PMS Question - Is a VC breather better than a line back to the intake?

Discussion in '1994 - 1995 Specific Tech' started by GRGT1994, Mar 4, 2006.


  1. GRGT1994

    GRGT1994 New Member

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    I have heard that it is easier to tune with a PMS if you do not connect the VC breather line back to the intake. The line I'm talking about connects at the oil filler neck, and runs to the cold air intake somewhere after the MAF.

    Is there any truth to this?
  2. Grn92LX

    Grn92LX Fidanza Man! Founding Member

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    I had it both ways and didn't notice much difference in my a/f. I have it how it came from the factory. I ditched the breather and returned it back to pep boys. You NEED to run that hose from the oil fill to the tb.
  3. GRGT1994

    GRGT1994 New Member

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    Even with a PMS? Because Danny from AFM says he prefers to dyno tune with a breather instead of the stock setup (especially if you don't have to pass emissions).
  4. vristang

    vristang Advanced Member

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    If you remove the tb/vc hose for a breather, and leave the pcv intact you will essentially create a vac leak via the crankcase/pcv.

    Why would you tune the motor in a state that it will not be driven in?

    I am sure the pro tuner has a reason, I just don't see it.
  5. GRGT1994

    GRGT1994 New Member

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    Vacuum leak? Huh?

    My understanding of that line was that it allows the release of pressure built up by blow by. Air is generally flowing out of the motor here - the opposite of vacuum. So this is not vacuum line. And the reason it flows that excess pressure back into the cold air intake so that dirty air can be combusted again, reducing emissions. But if you are not tested for emissions, then it does not matter that you leave that air to flow out through a breather.

    The way the tuner described it, you would drive the car with the breather.

    I don't mean to call you out, I just don't fully understand tuning very well at all. Can you clarify?
  6. zenboy99

    zenboy99 Founding Member

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    I've heard this from a couple of people and I thought it was the cause of my rough idle problem. I switched back to the line to the throttle body and the vacuum was 15 with the line and with the breather. So with my combo I didn't notice any change in vacuum using a breather.
  7. vristang

    vristang Advanced Member

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    Confusion is pretty common here so I will do my best to clarify.

    What causes problems is putting a breather (filter type) on the oil filler neck and capping the vc/tb nipple while leaving the pcv in place. There is a good vac signal at the pcv (in most cases) that will draw air from the breather on the vc, through the vc, and into the intake. This air is not metered by the TB, making it a vac leak. Even worse is that the fresh air entering the crankcase is not metered by the MAF. The computer has no idea that this air is even present in the intake.

    The way Ford designed the system was so that the tb/vc line would atleast pull the fresh air for the crankcase through the MAF before bypassing the tb. Vac supplied via the pcv is what pulls the air in through the pcv.

    Maybe the sn95 motors are a little different from the fox setup I have experience with?
    My concern is that unmetered air is being introduced into the intake.

    jason
  8. GRGT1994

    GRGT1994 New Member

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    Great explanation. And I think that the concern you describe echos what I have read from others here on Stangnet about VC breathers.

    The tuner that I spoke with said that the primary function of the VC to intake tube is to vent excess pressure due to blow by. That would lead to an airflow opposite from the one you describe. And I think that both are probably right. That is why when you look at a good diagram of the airflow system in a 5.0 motor, you see an indication that air flows both ways through the tube we are talking about.

    I guess I understood that air would flow out of the VC through this tube at WOT (since that is when the greatest volume of blow by pressure is created). And then at normal load there is either negligible flow or reverse flow in the direction you describe. If my scenario is right, then there is no risk of lean condition at WOT (air is going out not in), and there is possibly a leaner than optimal condition at normal driving load. But since the A/F ratio at normal load is so forgiving, the marginal impact should be insignificant.

    Now I clearly know nothing on this topic, and I am just trying to put together what I get from others who know much more. If I have my scenario reversed from reality (i.e., blow by is prominant at normal load, and there is significant draw into the engine at WOT through this tube) then I could see there being a significant problem with a breather.

    Any thoughts?
  9. 18mustangs

    18mustangs New Member

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    friends don't let friends use breather caps!

    Here's the write up I did on the subject a while back:

    How the PCV system works
    One an EFI engine and some later carburated engines, the PCV (positive crank case ventilation) system draws air from the crankcase into the intake stream and into the combustion chamber where it is burned in order to reduce emissions. This system does more than reduce emissions, it positively ventilates the crankcase prolonging engine life, whereas running a breather cap system even with the PCV valve capped or filtered to the outside is a passive system rather than a positive system like the PCV, and is not as efficient at ventilating the crankcase as the PCV system as our newer emissions friendly engines do not have road draft tubes like older muscle cars did in order to create the positive ventilation effect that the PCV valve does. Also remember that the PCV system is an emissions component, and if it is deleted or changed some states may fail the visual portion of your emissions test.
    Why not to run a filtered breather cap
    One should never run a breather cap on an EFI engine with a functional PCV system (with the oil filler neck capped or connected) as the breather cap allows unmetered air from the engine bay to enter the crankcase and then the intake charge via the PCV valve which creates a lean condition. This lean condition may cause detonation\pinging (a common problem with SN95s in stock form) to occur, and it is important to note that detonation is not always audible so you may not know that you’re pinging. This lean condition is created because the PCV system draws fresh metered air into the crankcase via the oil filler neck hose in order to replace the oil vapor laden air that the PCV valve drew out of the crank case into the intake charge. This is a 1:1 swap so the air fuel ratio is affected, and prevents a vacuum from forming in the crank case when the PCV valve draws the air out of the crankcase. If one runs a filtered breather cap the the intake charge still has 100% if the intake charge’s air as no air was diverted to the crankcase, and now the additional air from the crankcase is added to the intake charge leaning the air fuel mixture.
    Capping the filler neck hose
    One should not cap the oil filler neck to throttle body PCV hose in order to reduce oil in the intake charge as that hose is bi-directional (with the correct non breather oil cap or even with a filtered breather cap). The PCV system draws fresh metered air into the crankcase in order to replace the oil vapor laden air that the PCV valve drew out of the crank case into the intake charge. This is a 1:1 swap so the air fuel ratio is not off and prevents a vacuum from forming in the crank case. If the PCV valve cannot keep up with the pressure inside of the crankcase the extra pressure can be relieved via the oil filler neck hose. Capping off the PCV oil filler tube with the proper oil cap prevents the extra crankcase pressure relief, contributes to a vacuum in the crankcase, and contributes to a lean condition as no air is diverted to the crankcase from the intake charge while air is added to the intake charge from the crankcase. Capping the oil filler neck with a filtered breather cap allows unmetered air to enter the intake charge from the engine bay via the crankcase, through the PCV valve.
    One Possibility for filtered breather caps
    If one is dead set on running breather caps, you should cap off the oil filler neck\TB PCV connections and put a small breather on the PCV valve (capping off the intake vacuum ports that the PCV hose used to lead to) as well as on each valve cover. This system is not as efficient as running the PCV system but does prevent excess crankcase pressure from blowing out gaskets and causing oil leaks. This is not recommended as this is not a positively vented system, it only relieves excess crankcase pressure and doesn't ventilate the crankcase with fresh air extending engine life.

    Tim
  10. GRGT1994

    GRGT1994 New Member

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    Thanks for that expansive writeup. The parts about unmetered air entering the system and introducing a possible lean condition make some sense. But think about it this way, the primary source of air on my engine is an opening that is 75mm wide, while the opening at the filler neck is approximately the diameter of a straw. Not only that, but the path from CAI to combustion chamber is a direct route, while the path through the straw sized opening then moves through the top portion of the passenger side head to the lifter valley, then to the PCV system (where it is intentionally constricted by the PCV valve), and finally into the upper intake manifold. I simply cannot fathom how air pulled through that circuitous route could amount to any significant variation of air/fuel ratio compared to the massive amount of air passing through the intake track. But that's just how my crazy mind works. And I certainly defer to others who actually know about how this system works.

    I will discuss again with the tuner and see what he says.
  11. vristang

    vristang Advanced Member

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    I like that you are trying to understand the system, and not so much just looking for the "right" answer. :nice:

    Considering that the crankcase gasses are pressurized and that the intake manifold is a vacuum, I don't see the length of the circuit as causing any flow restrictions, at least not to the level you are envisioning. The pressure differential between the crankcase and the intake can be significant. The ~3/8" (I think?) vac line should flow plenty of gasses with such a pressure differential. But we are now rapidly approaching the limits of my understanding as well.

    I would love to hear what your tuner has to say.
    It's always possible that he knows something I don't :shrug:
  12. zenboy99

    zenboy99 Founding Member

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    This has been an interesting read, and I believe all of it to be true. But I just wanted to say that I've been running a breather on my valve cover for over 2 years now without a problem. Car idles at 850 like the Tweecer tells it too. PCV is routed to the intake

    I ran the breather because my Anderson Ford Power pipe didn't have a provision for the oil breather line and I didn't feel like tapping one in.
  13. vristang

    vristang Advanced Member

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    Congrats on your 2,000th post :nice:

    Disclaimer:
    I only ordered my tweecer last week (and it still hasn't arrived :mad: ), so I don't feel I have enough experience to really know what I am talking about, as far as tweecer tuning.

    I am glad you posted this, as it is always interesting to hear more.

    I am pretty surprised that you don't have idle issues, but have to ask if you had any trouble tuning the idle? Did anything seem out of place considering your modifications?
    I would guess that with the tweecer you could tune to compensate for what is essentially a vac leak?

    With the stock computer many people have reported idle control/fuel economy issues with such a setup. This is the first time I have heard of a tweecer using a breather, so I have to think that this is why you are doing ok.
  14. GRGT1994

    GRGT1994 New Member

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    I talked to Danny at Anderson Ford. He said that they generally run a breather if emissions are not an issue (track car). I'm still no expert on how this works, but I'll relay what I understood from his comments.

    The function of the line from the valve cover to the intake is solely to vent excess pressure resulting from piston ring blow by. This pressure builds up in the top portion of the engine, and vents back into the intake flow to be reburt in engine combustion. Since air is only flowing out from this engine opening, there is no vacuum leak as a result of using a breather on the valve cover. In fact, they will often use a breather on both valve covers to further facilitate the release of pressure resulting from blow by.

    These are not my opinions. Rather, this is my recollection of comments from someone who does enough Mustang tuning to know how this system works. So hopefully this helps clarify the issue a bit. But I encourage others who know more about this to please continue the discussion, because I would love to understand this and many other things better about my engine.
  15. final5-0

    final5-0 Mustang Master

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    I think we are our own worst enemy here :rlaugh:

    We are making this too difficult if you ask me :shrug:

    These breather threads always draw a lot of interest :)

    If we are looking at the whole system in its stock configuration

    Its a closed system. That means the only air flow anywhere in that system has been metered by the maf.

    If you do the breather thing, you now got air entering the system that has not been metered by the maf.

    I just don't see how it is any more complicated than that :shrug:

    If I have missed something here, I look forward to enlightenment :D

    Grady
  16. zenboy99

    zenboy99 Founding Member

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    Thanks! I didn't even notice that.

    I still have my stock valve covers, maybe I'll put those back on and do some more datalogs.

    Car ran fine right when I got the tweecer, idled the same as it did with my JMS chip. For a week my lower MAF values were climbing, but that was a result of my Performer intake being cracked. Once I get the new intake on I'll try some datalogs with breather and without.
  17. zenboy99

    zenboy99 Founding Member

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    Just some concerns, no flame intended. If its ring blow by then thats going to be vented via the PCV. Did they mean pressure blowing through the valve guides? I guess that would make more sense for the top half of the engine being vented via the oil breather. I would think ring blow by would be easily vented via the PCV instead of back up through the head to the oil breather, just some thoughts.

    As far as Anderson Ford Motorsports, they've got a great reputation. Being from the midwest alot of my friends have driven to their dyno for a tune. From what I've been told that dyno is constantly running. Alot of guys from Chicago, Wisconsin and Indy go there besides alot of NMRA racers. Just based on that you'd think these guys have their act together, I would bet they do. One thing concerned me about them though. I called them up before I did my head/cam swap asking about a tune. I told them I was worried about my finicky computer and wondered what kind of chip they would use to tune my car. The dyno operator told me they wouldn't use a chip at all, they would just tune the car by changing the fuel pressure and base timing. Now, I think alot of the tuner guys on here know how much more you can do to get a car running right beyond "base timing" and "fuel pressure" changes.

    It would be nice if an AFM guy would come on here and talk about the oil breather issue, just to get another opinion.

    I'm waiting on my Fluidyne radiator. Once the salts off the roads I'll run both closed, and open breather and report my results.
  18. vristang

    vristang Advanced Member

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    The first line I disagree with, because I have had more oil vapor blow out of my pcv than through the vc line. To me this suggests that the pcv is the primary vent of crankcase gasses. But that is a philosophical debate that probably can't be fully resolved here.
    Secondly the concern is that there is essentially a vac leak that enters through the breather, travels through the crankcase, and enters the intake via the pcv. When you have high vac and low crankcase pressure, there will be air moving into the breather; Idle is a good example of this.

    :nice: Agreed

    If they insist on using vc breather caps, then a smart customer would insist that the pcv is either capped, or replaced with a breather itself. As I see it that is the only way to make it "right."

    jason
  19. blackcloud50

    blackcloud50 New Member

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    My thought on this issue are as follows:
    With pcv still connected and using a breather, any unmetered air that gets into the combustion chamber and causes a "leaner" condition would be quickly addressed by the ECM's ability to adjust and correct a/f ratio based on feedback from the O2 sensors (O2s would see a lean condition). Otherwise no since in having O2s. Now if one is using a tuning device, a/f ratio can be adjusted to compensate for what would have been a very temporary leaner condition caused by using a breather with pcv still connected right from the start.
  20. final5-0

    final5-0 Mustang Master

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    I agree 100% with this line of reasoning :nice:

    however

    With the bolt ons and non stock levels of fuel pressure a lot of folks run ......

    The adaptive can only do so much and each combo can be different to say the least.

    I have seen several data logs from noobies that showed the adaptive at full on or close to the limit.

    Just trying to say some of the time, the pcm can't bail us out of a problem we may have caused.

    Grady

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