friends don't let friends use breather caps!
GRGT1994 said:
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?
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
), so I don't feel I have enough experience to really know what I am talking about, as far as tweecer tuning.
