Crankcase Ventilation ideas.
09-25-2003, 05:22 PM
#1
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Location: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
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Crankcase Ventilation ideas.
I have an '85 L98 vette, and oily residue getting into the throttle body has always been a problem. Any excess blowby the pcv can't handle will go up the pcv supply line, backing up into the throttle body, and making the passenger side valve cover oily.
The PCV system has to be a closed loop to prevent unmetered gas from entering the system. It's sealed to prevent blowby to escape and pollute.
Now what I'm thinking.. is why not just vent excess pressure to the atmosphere, put a check valve on that side, to prevent the pcv from sucking unmetered air in through it. And... just have the pcv suck from the crankcase.
Is it ok to suck solely from the crankcase? Will this pull in too much oil, or create TOO much crankcase vacuum? I thought crankcase vacuum was always a good thing, something carbed cars with breathers don't have, which is why they burn up more oil.
Here's an explanation of exactly how the PCV system works on TPI vettes if you're not familiar:
The PCV system has to be a closed loop to prevent unmetered gas from entering the system. It's sealed to prevent blowby to escape and pollute.
Now what I'm thinking.. is why not just vent excess pressure to the atmosphere, put a check valve on that side, to prevent the pcv from sucking unmetered air in through it. And... just have the pcv suck from the crankcase.
Is it ok to suck solely from the crankcase? Will this pull in too much oil, or create TOO much crankcase vacuum? I thought crankcase vacuum was always a good thing, something carbed cars with breathers don't have, which is why they burn up more oil.
Here's an explanation of exactly how the PCV system works on TPI vettes if you're not familiar:
The drivers side pcv line goes behind the throttle blades, (the vacuum side)
The passenger side pcv vent tube connects ahead of the throttle blades, (the atmospheric pressure side)
So.... flow will go from the passenger side port on the TB, down through the valve cover, head, crankcase, and up through the other side, through the pcv valve and into the plenum.
That is why you can't use one breather on these cars. If you did, the vaccum would just suck a bunch of air from the passenger side breather through the pcv valve on the drivers side... essentially a vacuum leak. (Un-metered air.)
The only time blowby goes UP the passenger side pcv vent line ahead of the TB blades, is under high load condtions. (Low rpm + high throttle = lugging)
This results in lots of cylinder pressure, lots of blowby, and not enough vacuum to pull all the blowby out. Soo.. it goes up the passenger side tube. I think it was designed this way on purpose... so that no blowby gasses could escape from the engine and pollute the atmosphere. Instead... they pollute the whole intake with nasty gooey slime.
The passenger side pcv vent tube connects ahead of the throttle blades, (the atmospheric pressure side)
So.... flow will go from the passenger side port on the TB, down through the valve cover, head, crankcase, and up through the other side, through the pcv valve and into the plenum.
That is why you can't use one breather on these cars. If you did, the vaccum would just suck a bunch of air from the passenger side breather through the pcv valve on the drivers side... essentially a vacuum leak. (Un-metered air.)
The only time blowby goes UP the passenger side pcv vent line ahead of the TB blades, is under high load condtions. (Low rpm + high throttle = lugging)
This results in lots of cylinder pressure, lots of blowby, and not enough vacuum to pull all the blowby out. Soo.. it goes up the passenger side tube. I think it was designed this way on purpose... so that no blowby gasses could escape from the engine and pollute the atmosphere. Instead... they pollute the whole intake with nasty gooey slime.
09-25-2003, 08:57 PM
#2
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Re: Crankcase Ventilation ideas. (CentralCoaster)
Sounds like you need new rings. There will always be some oil vapor in the PCV system but as blow-by is increased, so are the amount of oil particles in it. Venting it to the atmosphere, besides partially defeating the purpose of the PCV system to begin with, would probably eliminate the oily build up you're noticing but (unless you vented it to outside the engine compartment) those same oil particle would just build up somewhere else. Also, you might not like the smell of hot motor oil and unburnt gas that would be freed to stink up the air around (and in) the car.
09-26-2003, 07:24 PM
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Re: Crankcase Ventilation ideas. (CentralCoaster)
Now what I'm thinking.. is why not just vent excess pressure to the atmosphere, put a check valve on that side, to prevent the pcv from sucking unmetered air in through it. And... just have the pcv suck from the crankcase.
How 'bout an electric driven vacuum pump. How much do you want to invest in the mods to your oem system?
[Modified by arnold, 6:26 PM 9/26/2003]
09-26-2003, 08:49 PM
#4
Burning Brakes
Re: Crankcase Ventilation ideas. (CentralCoaster)
Try a 12 V vacuum pump set for around 20 inHg, you'll get better ring sealing and wont need the PCV system and your intake will be dry.
John
John
09-27-2003, 05:29 PM
#5
Race Director
Re: Crankcase Ventilation ideas. (CentralCoaster)
i connect a 1/2 or 3/4" hose from the inside of the air cleaner to one valve cover(thru a Mopar baffle sometimes). this has no PCV valve and works all the time :cool:
the PCV valve is left connected. Together they get the job done, even on the 67 galaxie i got in 1985; was blowing out oil. Never since; still not rebuilt :cheers:
the PCV valve is left connected. Together they get the job done, even on the 67 galaxie i got in 1985; was blowing out oil. Never since; still not rebuilt :cheers:
