PCV/Crank Case Evac

[luseboy]

So I know this has been talked about a fair bit but I'm having trouble finding any definitive answers.  I have an L28 (P79/F54 combo) with an AZC 4bbl manifold and an unknown header.  I'm trying to figure out the "best" solution for PCV/Crank Case Evac for mostly street use.  I have read about people using a crank case evac system pulling from the exhaust, but have heard this may pull too much vacuum and suck oil through the exhaust.  I've heard that plumbing a PCV valve into the intake manifold is "bad" because then you're burning some oil/getting oil residue in the intake. I can't think it would be such a huge problem if so many cars come stock set up that way. Obviously it's not a huge deal because a lot of people seem to just put little filters over the PCV fittings on the block and valve cover, but I would like to have some vacuum in the crank case to help make sure all my seals are seating.  So my question is, should I just plumb a PCV valve into the intake manifold and be done with it? Or would welding in a fitting to my exhaust for an evac system be a good option?  With either of these options, is there any reason I shouldn't plumb a catch can in?  And if I am going to pull vacuum through the crank case, should I pull from both the valve cover and the block?  Or should I put a filter on the valve cover, and pull from the block? Sorry for the 20 questions, I'm just trying to figure out what I should do here as I'm finally getting my car onto it's feet after too long of a build, and this is one aspect I haven't really figured out yet. 

Thanks!

-Austin

[NewZed]

AZC shows a simple way to get PCV.  Not very sophisticated, it's before the carb and depends on vacuum inside the air filter housing to create vacuum.  The worse the air filter the better the PCV pull.  Plus it oils up the carburetor.

 

http://www.arizonazcar.com/cableinstall.html

 

You could plumb the manifold and use a PCV valve.  That's how Nissan did it with the 240Z's.  Read up on them and you'll get some ideas.  The carb stays clean, gases are purged.  The crankcase has to be sealed, so you can't run a breather and a PCV.

 

Edelbrock has a PCV port on the carburetor itself - http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

 

Looks like some Holley's use the same port for PCV and brake booster.  You could do that.  http://documents.holley.com/199r7950-7rev7.pdf

 

Of course, if you use the 0-8007 390, you could just use the PCV port already there.   http://documents.holley.com/199r8108-2rev4.pdf

 

 A sealed crankcase and a PCV valve is probably the cleanest way to go.  The problem with the sealed crankcase is that you need a way to let off pressure from blowby at high RPM, open throttle.  Like the EFI systems do.  Not sure how the carb guys do that.  Probably why they run them open with catch cans.  Could be that they just run the hose to the carb, but no PCV valve.  Seems like the power brake system would get oiled from crankcase fumes if it's T'ed in there. 

 

If I was building a system, and wanted it to stay clean, but avoid over pressure in the crankcase, I'd probably put a check valve on the valve cover with a breather, one-way vent to the outside, and use a PCV valve on the manifold with the carburetor port.  Or run the valve cover line to the air filter, like AZC shows.  Still with a check valve though.  Not sure where you'd get that valve.

 

There's probably a thread out there somewhere in which somebody has gone over all of the PCV permutations for a carb setup.

 

[luseboy]

I know that's what AZC recommends but it seems like a very rudimentary set up to me.  They also just have a breather on the lower PCV inlet, which seems like not such a good set up, right?  I am using a plenum with a cone filter so would have to drill and tap my plenum to do a set up like that one.  And oil in the carb would not be such a great thing either, unless I'm mistaken.  

 

I think plumbing the manifold seems like a good solution, I have an extra port already and could probably find a PCV valve that would screw right into it.  I am using an edelbrock carb (it's a 500cfm carb though so I may need to swap it out for a holley 390 anyways), but for whatever reason I think I'd prefer to have the PCV plumbed separately from the carb, unless that means it already has a PCV valve in there?  I guess the advantage to this would be that the gasses from the crank case will go through all 6 cylinders equally, where as the port on the manifold would put it going through only the front 3 cylinders, so maybe that's the best place to plumb it in.  

 

I like your idea, that sounds like the way I want to go.  I like the look of a breather on the valve cover, and the fact that I can then pull the valve cover without undoing any hoses. I don't know a whole lot about PCV systems, but in my quick internet searching I found that the PCV valve is never fully closed, so I couldn't just use a PCV valve as my check valve.  How much pressure would I see at that valve cover inlet if I hooked the block inlet to manifold vacuum and had a check valve on the end?  I'm trying to think of where I might be able to find a check valve... I can't seem to think of any easy applications but I'm gonna have to start hunting one down.  

[Derek]

My understanding is the flow changes direction depending on manifold vacuum. At high vacuum it pulls fresh air from the air filter via the valve cover through the partially open PCV valve that is connected to the crank case. At high RPM the spring in the PCV valve opens it fully and the flow reverses pulling the oily air through the air filter housing. If my memory is correct and you look at pictures of the stock air filter housing you will see the end of the tube is cut at an angle to create a Venturi. Been a while since I've seen one of those up close though:)

 

Derek

[luseboy]

My understanding is the flow changes direction depending on manifold vacuum. At high vacuum it pulls fresh air from the air filter via the valve cover through the partially open PCV valve that is connected to the crank case. At high RPM the spring in the PCV valve opens it fully and the flow reverses pulling the oily air through the air filter housing. If my memory is correct and you look at pictures of the stock air filter housing you will see the end of the tube is cut at an angle to create a Venturi. Been a while since I've seen one of those up close though:)

 

Derek

 

Interesting, that makes sense. I don't really see any mention of flow reversal, but at high rpm the vacuum would be reduced in the manifold, but I didn't think it went all the way up to 14 psi, which would cause a flow reversal.  Maybe in a super/turbo charged application with enough boost, but I thought there were systems in place to avoid this.  

 

So far everything I've read has said that there is simply a breather on one side and the other side is hooked up to manifold vacuum (for a pcv system in general).  No mention thus far as to a check valve or anything like that.  I'm a little concerned that if I do the check valve on the valve cover breather and hook the crank case up to manifold vacuum that I may have too much vacuum running through the engine, possibly resulting in a seal being sucked in.  I'm assuming I'm just over-thinking and missing some key points but maybe I'm wrong.  I can also see, however, that running a breather on top and manifold vacuum on bottom will basically give me a vacuum leak, but if I messed up on any of my seals in my engine the same thing could/would happen, right? I guess this is why a lot of people just run breathers on top and bottom and don't worry about it, but I'd like to set up something if for no reason other than I'd like to avoid oil leaks, and PCV systems seem to help keep the engine healthy.  I guess I need to look further into the stock set up and get a better idea of how Nissan engineered it.  

 

Edit:  Just read the FSM, you were totally right.  The FSM says that under full throttle the manifold vacuum is insufficient for the amount of blow-by being produced, so the blow-by goes to the air cleaner where the inlet is.  Also, there is a flame arrestor in this tube near the front (not sure what that consists of or how important it would be for my setup, any ideas?) and under normal conditions the pcv valve pulls fresh air through the engine through this tube.  But there should be a small amount of vacuum in the air cleaner under normal conditions right? So I couldn't just put the breather on top and connect the crankcase to vacuum and call it a day, right?  I still like the check valve idea but am still worried that might be too much pressure.  I think it's safe to say I am over thinking this system, but was hoping for some more guidance on which route to go...   

Edited April 5, 2016 by luseboy

[Derek]

I just went through all this with my twin cam head. Right now I have half a system. PCV drawing through an air filter on the valve cover. Since I don't have an air filter housing there is no way to leverage the air flow produced on the incoming side of the butterflies at higher RPMs. If I had an air filter I would plumb it like the factory. Ultimately the header valve setup may be the best way to go for me. 

[NewZed]

So I couldn't just put the breather on top and connect the crankcase to vacuum and call it a day, right? 

You'll pull "unfueled" air in to the intake manifold that way.  It will not have passed through the carburetor.  It will lean out your mixture.  That's why the systems that use intake manifold vacuum are "sealed" systems, and the open systems empty before the carburetor.

 

The hot rod V8 guys have this same problem.  You might search around those sites.  I think that one reason for using a PCV valve on whatever you use to pull through the crankcase is that it controls flow.  If flow gets very high the spring will compress and the valve will close.  The PCV valve is essentially a two-way check valve.  You don't want huge volumes of air passing through pulling all of your oil out in vapor form.  That's why people use catch cans.

 

Fix on top of fix.  It can get complicated.  Before PCV they just left the crankcase open with a breather type filler cap.  Probably got messy with old engines.

[luseboy]

I just went through all this with my twin cam head. Right now I have half a system. PCV drawing through an air filter on the valve cover. Since I don't have an air filter housing there is no way to leverage the air flow produced on the incoming side of the butterflies at higher RPMs. If I had an air filter I would plumb it like the factory. Ultimately the header valve setup may be the best way to go for me.

DOHC os giken head? Your wallet must be a little lighter since you got that.

 

I may plumb into the air filter but I'd like to avoid oil residue on my carb if I can. Plus I'd like to have a setup that I can make work with triple webers in the future, which I guess may mean header valve

[NewZed]

He built his own head.  How'd you miss it?  I think he's running ITB's also.

 

I posted in front of you.  A good catch can would probably keep your carb clean.  Cleaner for longer anyway.  More hoses though.

[NewZed]

Here's some interesting reading - 

 

http://speedtalk.com/forum/viewtopic.php?f=1&t=37104&start=0

 

http://mewagner.com/

 

Here's a fair summary of the basics.  They still seem a bit confused about where "fresh air" comes from  - http://mewagner.com/?page_id=434

Edited April 5, 2016 by NewZed

[NewZed]

Actually, Wagner's description fits.  "Unfueled" air is drawn in to the intake manifold.  Seems very crude.  And this was 1981.  Apparently the PCV and carburetion system are tuned as a complete system.  Here's a pciture from the 1981 truck service manual.. They even call out the "dirty" air, apparently it's not even pulled from the filter side,

 

Funny story - I helped a friend get his car running after he filled it way over with oil, soaked his air filter through the PCV hose, and the engine wouldn't start.  I threw the filter away to fix it..  He didn't know anything about engines (hence the overfill) and had beat the piss out of it which caused much of the oil vapor to get pushed through the hose.  I told him to take it easy, the motor was on its last legs.  He beat the piss out it again that night ( a bunch of us met later that night and he was complaining about the smoke) because it ran so much better, parked it in front of the house with the room he was renting, and woke up to a muffled boom as the oil on the super-hot exhaust manifolds caught on fire.  Burned to the ground.  I think it was a Yugo.

Edited April 5, 2016 by NewZed

[luseboy]

I haven't really been on this site in the last couple years aside from the last couple weeks. Was this a ground-up head build or did it utilize two KA heads or something like that? Sounds pretty cool in any rate.

 

Well I have been doing some reading on honda forums (trying to decipher slang gets old though), muscle car forums (lots of 'merica and confederate flags sorta dilluted the point), FSM's, wikipedia pages, etc. The only thing I know for sure is that PCV is a good thing, and that I am over thinking my set up. So if the manifold vacuum is not too much for the engine seals, I still like the check valve idea. Seems like it solves all my problems and is a solid approach. I'd just like to make sure it isn't going to be too much. I'd be running an air compressor water/oil seperator just to diminish the amount of oil going into my combustion chambers. But I'd really like to avoid pulling seals into the engine, and am a little concerned that manifold vacuum at idle might be a bit much. That being said, would the PCV valve be sufficient at limiting the pressure? I guess it is made to do this very thing. And for check valve, how much pressure would I want it to open up at? Maybe 5psi (of positive pressure)? And would you think it'd be best to have the input to the manifold be centered so all 6 cylinders get the same amount of crank case fumes? Or would I be find just plumbing to one side of the manifold (first three cylinders would be getting the fumes/oil), I'm thinking central.

 

The story about your friend sounds funny! I think he solved his problem (of having a yugo) by it burning to the ground. This is not the outcome I want at all in any way shape form color or theme. Luckily I pay attention to things and don't forsee something like that happening (I'm sure he said the same thing).

[Tony D]

The Factory PCV Valve in the bottom of the manifold is a flow-regulating valve. 

It is almost CLOSED at idle, off idle it opens and with cruise it will 'float' to maintain a suitable draw on the crankcase to pull fresh air in from the air filter (connected to the top of the valve cover.) This limits 'overdraw' and sucking of excessive amounts  of oil into the intake tract. Some 'emissions' devices which are commonly removed also HIGHLY restrict the super-high manifold vacuum spikes of drop-throttle. Removing those usually results in increased oil consumption and people wonder why...

 

At idle, almost closed, there is no blow-by and very little vacuum is necessary to make this circuit flow. If you have blow-by on a worn engine, you will see smoke puffing out of that tube in the air cleaner.

 

At WOT, there is NO VACUUM in the manifold to produce flow, so the inlet air filter is creating a few inches of water restriction on the valve cover, it's a weak motive force but in reality the blowby is so much it simply vents the blowby back into the air cleaner.

 

There's no real flow reversal except in the valve cover hose..

 

In fact, there's two systems: "PCV" and "Road Draft".

 

When it's  dumping out the cover you are operating like most any vehicle did until 1967... pressure built up from blow-by is simply vented down a pipe and onto the roadway. In a Z, this is the upper hose into the Air Cleaner. Having the air cleaner like that, the  restriction of the air cleaner helps suck out the blowby fumes, but there is really no way for fresh air to get into the crankcase. Old Corvairs were like that, early models had a road draft setup, later models an orifice into the air cleaner.

 

The PCV ALWAYS flows from Air Cleaner, to Valve Cover, to Inlet Manifold for combustion. It is designed to draw fresh air through the crankcase 'positively' using Vacuum as a motive force.

 

In turbo cars, it's more pronounced than if you go WOT...not only do you not have vacuum to provide motive force there is pressure there and the PCV shuts to prevent turbo boost from pressurizing the crankcase and spewing oil everywhere... Under boost you have more blowby and you just road-draft it into the inlet before the turbo (after the inlet filter.) The turbo adds a nice suckage on that....

 

PCV can work with just a vent on the valve cover if you restrict the manifold draw...Mitsubishi used a 0.063" orifice to limit maximum draw (similar to the old Late-Model Corvairs!)

 

If you don't have a lot of vacuum, or are always driving at WOT, then an ejector through the exhaust (with a similar restriction orifice) will work fine to eliminate Blowby in the crankcase.

 

You want the fresh air in there to mitigate acid and condensation buildup, the sorts of nasties that can corrode the internal components. It's more pronounced when you take short trips, never warm up the engine fully, and live in colder climates where the engine can cool down from hot very quickly.

[luseboy]

The Factory PCV Valve in the bottom of the manifold is a flow-regulating valve.

It is almost CLOSED at idle, off idle it opens and with cruise it will 'float' to maintain a suitable draw on the crankcase to pull fresh air in from the air filter (connected to the top of the valve cover.) This limits 'overdraw' and sucking of excessive amounts of oil into the intake tract. Some 'emissions' devices which are commonly removed also HIGHLY restrict the super-high manifold vacuum spikes of drop-throttle. Removing those usually results in increased oil consumption and people wonder why...

 

At idle, almost closed, there is no blow-by and very little vacuum is necessary to make this circuit flow. If you have blow-by on a worn engine, you will see smoke puffing out of that tube in the air cleaner.

 

At WOT, there is NO VACUUM in the manifold to produce flow, so the inlet air filter is creating a few inches of water restriction on the valve cover, it's a weak motive force but in reality the blowby is so much it simply vents the blowby back into the air cleaner.

 

There's no real flow reversal except in the valve cover hose..

 

In fact, there's two systems: "PCV" and "Road Draft".

 

When it's dumping out the cover you are operating like most any vehicle did until 1967... pressure built up from blow-by is simply vented down a pipe and onto the roadway. In a Z, this is the upper hose into the Air Cleaner. Having the air cleaner like that, the restriction of the air cleaner helps suck out the blowby fumes, but there is really no way for fresh air to get into the crankcase. Old Corvairs were like that, early models had a road draft setup, later models an orifice into the air cleaner.

 

The PCV ALWAYS flows from Air Cleaner, to Valve Cover, to Inlet Manifold for combustion. It is designed to draw fresh air through the crankcase 'positively' using Vacuum as a motive force.

 

In turbo cars, it's more pronounced than if you go WOT...not only do you not have vacuum to provide motive force there is pressure there and the PCV shuts to prevent turbo boost from pressurizing the crankcase and spewing oil everywhere... Under boost you have more blowby and you just road-draft it into the inlet before the turbo (after the inlet filter.) The turbo adds a nice suckage on that....

 

PCV can work with just a vent on the valve cover if you restrict the manifold draw...Mitsubishi used a 0.063" orifice to limit maximum draw (similar to the old Late-Model Corvairs!)

 

If you don't have a lot of vacuum, or are always driving at WOT, then an ejector through the exhaust (with a similar restriction orifice) will work fine to eliminate Blowby in the crankcase.

 

You want the fresh air in there to mitigate acid and condensation buildup, the sorts of nasties that can corrode the internal components. It's more pronounced when you take short trips, never warm up the engine fully, and live in colder climates where the engine can cool down from hot very quickly.

 

I was hoping you would chime in! So for the sake of my setup (I know you'd never run a 4bbl but let's pretend), what would you do? Crank case fitting to Pcv valve to manifold vaccuum, and valve cover restrictor to breather? If so, any ideas on where I might find a suitable restrictor and if the same size as the mitsubishi would be ideal? Also any reason I can't use a universal PCV valve? I don't really want to use a stock pcv valve because that would entail finding a british pipe thread to npt adapter or drilling/tapping my manifold. Two things I don't want to do. I don't have any of the other pieces of smog equipment and would like to avoid trying to set them up, so I just want to make the PCV system work as best and as simple as it can short of being the factory set up. Also I haven't noticed any smoke coming out of the open pcv ports other than the few times it dieseled on me, so I feel hopeful that it won't be too much of an issue. As far as the exhaust ejector goes- I'm not building a race car, just a fun street car. It will not see a whole lot of WOT and will mostly see street driving and maybe some autocross. As such I really just want to have a functioning PCV system for engine health and to help prevent sealing problems from arising. So what would you do if you wanted to set up a simple but effective PCV system on an L28 with a 4bbl? I clearly am not as well read on this stuff as I could be but I'd just like to try and make sure I have things working right.

Edited April 9, 2016 by luseboy

[madkaw]

Not all Pcv,s are the same, so choose wisely. You can use a stock Pcv , just put it in the middle of the run and use hose clamped on to the threaded end.

I pulled a valve off the shelf at the parts store based on it fitting my set up, but it did not work the same. It bipassed quite a bit of air at idle vs. the OEM valve that hardly bypasses any air at idle. I was able to tune the car this way and it actually worked really well for idle and start ups, but didn't realize how different it was from stock until I put one on. I wish I knew what actual Pcv I was using ( for what car).

[luseboy]

Not all Pcv,s are the same, so choose wisely. You can use a stock Pcv , just put it in the middle of the run and use hose clamped on to the threaded end.

I pulled a valve off the shelf at the parts store based on it fitting my set up, but it did not work the same. It bipassed quite a bit of air at idle vs. the OEM valve that hardly bypasses any air at idle. I was able to tune the car this way and it actually worked really well for idle and start ups, but didn't realize how different it was from stock until I put one on. I wish I knew what actual Pcv I was using ( for what car).

Interesting, thank you for your insight! I was thinking that they would all be similar enough but now that I think about it they would probably be designed to work properly with the way the vacuum would change in different engines, which is not the same in all engines I would assume. I'm not so sure if I want to put the vacuum hose over the threads on the stock pcv valve but I guess if I can get itto seal it doesn't matter, I'd just be worried about it coming off in the middle of a drive giving me a nice vacuum leak. Maybe I could find a bspt thread to hose nipple adapter... I'll have to look into that.

 

Still looking for more guidance. I'm not anything near an expert on this shit, so what should I do with my valve cover breather?

[Tony D]

"So what would you do if you wanted to set up a simple but effective PCV system on an L28 with a 4bbl?"

 

Short Answer: Hook it up with the stock Nissan Components same as Nissan did.

 

Is there some aversion to using the Nissan-Designed Component on a Nissan-Similar Manifold? Nissan had both 2 and 4 bbl intakes available for the L-Series Engines, they all had the same valve used on the SU's, which is the same valve used on the EFI Cars, which is the same valve used on the 4 cylinder SU and DAF Downdraught carbs, which is the same as....

 

Nothing on the 4BBL setup precludes using the stock Nissan PCV valve screwed in at the base of the carb same as so many Domestic Setups, and the air can still be drawn in through your little predator triangular domed filter, or whatever.

 

It can be screwed in just about anywhere you can get it to fit, but under the carb gives the best chance that it's evenly distributed amongst all cylinders and no one cylinder suffers disproportionally.

 

That was the system we ran on the 4 BBL we had in testing back in 98/99.

 

I don't see what this 'reinvent the wheel' exercise is accomplishing. Why is this an issue? Everything you want already exists, you don't need anything different than what is in the standard parts cards  at the AutoZone. Get a Nissan PCV and screw it into your manifold. There is nothing special about your 4 barrel that makes it any different than a DAF 2 or 4 Barrel or the EFI Plenum, or the SU Balance Tube. It just needs Manifold Vacuum.

 

That's harder to do on ITB's. Hence alternate methods. If you want less oil in the intake manifold than the stock system provides, the answer is NOT a different PCV, it's installing a  BETTER PHASE SEPARATOR.... A vacuum tight can with brillo pads in it should suffice, with a drip leg and auto drain to let the condensed oil go back to the sump. Do not use the compressor water oil filter, terrible mismatch of components.

Edited April 10, 2016 by Tony D

[luseboy]

"So what would you do if you wanted to set up a simple but effective PCV system on an L28 with a 4bbl?"

 

Short Answer: Hook it up with the stock Nissan Components same as Nissan did.

 

Is there some aversion to using the Nissan-Designed Component on a Nissan-Similar Manifold? Nissan had both 2 and 4 bbl intakes available for the L-Series Engines, they all had the same valve used on the SU's, which is the same valve used on the EFI Cars, which is the same valve used on the 4 cylinder SU and DAF Downdraught carbs, which is the same as....

 

Nothing on the 4BBL setup precludes using the stock Nissan PCV valve screwed in at the base of the carb same as so many Domestic Setups, and the air can still be drawn in through your little predator triangular domed filter, or whatever.

 

It can be screwed in just about anywhere you can get it to fit, but under the carb gives the best chance that it's evenly distributed amongst all cylinders and no one cylinder suffers disproportionally.

 

That was the system we ran on the 4 BBL we had in testing back in 98/99.

 

I don't see what this 'reinvent the wheel' exercise is accomplishing. Why is this an issue? Everything you want already exists, you don't need anything different than what is in the standard parts cards at the AutoZone. Get a Nissan PCV and screw it into your manifold. There is nothing special about your 4 barrel that makes it any different than a DAF 2 or 4 Barrel or the EFI Plenum, or the SU Balance Tube. It just needs Manifold Vacuum.

 

That's harder to do on ITB's. Hence alternate methods. If you want less oil in the intake manifold than the stock system provides, the answer is NOT a different PCV, it's installing a BETTER PHASE SEPARATOR.... A vacuum tight can with brillo pads in it should suffice, with a drip leg and auto drain to let the condensed oil go back to the sump. Do not use the compressor water oil filter, terrible mismatch of components.

Alright alright this is what I was trying to avoid lol. I'm not trying to re-invent the wheel, and I'm also not claiming I have some super special set up. My whole point was that I'm just trying to figure out a simple yet effective PCV set up.

 

So the stock PCV valve is bspt right? I guess I can find an adapter to use that to plumb into the carb, simple enough. I'm still mostly trying to determine what I should do with the breather on the valve cover, I don't just want to leave it open with a filter right? Should I use a check valve on there like was suggested earlier, a restrictor like you mentioned mitsubishi uses, or something else? If I can answer that I can figure out the rest.

[Derek]

 

 

So the stock PCV valve is bspt right?

 

Mine screws right into 1/4 NPT and it's factory Nissan.

 

Derek

[luseboy]

Mine screws right into 1/4 NPT and it's factory Nissan.

 

Derek

Awesome, that's simple! Everything I found online said bspt so that's what I had assumed it was, which would make sense seeing as how a few other engine plumbing threads are cut in bspt.