Positive Crankcase Ventilation 101 (TECH)

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OK all i have on my sbc is the typical breather tube at the front of the engine so exactly how do i get around this?please explain in real simple terms as i am a simpleton and also it must look quite 50's trad also explain what i need to buy and what it has to go to and from..........Marq

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The design of the Chevy small block was changed in 1968(?)...if you have an early block, it will have a hole in it behind the intake manifold. This is where the draft tube went (1956-61) and later the PCV valve hose connetor (62-67). If you have a later block, it won't have the hole there.

To do it right, you need an intake and valve covers that match the style of the block...early block, early intake with the filler tube, and non-vented valve covers go together.

No hole at the back, no filler tube, and holes in both valve covers go together for later style engines.

To put PCV on the early sbc, you need the fitting to put a hose onto that hole in the back, and you need a pcv valve screwed into the back of the base of the carb.

For a closed system you also need a filler tube with a hose spigot on the side, and a non-vented cap.

Here's a scan of the 66 Chevy service manual showing some of the creative ways they plumbed the SBC for PCV.

 

Here's a quick question. For some time I had a PVC on one valve cover and a breather on the other other (early 70's SBC). Only problem is I would get lots of oil on the valve cover through the breather.

I got rid of the breather and put a 2nd PCV, so now I have one on each side, with no breather. I'm gathering that this isn't a good setup, correct? I think I need to find a better-baffled breather or something?

Sorry for the dumb question...

 

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Here's a quick question. For some time I had a PVC on one valve cover and a breather on the other other (early 70's SBC). Only problem is I would get lots of oil on the valve cover through the breather.

I got rid of the breather and put a 2nd PCV, so now I have one on each side, with no breather. I'm gathering that this isn't a good setup, correct? I think I need to find a better-baffled breather or something?

Sorry for the dumb question...

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Two things I would check..(and this would be based on a CLOSED system)
1. Is the first PVC valve operating? (is it plugged?)
2. Remove both of the two hoses (PVC and breather) and look to see if oil is present in either (or both) hoses. If there is oil at the ends of the hoses, you might be getting some excessive blowby. (worn rings?) And the ventilation system can't relieve the pressure fast enough.

 

Brian, THANKS! Makes sense. I just scored an Edelbrock 2V intake for using a 4V carb, after some of these projects clear up a bit, I'm going to swap manifolds and try to hook up a proper PCV setup.
B*** rules.

 

Ok this is what i have.I have a mid 70's sbc trying to look like it's older.I have no breather at the back of the block i have a filler tube and non vented covers.I gather from the scan that if i use a spigotted breather with a pcv valve going to the intake on a carb vac takeoff and a non vented filler tube cap this should be ok?......Marq

 

“should be ok?......Marq”

Not really, if you look at the first picture there is no PCV Valve, only the lower two and they have open fillers.

If you should list the ways to do this in order of environmental correctness if would go some thing like this, for a street engine:

Number one:
One connection from crankcase to manifold vacuum via PCV valve and,
One connection from crankcase to air cleaner
Drawbacks: May be hard to incorporate on a traditional looking Hot Rod engine

Number two:
One connection from crankcase to manifold vacuum via PCV valve and,
One connection from crankcase to air, with some filtering. As in vented oil filler tube in front of manifold on old SBCs
Drawbacks: During high blow by situations you will get a goey filler tube since the excess air will come out there.

Number three:
One connection from crankcase to road draft tube
One connection from crankcase to air, with some filtering. As in vented oil filler tube in front of manifold on old SBCs
Drawbacks: Oily everything but very traditional.

You need an overflow path for the situations where the PCV valve alone can’t handle the flow.
You either put the overflow into your air cleaner or into the air depending on how messy you like to be.

 

so if i want to go real trad the filler tube and a crank case breather with a one way flap valve to the road as i care little of mess.......Marq

 

That is what I got, don't know of any flaps though?
I'm not sure where you'd put the road draft tube on a post 1968 small block since they took the hole away next to the distributor?

 

That's a nice looking setup there tunglegubbin.

Here is a lesson I learned a long time ago, PCV is a good thing.

 

Couldnt i just put some breathers on the back of the valve covers.I know a lot of the biker boys done it over here in the 50's as well as some of the car lads but would it be period for American stuff...........Marq

 

Wow...I think I just wet myself. Tunglegubbin, that has to be one of the best looking small-blocks I've ever seen. Beautiful!

One thing about your road draft set-up...do you have the stock canister that goes under the intake, and baffles to road draft tube? If not, you should try to find one...or that nice chrome bellhousing will be covered in oil in no time.

And Jeem...you're welcome, buddy!

 

Sorry to have to go back to this, but I do. I have a SBC with an Air Gap RPM that has been modified with an oil fill tube in the front and was drilled back by the distributor to allow a PCV valve. The oil fill cap is an Eaton that has holes all around the bottom and is filled with wire mesh (vented?). The PCV is sized for an SBC and is connected to the large port under the back of Holley. I thought this would bring air in the oil fill, and pull it through the PCV and back under the carb. I had the car on the highway and there was tons of oil coming out the fill tube! I checked the PCV and it's not clogged. Any thoughts on why this is happening and what I should change? Thanks, Dave

 

Would seem to me that you don't have crossflow through the engine. The oil filler tube (inlet for fresh air) and the pcv (outlet for crankcase) are both just under your intake. That would be like putting the inlet on one end of a valve cover and the outlet on the other end of the same valve cover. Usually the inlet is on one valve cover and the outlet is on the other valve cover, pulling a path of ventillation throughout the engine. You have a pressurized crankcase (which everyone has as the bottoms of the pistons are pressurizing the bottom end) and that pressure is trying to get out and the easiest path is your filler tube (where there is lots of splashing oil). Short of going with new valve covers, I don't know where you could pull from (where to hook up the pcv/carb part of the system to the engine).

 

1-In theory yes, not such the leak in reality as suspected.
2-In really high blowby conditions? Fix the motor, cause there shouldn't be such conditions

 

I still get confused by this too, I have a sbc '78 block with an early Edolbrock with oil fill tube at the front. If I have sealed valve covers with no breathers then where would the PCV go ?

 

I thought that by having the air intake in the front and the valve in the back of the intake being aided by the vacuum, it would create ventalation for the whole block. Why would this be different than tapping into a valve cover? Does this create a vacuum problem as some have said above?

 

The main reason that crankcase vacuum increases power is that is reduces "pumping loss". When a piston moves down the cylinder, is must displace the air underneath it; when it rises, it ****s air back up with it. When there is less air in the crankcase (ie: va***m), there is less air to be moved and hence less energy wasted moving it. This is compunded by oil mist in the air. When all the air is moved around in the crankcase, it moves oil mist with it - consuming even more power. Windage trays do their best to prevent this.

Another way to get crankcase vacuum is with an "overscavenged" dry sump oil system - NASCAR style. With this system, multiple scavenge pumps are used to draw oil out of every nook and cranny of the engine. Oil mist and air is collected as well, creating a vacuum in the crankcase. The air is then separated from the oil in the oil tank.

 

Know what I found out the other day?
MANY newer cars DON'T have PCV's anymore!!!!
Daughters Boyfriend has an intake tube on his 2000ish Cavalier...cone filter is soaked with oil. I removed the vent line and sure enough he was right...blowby and "pond s***" running down the tube to the filter.

Figured the PCV was plugged...he says he tried to buy one already, no listing.
I thought that was odd so I looked it up on a Cavalier board and he was RIGHT!
NO PCV!!!!

The guys have to make a catch can and mount it to the firewall to catch the oil that gets blown out...with a hose to the bottom of the car to let the fumes and blowby p***!!!!
Non-blown engines guys...with a turbo the problem is much worse.
What the hell happened to the enviromental issues!?!?

I figure the engine management computer can't deal with the eventual (as the engine wears) addition of blowby pressure into the intake, so the manufacturers took the easy (cheap) way out.
Should have some form of idle air compensation control to add or subtract from the throttle air flow or something.

I was asking the Mercedes Mechanic at work about it and he says its getting common! Even some MERCEDES are running without PCV systems!

The ONE reliable, user friendly emissions device that didn't hurt performance a bit...maybe added to it...gone to save a buck.

Something is WRONG there in my opinion.

 

I'll chime in here too--
I just dressed out a new GM 350 crate engine to look like a '62 327; Chrome stamped steel valve covers with no breather holes, Edelbrock C3B intake with oil fill tube, new 350 block with NO canister or port at the back of the block for PCV or road draft tube.

I bought a new oil fill tube with sealed cap from YearOne. They have two tubes: one has a screw-in hose ****** at the top with a cap that has two ears that fit inside the opening of the tube and screws on (like a gas cap). The other tube they have has a small ****** installed on the tube and uses a screw-on cap that screws down over the OUTSIDE of the fill tube's mouth. This is a square cornered cap, like a 409. I went with the other version. Spend the extra dough and get the resto fill cap, not the cheap Mr. Gasket cap, because it doesn't seal.

The ****** on the oil fill tube needs to run to the PCV valve. Chevrolet vehicles in California used a closed PCV system in '62. YearOne and your local parts store both carry a PCV valve that screws into the base of your carb. It has a threaded end with wrench flats on one end, the hose connects to the other end. YearOne also sells a long PCV hose with a 90-degree bend in it; cut to fit. To install the PCV in the carb base, I used a 45-degree br*** fitting as an adapter, to move the hose out from the linkage of the carb a little bit.

So; screw-in PCV into the base of the carb, running to port on the oil fill tube.

Next, I needed to add the "In" side of the system. On a 283 or 327, this would be in a can at the back of the block. I drilled a 1/2-inch hole through the back of the intake manifold, between the runner and the distributor, and welded in a 2-inch tall piece of 1/2-inch dia. aluminum tubing. This opens right into the lifter valley. There was webbing cast into the intake where I installed my port; I welded a piece of thin aluminum plate across the webbing, with a single 1/4-inch hole drilled in the center of the plate. This created a blaffle to keep oil spray out of the tube.

This port in the intake manifold runs to another port I welded into the air cleaner base, inboard of the air filter element. It's an open-element air filter. A rubber hose connects the air cleaner port to the port on the intake manifold. With the port installed between the air cleaner and the carb mouth, it draws filtered air.

So, my "circle" is: PCV draws from base of carb to sealed oil fill tube. Crank case draws from back of intake manifold to base of air cleaner.

I've basically recreated the '62 closed PCV system, only using my own port in the valley of the intake, rather than a can at the back of the block.

-Brad

 

Which direction is your PCV flowing? I had the same problem on mine...if it flows the wrong way, you'll push out out your vent.
I bought a valve that screws into the base of the carb so it draws into the carb. (OE application for closed '62 PCV system)

The oil fill tube on my SBC has a threaded port in it for PCV hose. I followed the wrong diagram in a catalog, and installed my PCV in that port, then ran the hose to the base of the carb. The result was the flow through the valve was the wrong way, and it allowed oil to get ****ed up the hose at the back of the intake that runs into the air cleaner base. The vacuum at the base of the carb closed the PCV valve, then pressure in the crank case built up and flowed out into the air cleaner port at the back of my intake.

-Brad

 

Brad's fix seems to be dead on right.

In all of this exchange I do not remember seeing anyone say anything about the sealed up engine being free of oil leaks. An open-to-the-atmosphere system is hard to keep from dripping oil all over the place. And it lets cylinder ring blowby and valve guide dribble burn along with your fuel.

We used to run an active "PCV" system on a dragboat years ago. Put some venturi tubes about an inch past the last zoomie tube on each side of the motor, -12 hose to the VC on each side. The draw at full throttle would help keep the oil on the bottom side of the oil rings and help prevent detonation / oil contamination of the fuel air mixture / and backsiding of the ring lands. And we could easily tell the general condition of the motor by the amount of oil or lack of oil present in the -12 hose.

And as far as the belt pump deal, makes tons of HP increase the more vac you pull due to increased ring seal. But then the oil tries to boil as the pumps (both air and oil) fight for fluid to pump and the bearings loose out in that fight because the OP will start to cavitate, this happens at about 21-22 inches Hg. K

 

My old motorhome (87 W/Ford 460) was getting oil in the air cleaner from the breather and at the same time my PCV would rattle while idling.

When trying to find why all this was happening and what to do about it , especially since I just put new heads on it, I found a aftermarket PCV called an "ENvalve". It's CARB (California Air Rescources Board) approved and it stopped all the problems.

It replaces the PCV exactly. It doesn't rattle when you shake it because it's not supposed to.

http://www.misterfixit.com/envalve.htm

 

Boy! Talk about the POWER of the HAMB!!
It's just over THREE YEARS AGO!! that this thread first started!
(shows how the "search" DOES work!!)

 

Yes I would like to know how race car engines create the necessary crank case vacuum

**** bernard

 

Yes.... searching through the closet so back TTT again for a little help.

Objective: 64 Chevy truck w/a mid 70's sbc & Perf Air-Gap intake. I want to run the 'no-holes' script valve covers & need to create a 'hidden' ventilation system. I was going to drill/tap the intake & run a pcv valve similar to a 67 truck @ the carb base & then plumb fittings @ the back of the VC's to a remote breather.

My buddy suggested just installing the fittings @ the rear of the VC's to a catch can that has an attached breather (said catch can would be out of sight) & don't mess w/the pcv.

Seems my method would be better for the motor right? Any other suggestions? Possible pics of 'hidden in plain sight' type set-ups.
Thanks.

 

One of the most important jobs that a PCV/or road draft system performs is to remove water vapor from the blow by g***es that condense inside the engine when it cools down just like the dew on the lawn. If the water vapor is not removed it builds up inside the engine.

People mistakenly think a PCV system is a pressure relief system. It is actually an air flow system. Air enters through a breather (to keep out dirt) on one side or end of the engine and is ****ed through the PCV valve on the other side or end of the engine into the intake to remove harmful g***es that mix with the water vapor to form acids and sludge inside the engine. Race cars get their oil changed much more frequently than street driven cars. The long term effects of a poor breather system on a race car are not as much a concern as a street driver that puts on a lot more miles and hours on an engine.



I like this old flathead picture because it uses arrows that trace the airflow through the engine on a road draft system. This is what you need to duplicate with any breather system. The road draft tube just gets replaced with a PCV valve. Always think about how the fresh air will travel through the engine on it's way to the PCV valve. Without this air flow you get sludge and acid build up.