passing through a narrower passage) will create a lower pressure. hence the reason the carb will pull fuel out of the primary booster(the round thing in the bore of the carb hanging out in the airflow) If you look down a carb you will notice the bore diameter is narrower in the middle than at the ends. that gives the venturi effect of the air passing through the narrower passage with the throttle plate open.

[Rooster's 67]

as far as a pcv use one for a mid 70 chevrolet truck.... it will work just fine.

[Tx Firefighter]

Rooster, how long you been out of college? :p

It's not Boyle's law, it's Bernoulli's Principle.

Bernoulli's Principle states that an increase in the speed of the fluid results in a decrease in the pressure.

Air is a fluid in this case.

As they taught me in jet engine school "As velocity increses, pressure decreases"

All of those 350 cubic inches of air has to go real fast through that little crack in the throttle blades, so the velocity increases, causing a pressure drop (vacuum).

Just hackin on you Rooster, don't send that healer after me.:p :p
It's been a long time since we had a stimulating discussion on this board.

[67sub]

Rooster and TX thanks for some good info, I've been thinking on the same question Orion had, just had not asked yet.

Orion - I've got the same carb you are running and I'm curious about what type gasket you are running between it and the manifold. Mine had a 1/4" thick 4 hole gasket on it but it broke during the overhaul and none of the local parts stores can find the right one. Even with a Holley part number the local "performance" shops are no help.Just wondering what you are running. -- 67sub

[Bernie]

It seemed to me that Bernoulli's Law states mass flux is proportional to velocity and cross sectional area so if you keep the mass flux constant and decrease the cross sectional area then you increase the volume and create a vacuum. As a Math Professor I'm big on the theory and short on the details. I bought two new valve covers, each with one hole (for a 350) I plan on a PCV for one hole, but does that mean that the only difference between an open or closed system is whether or not the filler cap is vented ? For a closed system wouldn't there have to be some adjustment to the carb ?

[Tx Firefighter]

Curtis, you have PM from me coming

[cableguy0]

if you run without a pcv your motor will sludge up inside and in a hurry. especially in cold weather its nasty stuff. no more than 100 miles and you will have close to 3/8 of an inch of sludge on the top of the valve covers. and as the other guys said pcv is a good thing and it is needed two breathers will not do the job

[PanelDeland]

Firefighter/Rooster,
I just want to say that this post is the type that makes me keep coming to the board.An intelligent discussion on one of the principles of the internal combustion engine.What could be more topical?Good info and though the theory is less important to most than the practical application, knowing the theory helps to make the right decisions when applying.
We should make it a point to try to keep a thread like this going.However some topics could outpost the Babes post that was on the general discussion board.(think cam theory).

Thanks for the thread!!

[Orion 762]

Hey folks . . . ya'll have really cleared up a lot of issues for me on this. They key, then is the throttle plate position!

Let's see if I have this close to right: When at idle, the throttle is "near closed" and the engine is then "sucking hard" trying to get get more air past those throttle plates. Since they are "near closed", we get instead a "high vacuum" in the intake manifold. As the throttle opens (higher rpm), then the engine can suck the air/fuel mixture a lot easier and hence does not generate as much vacuum. Meanwhile, as the air flow rate is now increased (at high rpm) the higher flow rate pulls more gas from the primary booster. This is then how the throttle plates "position" controls the rpm!

Hope I got that right anyway. Now that I understand the intake manifold vacuum levels (high vac at low rpm, low vac at high rpm), does the vacuum advance to the distributor work the same way?? I always thought (probably wrong) that higher rpm gave you more "suction" on the vac advance line . . . hence advancing the timing. Do I have this one "backwards too", or is the vac advance pulling from a different setup (venturi effect) for example??

This has been an informative thread indeed!! Thanks.

Orion 762

[71-longbed]

the vacuum advance is there to advance timing during mild cruise situations ....ie high vacuum .... when you romp on it ..... the vaccum drops ...,..race and dragster dont have a use for vacuum advance ..... becuase why in heavens would you want to save gas while your trying to go fast ..

[Huck]

For those with longer experiences and less education----if you ever had vacuum wipers, you would quickly understand the principles of what happens and why. Uncle Ken's Auto Wrecking yard -class 101----idle = high wiper speed. Give it gas going up a hill in the rain = stick you head out the window as your wipers hardly move.
Now that we have the theory down pat, can we make the sob run good??
Kind of reminds me of my neice who works for NASA and is truly a rocket scientist--doc in aero eng--she honestly doesn't know how to turn a screw driver. But ask her about the theory of scram or scatter jet propulsion for aircraft in the 21-22nd century---she'll blow you mind. Currently working on things that wouldn't see reality for 20-50 years.
Another cool way to get an engine to idle is to attach a vacuum gauge to it and adjust idle mixture on carb in concert with the timing. Believe GMC Paul's technical tips has a good description of how to do this. By the way, most PCVs are very similar in capability but slightly different in size of bottom/top/angled or straight.
Now if someone could explain why men are NEVER right in the minds of women or why Murphy's law is so inherently strong, I would love to hear that.
Thanks guys for some great discussion and education. It's nice to know someone can link old junk to education and design.

[deadof_knight]

Pcv valve also helps with ring sealing ,I really like the tecnical explaniations you guys gave but I think you might have overloaded him with info. its pretty simple depending on which pcv valve you get is wether it will rattle or not pcv valves are weighted in side not sprung. The pcv valve with the black L makes for a nicer looking hose routing. Pull it apart its the same except it has a plastic L snapped on top .I have always used the one with the largest hole underneath cant remember the part number now. It should make a difference when you plug it up .I think you need to look for a vacuum leak somewhere on the other lines personally. Back to the breather issure the newer vehichles breather is vented in to the air cleaner not to the good ole outside there is a difference in air pressure here negative compared to the engine compartment. rather than just pull gases thru the crankcase with virtually no difference in pressure. A negative pressure in the crankcase helps with ring sealing ,you will always get some kind of blowbye this is where the (BAD GASES) come fromthe negative pressure helps with this.


Now look at me I went off the deepend also uggggg!


PS. the breather needs to be vented beneath the air cleaner like the original to be as effective and use the larger L type pcv from autozone valve. theres like 4 or 5 of you really cant go wrong .