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  #31 (permalink)  
Old 01-25-2010, 01:43 AM
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keiser, we aren't arguing, we're having a discussion.

Intense,
a pcv valve is a calibrated, variable vacuum leak with integrated backfire protection which also seals the crankcase off from the intake under boost on forced induction motors.
how is that for a description of the PCV valve?

from one of the first classes i took years ago, "the PCV system is an emissions component, its job is to vent crankcase vapors into the intake to be burned instead of venting them to the atmosphere"
before the PCV valve, engines used road draft tubes which vented to the atmosphere.
before the road draft tube, there was just a vent tube to the atmosphere.
before the vent tube, engines were sealed to contain the blowby. yes, they leaked oil, and they didn't last very long.
some engines don't have a PCV valve, they use a fixed orifice with a vent to the air inlet.
some engines don't use a fixed orifice or a pcv valve, they use an oil/air separator thats routed to the air intake tract. these are modern engines.
it may have not been complete, but again i gave no misinformation.

here is a test you can do if you want to see just how well the pcv valve actually relieves crankcase pressure vs the vent tube.
remove the crankcase vent hose at the valve cover and plug the valve cover and leave the pcv valve and its hose completely intact to the intake, pull the dip stick and hook a vacuum/pressure gage to the dip stick tube and put the gage where you can read it. now go drive the car.
at idle and light throttle, you will see crankcase vacuum. on a good motor that has good seals and gaskets, you may even see up to full intake vacuum. you can expect to hear the motor whistle with the crankcase sealed like this, and it isn't good on the seals either.
now, if you remove the pcv valve and plug the valve cover and leave the crankcase vent hose intact, you won't see crankcase vacuum.
you also won't see crankcase pressure unless there is a flow problem with the vent side or if it has excessive blowby because of bad rings that overwhelms the vent side.

you shouldn't see any notable crankcase vacuum with a properly working PCV system.
the highest specs i can think of are on some Volvos, about 7 inches of water max IIRC. i believe they also use a restricted vent tube.
to check if the PCV valve is flowing the proper amount, find the specs for your motor and see if it matches the RPM drop.
if you'll post what you have, i may well have the specs here at home.



eric32, if they are not removed, crankcase vapors can cause acids to be formed and breaks down the oil faster.
it helps to make the vehicle cleaner because the crankcase vapors are burned in the engine instead of vented to the atmosphere. venting to the atmosphere can make up to around 20~25% of total vehicle emissions on vehicles without a cat.
for cars with a cat, i've read total vehicle emissions can increase up to 60~80% if crankcase vapors are vented to the atmosphere instead of the motor.
a wrong or defective PCV valve can pull too much from the crankcase causing excessive oil consumption.
if you have aftermarket valve covers, they could have poorly designed baffles in them. if you bought them used, the

for your honda friends, in my 30+ years of auto repair i've seen more PCV valve failures on hondas where the valve has came apart internally than all other vehicles put together.

cut away of an average PCV valve,
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  #32 (permalink)  
Old 01-25-2010, 05:47 AM
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Good info Soul,
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  #33 (permalink)  
Old 01-25-2010, 06:29 AM
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my $.02:
you guys are not explaining a pcv valve operation "complete" enough...
(I did not carefully read all the posts but many are confusing)

yes, at high Hg no load very low idle rpms it is just barely open BECAUSE there is very little total CFM going thru the whole motor pulling against the spring and weight resistance....

at any cruise speed, with higher rpms=more total cfm, light throttle high hg condition, it is most definitely a major cfm source into the motor (pulling a vacuum in the crank case and expelling the crap),,,because there is alot more total cfm thru the motor pulling on the spring and steel piston weight...
infact the carb fuel feed curve for A/F mix is factory set to include the huge additional 3/8" line pcv air cfm supply
(that's why there are so many different pcv model units,,,match the spring rate to the cubes cfm and cruise rpms for the particular car normal cruise Hg)

there is next to no Hg (approx 2-5Hg) present WOT so the pcv doesn't pull any crankcase vacuum regardless of rpms when the pedal is to the floor it's in the full down position....
when you do back off the pedal quickly the Hg goes to max approx 25Hg and the valve is still shut off but in the full up position....

normal acceleration from a stop sign the Hg drops to about ten, and the pcv is closed, while the carb feeds a rich mix to make acceleration power, and as the Hg gradually recovers with more rpms the pcv gradually opens...

"if" there is major blow by causing crank case pressure, and you use breathers only,,,you are blocking the oil return holes in the heads to the crankcase which is where and how the air does escape from the block...
oil can't flow down to the pan with air flowing up the same "pipe" well at all...

not street legal,,,, but race motors are often configured to pull a neg crank case pressure using the exhaust flow cfm as a siphon...
sealed valve covers with pipes or hose connected to bungs on the exhaust pipes...
the fast moving exhaust at high rpms does pull signifigant vacuum on the pipe

on a strong street motor a better choice is mount a vacuum pump which pulls from the crankcase direct and helps the oil flow...

Last edited by red65mustang; 01-25-2010 at 04:58 PM.
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  #34 (permalink)  
Old 01-25-2010, 08:03 AM
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Quote:
Originally Posted by Soul_Hunter
keiser, we aren't arguing, we're having a discussion.

Intense,
a pcv valve is a calibrated, variable vacuum leak with integrated backfire protection which also seals the crankcase off from the intake under boost on forced induction motors.
how is that for a description of the PCV valve?

from one of the first classes i took years ago, "the PCV system is an emissions component, its job is to vent crankcase vapors into the intake to be burned instead of venting them to the atmosphere"
before the PCV valve, engines used road draft tubes which vented to the atmosphere.
before the road draft tube, there was just a vent tube to the atmosphere.
before the vent tube, engines were sealed to contain the blowby. yes, they leaked oil, and they didn't last very long.
some engines don't have a PCV valve, they use a fixed orifice with a vent to the air inlet.
some engines don't use a fixed orifice or a pcv valve, they use an oil/air separator thats routed to the air intake tract. these are modern engines.
it may have not been complete, but again i gave no misinformation.

here is a test you can do if you want to see just how well the pcv valve actually relieves crankcase pressure vs the vent tube.
remove the crankcase vent hose at the valve cover and plug the valve cover and leave the pcv valve and its hose completely intact to the intake, pull the dip stick and hook a vacuum/pressure gage to the dip stick tube and put the gage where you can read it. now go drive the car.
at idle and light throttle, you will see crankcase vacuum. on a good motor that has good seals and gaskets, you may even see up to full intake vacuum. you can expect to hear the motor whistle with the crankcase sealed like this, and it isn't good on the seals either.
now, if you remove the pcv valve and plug the valve cover and leave the crankcase vent hose intact, you won't see crankcase vacuum.
you also won't see crankcase pressure unless there is a flow problem with the vent side or if it has excessive blowby because of bad rings that overwhelms the vent side.

you shouldn't see any notable crankcase vacuum with a properly working PCV system.
the highest specs i can think of are on some Volvos, about 7 inches of water max IIRC. i believe they also use a restricted vent tube.
to check if the PCV valve is flowing the proper amount, find the specs for your motor and see if it matches the RPM drop.
if you'll post what you have, i may well have the specs here at home.



eric32, if they are not removed, crankcase vapors can cause acids to be formed and breaks down the oil faster.
it helps to make the vehicle cleaner because the crankcase vapors are burned in the engine instead of vented to the atmosphere. venting to the atmosphere can make up to around 20~25% of total vehicle emissions on vehicles without a cat.
for cars with a cat, i've read total vehicle emissions can increase up to 60~80% if crankcase vapors are vented to the atmosphere instead of the motor.
a wrong or defective PCV valve can pull too much from the crankcase causing excessive oil consumption.
if you have aftermarket valve covers, they could have poorly designed baffles in them. if you bought them used, the

for your honda friends, in my 30+ years of auto repair i've seen more PCV valve failures on hondas where the valve has came apart internally than all other vehicles put together.

cut away of an average PCV valve,
Much better! Thanks for seminar.
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  #35 (permalink)  
Old 01-25-2010, 08:15 AM
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Quote:
Originally Posted by red65mustang
my $.02:
you guys are not explaining a pcv valve operation "complete" enough...
(I did not carefully read all the posts but many are confusing)

yes, at high Hg no load very low idle rpms it is just barely open BECAUSE there is very little total CFM going thru the whole motor pulling against the spring and weight resistance....

at any cruise speed, with higher rpms=more total cfm, light throttle high hg condition, it is most definitely a major cfm source into the motor (pulling a vacuum in the crank case and expelling the crap),,,because there is alot more total cfm thru the motor pulling on the spring and weight...
infact the carb fuel feed curve for A/F mix is factory set to include the huge additional 3/8" line pcv air cfm supply
(that's why there are so many different pcv model units,,,match the spring rate to the cubes cfm and cruise rpms for the particular car normal cruise Hg)

there is next to no Hg (approx 2-5Hg) present WOT so the pcv doesn't pull any crankcase vacuum regardless of rpms to prevent blow-by when the pedal is to the floor....

when you do back off the throttle the Hg goes to max possible (approx 25Hg) as the motor winds down and the pcv is totally open during that period to relieve any pressure ASAP and suck out the crap ASAP....

normal acceleration from a stop sign the Hg drops to about ten, and the pcv is closed, while the carb feeds a rich mix to make acceleration power, and as the Hg gradually recovers with more rpms the pcv gradually opens...

"if" there is major blow by causing crank case pressure, and you use breathers only,,,you are blocking the oil return holes in the heads to the crankcase which is where and how the air does escape from the block...
oil can't flow down to the pan with air flowing up the same "pipe" well at all...

not street legal,,,, but race motors are often configured to pull a neg crank case pressure using the exhaust flow cfm as a siphon...
sealed valve covers with pipes or hose connected to bungs on the exhaust pipes...
the fast moving exhaust at high rpms does pull signifigant vacuum on the pipe

on a strong street motor a better choice is mount a vacuum pump which pulls from the crankcase direct and helps the oil flow...
We had this problem on a dry sump Hot Street engine. We were pulling the vacuum from one valve cover with a pop off valve on the other. Well, this motor had a sealed cam tunnel with ~5/16" drain back tubes. Took a day or so before we figured out what was going on. Almost forgot about the engine being sealed top to bottom..except for these drain backs past the crank. We ended up welding a new fitting onto the pan to pull the vacuum there. It started with the engine running well all season, but unusually low oil pressure. We found out what was wrong when they sent it to us!
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  #36 (permalink)  
Old 01-25-2010, 09:56 AM
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Honestly everyone here has some bits and pieces that are correct about the operation of a PCV valve and how it works, and there's also bits and pieces that are incorrect. I found a Perfect explanation in my ASE Certification Books but with no Digital Camera no way to post a picture of it here. So I went and searched the internet to find something everyone can use that Explains the Correct operation and purpose of a PCV System.

This is by far the most Detailed Explanation I've found, even more so then my Automotive Books and ASE Certification books. This information is provided by Toyota Motor Co. and Has Tons of Fully detailed Pictures and explains the operation to the T of a PCV valve and shows how it operates. Hope this helps everyone!

http://www.autoshop101.com/forms/h63.pdf

Being a hardcore Ford and Chevy guy I know its disappointing it was made by Toyota but hey, I have to give them credit for a nicely laid out piece of information. Also in some ways this information is better then the Explanation my Mercedes Benz books gave.
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  #37 (permalink)  
Old 01-25-2010, 10:25 AM
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smoke,
thanks for posting the article link

it pointed out a (dumb) error I made in my post which I corrected...
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Old 01-25-2010, 10:59 AM
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Quote:
Originally Posted by red65mustang
smoke,
thanks for posting the article link

it pointed out a (dumb) error I made in my post which I corrected...

No problem man, I hope everyone else gets a chance to look at this
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  #39 (permalink)  
Old 01-25-2010, 12:32 PM
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Good link. I got an A I think. Common sense would lead you to conclude the valve should, again, should be halfway area of travel in cruise. My main concern right now is getting a slight vacuum on the crankcase at idle. I'm going to run down to the dealer and get an oem pcv and see if this corrects the situation with the Purolator brand pcv.

My engine has a wide operating range due to the stock stall limit right now. If I can pull some vacuum it will help with my slight rear main leak and get a "head start" on crankcase pressure. I hope to better ring seal. Don't get me wrong, the engine has 65k and is in great condition. Looking to squeeze more out of it. Looking for high 12's with stock shortblock including cam, converter, gears at almost 4k pounds. I'm getting closer, corrected et is 13.4-13.5.
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  #40 (permalink)  
Old 01-25-2010, 01:46 PM
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to get negative pressure at idle you need to restrict the inlet flow total cfm severely (dip stick and breather) and find a weaker rated spring pcv

and it will take awhile to pull enough air out of the crankase to have a negative pressure....

the main seal leak is because the crank is literally throwing the oil with force at the seal....

always glad to learn so do post your results...

Last edited by red65mustang; 01-25-2010 at 01:56 PM.
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  #41 (permalink)  
Old 01-25-2010, 02:30 PM
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Quote:
Originally Posted by red65mustang
to get negative pressure at idle you need to restrict the inlet flow total cfm severely (dip stick and breather) and find a weaker rated spring pcv

and it will take awhile to pull enough air out of the crankase to have a negative pressure....

the main seal leak is because the crank is literally throwing the oil with force at the seal....

always glad to learn so do post your results...
Can't do it by restricting the fresh air side because I want the gases to be able to go back up, if needed, just as it was designed to work if there is excessive blowby. Everything is sealed, one piece gaskets, Right Stuff, including factory o ringed dipstick. I think a pop off valve like used with a vacuum pump or dry sump system may work.

A stronger, not a weaker spring would help more at idle. With no volume, pcv closed, there would be no negative pressure. That would be working in the right direction, OR a larger orifice allowing more air to "leak" past while the pcv valve is up(against spring) and closed.

Yes, if I don't have enough volume pulled out pcv valve to intake it could take a little bit of time. Never seen it first hand, but it can't be that long. Open an air compressor with a 3/8 hole and it decompresses quite quickly. at 14 psi which would be 1" Hg of mercury or vacuum. I'd be happy to have that.
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  #42 (permalink)  
Old 01-25-2010, 02:36 PM
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The main seal is leaking because A it's defective, B it's improperly installed C it's a Ford and needs .001 under bearings.
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  #43 (permalink)  
Old 01-25-2010, 03:56 PM
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typical oriface opening is approx 1/4" diameter,,,there are some with close to 3/8" diameter,,,problem is the larger diameter has a bigger and heavier piston!!

to work at idle you need a very light piston so it will lift easier and/or a weak spring so there is less lift resistance on the steel piston with the available suction cfm which ain't alot at idle...
the carb venturi/throttle blade larger total "oriface" area is a easier path for the air to flow with the available suction from the slow moving pistons...

how long will it take to get to signifigant negative crank case pressure would be about like how long it takes to fill a 5 gallon bucket with only a 1/4 diameter water hose...

they are cheap enough at the chain parts stores to buy several different ones and tear them apart to make your own "combo" with a piece of aluminum tubing to experiment with????

Last edited by red65mustang; 01-25-2010 at 05:09 PM.
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  #44 (permalink)  
Old 01-25-2010, 05:07 PM
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dang CRS!!!

forgot to post:
the suction volume limiter "bottle neck" is usually the 3/8" hose nipple on the carb,,,that tube I.D. is only approx 1/4" actual....
so a bigger oriface hole won't help....

Last edited by red65mustang; 01-25-2010 at 05:14 PM.
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Old 01-25-2010, 07:06 PM
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Quote:
Originally Posted by enjenjo
The main seal is leaking because A it's defective, B it's improperly installed C it's a Ford and needs .001 under bearings.
No...didn't start until I switched from syn to semi syn to save a couple bucks, put stock 192* thermostat back in, swapped to 4bbl M1 single from 2bbl single plane. Didn't check manifold vacuum before the swap but I have 18" at the plenum right now. It's a 99 Dodge with 5.9 Magnum.
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