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LM PCV system confusion
Posted by: KenHenry ()
Date: September 15, 2020 07:15AM

I understand that for the LM PCV system, engine vacuum will produce some flow of crankcase gasses through the fixed orifice of the balance tube and into the intake manifolds. Why, then, does the inlet to this system split with one line going to the crankcase (pulling out vapors as it should) and the second line going to the air cleaner? It seems that this second line to the air cleaner would reduce the effectiveness of the system at removing crankcase vapors.

I do understand the need for a line to the air cleaner: as source for filtered air to the crankcase and a path for excess blowby to get into the intake manifold when there is little or no vacuum. It just doesn't make sense to me that the line runs from the air filter to the orifice tube rather than to to oil filler tube or some other access point to the crankcase, which would produce some flow of clean air through the crankcase as in most other engines.

Is there some advantage I'm not understanding for the factory design, or should I consider revising as part of my EFI conversion? Thanks! Ken

Ken Henry, Rochester, NY USA
1965 Corvair Corsa
140 hp, 4 speed

Re: LM PCV system confusion
Posted by: Spectre ()
Date: September 15, 2020 07:27AM

While I do not know what the real answer is, if the fixed orifice that is routed to the vacuum balance tube got plugged, you'd have a mess with the main portion going to the aircleaner.

David Clamp


1965 Corsa convertible - 140 4spd/3.55/AM-FM

2013 Mustang GT convertible - 5.0, 6spd auto, Procharger i-1

2003 Miata SE - 6spd manual (wife's toy)

"Victory is mine!" - SG

Norman, OK

Re: LM PCV system confusion
Posted by: TerribleTed ()
Date: September 15, 2020 08:07AM

The orifice may not be able to take all the fumes that come out of the engine in all operating conditions. There are also operating conditions where there is very little manifold vacuum and at these times nearly all fumes expelled by the engine are recirculated thru the engine via this PCV system piping. The piping or hoses going to the air cleaner are to accommodate these excess gasses and keep them in the engine rebreathing and reburning them resulting in fewer engine emissions as well as keeping this possibly oily sooty air from being expelled all over the engine bay. Most old school PCV systems have this type piping.

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Avid Corvair hobbiest since 1984.
I have personally performed ground up restoration on over 20 Corvairs.
I currently work full time at restoring and repairing Corvairs.
Located in the Atlanta Georgia area.
[www.facebook.com]

Re: LM PCV system confusion
Posted by: cnicol ()
Date: September 15, 2020 08:27AM

The air cleaner connection serves two functions.

1) During conditions of low engine load and high vacuum (Idle and part throttle), the air cleaner connection protects the crankcase from high vacuum by providing filtered air to the orifice. In other words, air flows out of the air cleaner and into the orifice.

2) During conditions of high engine load, high blow-by, and low vacuum (WOT), blowby flows to the carburetor intake.

3) Blowby flow is variable. without the air cleaner source the PCV's influence would also be variable and unpredictable.

The fresh air connection in the early PCV design, as you note, is not like later systems that flow through the crankcase or valve covers. The newer "flow through" designs do a better job of clearing acidic blowby gas from the crankcase. I have a modified system on my Rampside that connects the air cleaner to the oil filler tube to provide flow-through ventilation.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

Attachments:
Re: LM PCV system confusion
Posted by: cnicol ()
Date: September 15, 2020 08:45AM

BTW: Kudos to Rex Johnson for the hand-fabbed air cleaners and PCV mod shown above!

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

Re: LM PCV system confusion
Posted by: KenHenry ()
Date: September 15, 2020 09:08AM

Thanks everyone for your replies. I think I'll do something like on your Rampside, Craig, with the line going from the air filter to either the oil fill tube or original fuel pump location. Craig: are you running a PCV valve or still using the orifice on the balance pipe? Ken

Ken Henry, Rochester, NY USA
1965 Corvair Corsa
140 hp, 4 speed

Re: LM PCV system confusion
Posted by: 66vairman ()
Date: September 15, 2020 09:44AM

A lot of good information.

I'll clarify Craig's statement "the air cleaner connection protects the crankcase from high vacuum by providing filtered air to the orifice."

In other words, air flows out of the air cleaner and into the orifice. This is correct if the engine piston blow-by is less than the airflow through the orifice to the engine vacuum balance pipe.

Another way of looking at it is the air pressure at the entry to the orifice must be the same as outside air pressure conditions so excess blow-by is routed TO the air cleaner and ingested and burned in the engine and when there is a lack of blow-by to enter the orifice then "filtered" air from the air cleaner is drawn into the orifice and to the manifold to mix with the fuel/air from the carburetor and to ignite in the cylinder.

The orifice must have normal air pressure on one side so the engine has a "fixed" vacuum leak and the carburetors are calibrated to compensate. Otherwise you would have a idle issues.

BTW - check the PCV orifice - they can plug up, especially on an older engine. It's part of the maintenance procedure per the manual.

Re: LM PCV system confusion
Posted by: cnicol ()
Date: September 15, 2020 09:57AM

KenHenry Wrote:
-------------------------------------------------------
Craig: are you running a PCV valve or still using the orifice on the balance pipe?

Craig replies: My system uses a PCV valve.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

Re: LM PCV system confusion
Posted by: corventure Dave ()
Date: September 15, 2020 12:01PM

Craig,

I have always really liked those custom Rex Johnson style 140 air cleaners. thumbs up
If I was ever to go with a 140 in my FC's, that's the way to go.

Corventure Dave

Re: LM PCV system confusion
Posted by: Seth Emerson ()
Date: September 15, 2020 02:56PM

Just as a note. If you drill out the orifice in the PVC tube, You will forever be chasing idle issues during tune ups. If you are seeing evidence of an air-leak, but cannot find any other loose or split hose. Remove the hose and look at the hole size. It should be small - but open.

Re: LM PCV system confusion
Posted by: JimBrandberg ()
Date: September 17, 2020 07:58AM

I've got a 140 going in a Lakewood. I would like to know more about the flow through ventilation with the hose to the filler tube. Would that negate the too much vacuum scenario in Item 1 above? Would it work with an orifice rather than a PCV valve? PCV valves seem to be one of those things where the quality of the part has gone down in modern times.

I don't mind buying something from Rex Johnson if it's proprietary. I could spend a lot of time doing a first attempt if he's got fabricating it down to a system. I've got some cores.

Jim Brandberg
Isanti, MN
CorvairRepair.com



Re: LM PCV system confusion
Posted by: RexJohnson ()
Date: September 17, 2020 10:48AM

Well I always thought that in my spare time I would make some more of those air cleaners. That was the second set I made and so far the last. Some people think that in my spare time I should paint my trailer and that hasn't happen yet either. For the system above I made it into a real system. The stock Corvair system is not very good when everything going in and out of the engine goes thru the same hole. By adding the clean air to the oil fill tube (easy place to do it) you now have clean air enter the engine at one place and the bad stuff coming out at another. In other words a POITIVE Crankcase Ventilation System. On the Corvair system the only way the bad stuff comes out of the engine is if the engine pushes it out. In other words a glorified road draft tube. The vacuum in the system can just pull in the clean air from the air cleaner. Almost all other cars use a system with 2 ports in to the engine. My brother has a '96 GEO and I saw that it has a single inlet system also and we all know those are high dollar cars. My understanding of a PCV valve is that at high vacuum the opening in it is sucked almost shut so not to have a large vacuum leak at idle like a small fixed orifice. But at lower vacuum there is a spring that pushes a plugger open to allow for more volume of blow-by gases to come out of the engine. Were as the small orifice is still small. So for a long answer I would use a valve over an orifice. The valve on Craig's FC I believe came off of some under 2.0L 4 cylinder engine in the wrecking yard. On my latest car (110) I got a smog crossover for the air cleaner as these had the nipple for the PCV system attach at the base of where the air cleaner sits facing to the front right of the car. I flipped the crossover around so that the hose nipple faced the rear left and then added a hose nipple to the oil fill pipe. The car has an electric fuel pump with the fuel lines to the front so it was easy to do. So with a short piece of hose to the filler tube I now had clean air into the engine or bad out if needed. I cut off the regular breather tube and installed I believe a stock PCV valve between it and the balance tube and now I have a real POSITIVE Crankcase Ventilation System.

RJ tools
Salem,Oregon

Re: LM PCV system confusion
Posted by: KenHenry ()
Date: September 18, 2020 04:46AM

Jim, I agree about the low quality of some valves. I had a new one from NAPA that made a racket, in my flathead Ford. I replaced it with an OEM Toyota valve (3.0L, 93-01 Lexus GS300) and it seems much higher quality, no noise, etc. I plan to try this same valve on my Corvair. Ken

Ken Henry, Rochester, NY USA
1965 Corvair Corsa
140 hp, 4 speed

Re: LM PCV system confusion
Posted by: JimBrandberg ()
Date: September 18, 2020 07:19AM

"Some people think that in my spare time I should paint my trailer and that hasn't happen yet either."

LOL. Mine needs fenders and more. Complicated by not good to weld when it's cold.
Thanks for posting information, I need to read a few more times to take it all in.

Jim Brandberg
Isanti, MN
CorvairRepair.com



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