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  1. #1
    Senior Member BasicQ's Avatar
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    Default Breathers v Catch cans v PCV with separator

    Discussion starter,

    Concerning the three ways of dealing with blow-by gasses and crankcase pressure relief listed above I think I might need some education on the catch can set up. It seems everyone is using that set up these days so maybe there is something more to it than I see. In the pics below is an engine with two breathers that has the breathers relieve crankcase pressure and vents gasses to atmosphere. The second pic is what I understand. PCV valve to draw gasses out and recirculate through the engine with an air/oil separator in line to prevent any oil mist going into the intake system. It also provides a helpful negative pressure in the crankcase. The last pic is the common type catch can set up I have been seeing around for a few years now on street driven cars. I see it as relocating the breathers from the engine to elsewhere in the engine bay and a place where oil mist carried by the blow by gasses are captured and stored.

    Am I missing something with the catch can system and, what are itís advantages?

    [img]https://i.ibb.co/YZX7HbB/85-E8-DB6-B-8-BFC-491-D-9587-8-D1-E4-FC2-C6-B2.jpg[/img]

    [img]https://i.ibb.co/XVtpNkr/2428-D832-6506-4235-880-A-22-B8400-E2-B27.jpg[/img]

    [img]https://i.ibb.co/R9ytNBH/05010678-4487-4077-A79-C-E2-CCA7-AE582-D.jpg[/img]

  2. #2
    Do you ever leave? immortality's Avatar
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    The standard PCV system lets a fair amount of oil vapour back into the intake, even ones with oil/air separators. A quality catch can added into the standard PCV system stops that oil going back into the intake. This is good for street driven cars as it is EPA friendly and also prevent those engine blow by gas smells from entering the cabin (missus friendly).

  3. #3
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by immortality View Post
    The standard PCV system lets a fair amount of oil vapour back into the intake, even ones with oil/air separators. A quality catch can added into the standard PCV system stops that oil going back into the intake. This is good for street driven cars as it is EPA friendly and also prevent those engine blow by gas smells from entering the cabin (missus friendly).
    Thanks immortality.

    What is the purpose and advantage of the catch can installs I am always seeing with no vacuum connected? They just seem to be a remote breather and a receptacle for oil pushed out the engine by blow by pressure.

    Iíll put this another way. If I had the set up in pic 2 for a street strip car can you sell me the set up in pic 3 as being better for a street strip car?


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    Do you ever leave? immortality's Avatar
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    Personal preference? I guess people don't want the oil in the intake. External catch can with breathers are tidier and simpler than a full plumbed PCV/catch can system and probably tidier in the engine bay.

    There is the argument that the blowby gasses in the crankcase isn't good for the oil and makes it more acidic? Other than that and for a street car you technically need an enclosed PCV system there isn't any real argument for it.

  5. #5
    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by immortality View Post
    The standard PCV system lets a fair amount of oil vapour back into the intake, even ones with oil/air separators. A quality catch can added into the standard PCV system stops that oil going back into the intake. This is good for street driven cars as it is EPA friendly and also prevent those engine blow by gas smells from entering the cabin (missus friendly).
    EPA friendly... ? not in Australia in any state

    all the wankers who think a catch can on their piece of street metal is cool forget
    that no state EPA or federal ADRS permit the use of a catch can
    Plod in Victoria know and do check for these.. and yes, you will get a canary!

    and also
    catch cans were designed for off road activities (especially motorsport*)
    where there is the possibility an engine may over-pressurise and something
    needs to be attached to catch the oil/oil fume combo (hence the name)

    Also note -
    No OEM vehicle manufacturer provides a catch can on any vehicle... anywhere globally
    because any such vehicle will never comply with any air quality rules


    *There are rules in place stating the size and required plumbing of catch cans
    in Motorsport here in Oz. Motorsport Australia (was CAMS) provide a lot of
    info in their competition handbook .. and yes, the scrutineers do check during
    scrutiny before events
    Last edited by Smitty2; 23-12-2020 at 08:19 PM.
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  6. #6
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by Smitty2 View Post

    and also
    catch cans were designed for off road activities (especially motorsport*)
    where there is the possibility an engine may over-pressurise and something
    needs to be attached to catch the oil/oil fume combo (hence the name)

    *There are rules in place stating the size and required plumbing of catch cans
    in Motorsport here in Oz. Motorsport Australia (was CAMS) provide a lot of
    info in their competition handbook .. and yes, the scrutineers do check during
    scrutiny before events
    I wouldn’t have thought pic 1&3 would be road legal and not since maybe late 60’s early 70’s?

    Reading between the lines from what you said Smitty the set up in pic 3 has been adopted from racing to the street. And from what you indicate a system employed as a preventative measure to avoid oil on a race track. Has it been misconstrued as a performance enhancing application?

    How much power would pic 2 legal system cost over pic 3 system?

    What piqued my curiosity more and had me enquiring if there was more to it than what I see is an advertisement out there from a guy that makes 6 litre capacity catch cans with his target market seemingly the street strip guy.

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    Senior Member LXCHEV's Avatar
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    For what it's worth; many years ago when I had the drag racing bug and thought I needed to chase down every last ounce of power, we decided to perform back to back pulls with the Torana on the chassis dyno one evening;

    Test #1: PCV connected (traditional setup of PCV valve directly connected from one rocker cover to full manifold vacuum (can't recall if that was a carb port at the time, or a bung in the intake), with a breather on the opposite rocker cover.

    Test #2: PCV valve removed and plugged, with a second breather installed onto that rocker cover in it's place.

    I still have my notes - power-wise, both runs were identical - there wasn't a single kW in it. That's on the previous incarnation of my 383 SBC - another mild 450 flywheel HP combo. The dyno operator did comment that on the run without the PCV, the air:fuel ratio curve straightened up ever so slightly (ie. the tiniest bit leaner) - but we're splitting hairs here and talking bugger all difference.

    That particular engine had become a bit of a heavy breather and I always preferred running the PCV. If I only ran the 2 rocker cover breathers, I could not only smell it, but also see wisps of vapour exiting the breathers.

    With my new and current engine (much the same ~450HP 383) - I have been running 2 rocker cover breathers since it was built which has now been 9 years. This was purely because I had no vacuum ports. However in the past few months, I've started paying a bit more attention to this stuff (it's always been a rich runner that smells fuelie and makes a mess of the plugs) - so I'm trying to get a much cleaner running setup. Part of this was my recent carb swap and I now have a dedicated PCV port and so the PCV valve is connected again. I'm yet to perfect the carb tune, and haven't got a lot of driving in yet, but so far it's certainly less fuelie (I don't tend to smell fuel/oil at all now).

    I'm actually curious to hear from anyone else who runs a PCV on a hottie street motor - if the particular PCV valve part number matters much or not (ie. different spring pressures/flow rates etc) - which all behave differently on different combo's (with varying vacuum readings). I'll need to check - but the right angle one I picked up recently from REPCO I think is a #2072. I have low idle vacuum (6") and I reckon I can hear the PCV valve whistling away - it's certainly working hard. It has an incredible amount of suction if you put your finger over the end. I need to get more driving in. My engine also tends to go through a little bit of oil, so after 9 years without the PCV, it'll be interesting to see if having one now increases oil consumption or not.

    Anyway - it's an interesting thread. I too have never particularly understood or been drawn to the catch can setups - but if combined with a PCV, I can see the idea of trying to eliminate the gases from re-entering the intake. I've never actually considered running a separator before - that's certainly something I might look at too.

    For me personally (I'm getting old) - I like the idea of the PCV for many reasons - elimination of smells and vapour, keeps everything 100% legal (not that I'm worried about that aspect), relieving of crankcase pressure, simple and clean engine bay, and of course the cleaner engine oil too.
    Last edited by LXCHEV; 24-12-2020 at 12:22 AM.

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    Senior Member LXCHEV's Avatar
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    By the way, there is also a 4th option - vacupans. More of an all-out race car thing yes - but the idea of them makes sense - relieve all that crankcase pressure (especially at high RPM when the exhaust is flow is creating the vacuum) and sucking all those blow-by gases straight out the exhaust instead of back into the intake. I would imagine following behind a car with vacupans installed though might smell? Depending on how heavy it breathes I guess.

  9. #9
    Do you ever leave? immortality's Avatar
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    Quote Originally Posted by Smitty2 View Post
    EPA friendly... ? not in Australia in any state

    all the wankers who think a catch can on their piece of street metal is cool forget
    that no state EPA or federal ADRS permit the use of a catch can
    Plod in Victoria know and do check for these.. and yes, you will get a canary!

    and also
    catch cans were designed for off road activities (especially motorsport*)
    where there is the possibility an engine may over-pressurise and something
    needs to be attached to catch the oil/oil fume combo (hence the name)

    Also note -
    No OEM vehicle manufacturer provides a catch can on any vehicle... anywhere globally
    because any such vehicle will never comply with any air quality rules


    *There are rules in place stating the size and required plumbing of catch cans
    in Motorsport here in Oz. Motorsport Australia (was CAMS) provide a lot of
    info in their competition handbook .. and yes, the scrutineers do check during
    scrutiny before events
    So you are saying that adding a catch can into a factory PCV system is illegal when the full function of the PCV system is retained and nothing is vented to atmosphere?

    Better for the environment if excess oil is burnt up via the combustion process rather then emptied and properly disposed off with the rest of the oil that is drained from the sump?

    I definitely know and understand the use of catch cans or puke cans on race cars and fully agree of their need but that doesn't discount the fact that when done correctly there are benefits for road cars.
    Last edited by immortality; 24-12-2020 at 05:41 AM.

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    Do you ever leave? immortality's Avatar
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    Quote Originally Posted by BasicQ View Post
    I wouldn’t have thought pic 1&3 would be road legal and not since maybe late 60’s early 70’s?

    Reading between the lines from what you said Smitty the set up in pic 3 has been adopted from racing to the street. And from what you indicate a system employed as a preventative measure to avoid oil on a race track. Has it been misconstrued as a performance enhancing application?

    How much power would pic 2 legal system cost over pic 3 system?

    What piqued my curiosity more and had me enquiring if there was more to it than what I see is an advertisement out there from a guy that makes 6 litre capacity catch cans with his target market seemingly the street strip guy.
    No performance benefit? Occasionally I like to let the blower work on the daily driver, SH16 is a great bit of road (for any of those playing in NZ), lots of hills, hairpin turns, a real good workout for the blower and the brakes. Without the catch can fitted I would notice a loss of performance when I started to get a little oil fouling on the plugs, so much so the engine would loose a little vacuum and the brakes would go away and the idle would suffer and it was time to back off and cruise, once the plugs cleaned up everything was good to go again. With the catch can fitted as part of a fully functioning PCV system that problem disappeared.

    Personally, I'm all for catch cans fitted into the factory PCV system. I like playing with older cars but you do see the results of the factory PCV system with oil gunk built up in throttle bodies and intake manifolds. LS motors are notorious for filling the intake with oil from the breather system.

    Anything with direct injection needs a catch can simply because you don't have fuel washing the intake valves like you do with port injection and the valves will get gunked up with crap. Definitely a known issue.
    Last edited by immortality; 24-12-2020 at 05:45 AM.

  11. #11
    Sure why not? 76lxhatch's Avatar
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    Quote Originally Posted by LXCHEV View Post
    I'm actually curious to hear from anyone else who runs a PCV on a hottie street motor - if the particular PCV valve part number matters much or not (ie. different spring pressures/flow rates etc) - which all behave differently on different combo's (with varying vacuum readings).
    Fully adjustable PCV valves are stupid expensive but it works:
    http://mewagner.com/?p=444
    If you can find a factory valve that suits your engine vacuum that will be a lot cheaper, but I tried a few and couldn't get it right. Bought one of these and dialled it in as per instructions, no longer have a massive air leak at idle which gave a lot more control to get it idling nicely while still giving PCV operation when it needs it.

    As Immortality says running the PCV valve in a secondary separator (a separate can is easiest but even just a good air/oil separator in the rocker cover can help) gives you the benefits of the PCV system with a little less oil re-introduced in the intake. Don't forget that the breather still needs to work well at the other end; its both the intake at low rpm and vents excess pressure at high rpm.

    On a high-revving large displacement engine replacing the PCV system with purely atmospheric breathers requires the breathers to be HUGE in comparison to the PCV connections otherwise you are just pressurising the engine and it will not only smell but start blowing out seals. Don't even consider it without cutting holes and using much bigger fittings and hoses. And you still lose the benefit of crankcase vacuum, not something I'd do on a street car.

  12. #12
    Senior Member BasicQ's Avatar
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    Default Breathers v Catch cans v PCV with separator

    So immortality you are describing your driving experience with direct pcv to manifold v inline catch can? The inline is something I get and makes sense to me too. I donít quite remember but doesnít a boosted application stuff with the function of a pcv like it does with carby power valves so you would see a greater benefit of inline system?

    Terminology. Is what you are calling a catch can in an inline situation essentially an air oil separator or is what you are calling a catch can something that functions differently?

    My original post is trying to understand the prevalence of the non pcv integrated catch cans plumbed with a single line to the front of the engine bay. Being done so often these days am I missing something?


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    Last edited by BasicQ; 24-12-2020 at 07:47 AM.

  13. #13
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    hi
    Catch cans are legal when installed correctly. Some are OEM fitted .
    1/ illegal catch cans installed to vent to atmosphere
    2/ legal when catch cans are installed inline with Pcv line . All vented gases are recirculated .

    All this applies with ADR rules
    1/ prior to approx 1970 no PCV required
    2/ Open crank case ventilation uptill 1977
    3/ 77 on closed crankcase ventilation

    Then there are motor sport rules

    Depending on how your engine is built but a little amount of oil lubes the valves etc

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    Do you ever leave? immortality's Avatar
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    I am. It actually took me ages to figure out what was going on but once I figured it out then the catch can was a no brainer, really it wasn't anyway because when I had the blower off you could see a nice coating of oil in the intake manifold, all nice and clean oil, no gunk which was a definite sign the PCV system was sucking oil into the blower. Blower cars certainly do benefit more as there are probably more blowby gasses but I found the main reason for my plug fouling issue was engine braking as this creates the most vacuum in the intake

    Yes terminology may be an issue. An inline oil/air separator may be a better name but most people call em catch cans which probably does confuse the issue.

    Smitty2 is correct that catch cans (or puke cans) are a racing requirement for most motorsports events and it's something that has found it's way into street cars. It also does away with the PCV plumbing and oil in the intake issues that most show cars or weekend warriors don't feel the need to deal with. I'm no greeny but I also don't see the need to pollute any more than necessary so a working PCV system with catch can (or inline oil/air separator) is my cup of tea.

  15. #15
    Senior Member BasicQ's Avatar
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    Default Breathers v Catch cans v PCV with separator

    Quote Originally Posted by immortality View Post
    I am. It actually took me ages to figure out what was going on but once I figured it out then the catch can was a no brainer, really it wasn't anyway because when I had the blower off you could see a nice coating of oil in the intake manifold, all nice and clean oil, no gunk which was a definite sign the PCV system was sucking oil into the blower. Blower cars certainly do benefit more as there are probably more blowby gasses but I found the main reason for my plug fouling issue was engine braking as this creates the most vacuum in the intake

    Yes terminology may be an issue. An inline oil/air separator may be a better name but most people call em catch cans which probably does confuse the issue.

    Smitty2 is correct that catch cans (or puke cans) are a racing requirement for most motorsports events and it's something that has found it's way into street cars. It also does away with the PCV plumbing and oil in the intake issues that most show cars or weekend warriors don't feel the need to deal with. I'm no greeny but I also don't see the need to pollute any more than necessary so a working PCV system with catch can (or inline oil/air separator) is my cup of tea.
    Thanks. Great reply and clears up the discussion.

    Since I learnt of the pcv system at trade school back in the early 90ís I have always used a pcv valve no matter what the engine build. Back then when my mates and I were stuffing around trying to get more zip out of our 70ís Holdens the pollution equipment was always the first to go but the pcv always stayed for its functions beyond helping the environment.

    So back to my fundamental question of Ďam I missing something in pic 3?í. Iíve never had anything to do with the set up in pic 3 that is now common to see at car meets and some people are putting on their new builds no questions asked. I have the pic 2 set up with the same Moroso air/oils separator. I moved away from the modified car scene for quite a few years and am just getting back into it over the past 5 or so years. Reading through the likes of Street Machine and wandering around car meets and shows I now see pic 3 set up quite a bit on all levels of modified stuff. My overly logical mind looks over the system in question and canít work out why itís used on a street driven car. So is there something to it I canít see because I have never been up close and personal to an install or is it purely fashion and those that choose it sacrificing the benefits of a pcv with an inline separator purely for looks?
    I do know of some guys experiencing leaks and another with crap under the valve covers and they are using pic 3 set up over a pcv. They laugh off the mention of employing a pcv valve.


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    Last edited by BasicQ; 24-12-2020 at 09:30 AM.

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