bripab007

Maybe that's true; I don't quite know how low a pressure is needed to evacuate the crankcase to the point of creating vacuum in the crankcase. It does help evacuate the pressure build-up in the crankcase, however, so we shouldn't even be concerned with the precise amount of vacuum/low-pressure between the compressor inlet and the AFM/air filter because the end result is the important piece of the puzzle.

I don't believe I've said in this discussion that there's enough vacuum pre-compressor to be able to pull a vacuum reading in the crankcase; I've merely stated that you're using the vacuum to rid the crankcase of pressure build-up. On a turbocharged car, you've just gotta do everything possible to help that pressure escape.

And, yes, Tony, I neglected to mention the other important reason for connecting the vent line post-AFM and pre-turbo, and that's because the crankcase air has also been metered. Thank-you for pointing that out

VRTSid

it doesnt have to be a vacuum to flow, just lower pressure.

and if you have a compressor, to compress it need to suck air in, thus creating lower pressure at the inlet than the surrounding air. if the crankcase pressure is *higher* than the pressure in front of the turbo, it will flow there... and if at idle the pressure on the intake manifold side is lower than the crankcase the PCV opens and then flows there.

left field

iTrader: (1)

no pressure is needed to vent crank case. it will vent its self. in olden days cars vented to atmosphere. only reason a pcv exists is environmental and less odor.

mrtonyg

iTrader: (1)

Brian this is a quote from you (13th post)

"No, "fresh air" would never enter the driver's side vent, as it is intended to be connected to a source of constant vacuum (you can see this on any stock car, including the Miata)."

"connected to a source of constant vacuum" that is simply untrue, and the reason why I posted on here.

This is not my "opinion", but a fact. Pick up any BOOK on the subject of automobile repair and read the section of PCV systems.

VRTSid

because the pressure inside the case is theoretically higher than the surrounding air.

at sea level the air pressure is ~14psi (right?). that means the air pressure in the crankcase would have to be higher than 14psi in order to "vent". if the air pressure in the crankcase is not, it needs pressure lower than itself in order to flow. so its hooked to a *lower* pressure source, since it is self contained its only affected by the pressures within the system, so it works under most conditions. the rate at which it "vents" would be in direct correlation to the pressure difference... the closer the pressures are the slower it would go, but I seriously doubt that they would ever be even as to stop flow completely because as long as the turbine is functioning (properly) the air pressure going into the system will always be higher than the air pressure at the inlet. and at idle the PCV would be open.

I think the term "vacuum" is the issue of confusion on this whole topic.

PAT!

iTrader: (1)

John Carmack picked up something close to 300whp by removing the air filter assembly on his turbo Ferrari.

Even without a filter you will see vacuum pre-compressor on a turbo car.

bripab007

Tony, you're right in that I should've been more clear in my description. A "constant-source of low-pressure" would be more accurate wording to use. However, depending on how restrictive the flapper-door AFM and air filter are, it could still be a source of low vacuum, even at higher-loads/throttle openings.

mrtonyg

iTrader: (1)

Brian, sounds good now buddy!

Alright, I just sat here in my desk and performed a silly little experiment:

Low pressure vs vacuum can best be illustrated by sucking in through your mouth (low pressure or pressure differential but no vacuum) and then holding a finger closer and closer to your mouth (building vacuum) until it's shut (full vacuum).


Pat!, you are interchanging the words "vacuum" and "low pressure". Like I said the only way you have vacuum on the front of the compressor wheel is if you have a restrictive air inlet. In fact, that is an old racer's test to find out if you have a properly sized carburetor. Wide open throttle (WOT) should see zero vacuum on a vacuum gauge. If you have anything over that, there is a restriction on the air inlet (air filter and such). Same goes for forced induction cars. See my little experiment above.

On that Ferrari...how much horsepower was it putting out? I guess a chain is only as strong as it's weakest link!







Tony

PAT!

iTrader: (1)

I fail to see the difference. Vacuum = pressure differential from the point of reference. In this case atmospheric pressure, therefore any space, entirely sealed or not, containing a lower pressure than that of the surrounding atmosphere can be said to be or contain vacuum.

The distinction that you make seems to be between true vacuum, complete lack of pressure, and the measured pressure differential between two referents. If that is the definition that you use than there is no point in an internal combustion motor from which you can pull vacuum, as there is ALWAYS a flow of matter, in this case "air" or Earth's atmosphere, into the motor and as such true vacuum cannot be present. It could be argued that once the throttle plate is closed, and provided that there is no bleed orifice in the plate or throttle body for idle air ingress and that the injectors are not fired, that true vacuum could be reached during engine braking. But in reality unburnt gases would be sucked from the combustion chamber when the intake valves open by the pressure differential in the intake plenum. It would take an engine that runs at 100% efficiency to escape this. Any measurement of complete and total vacuum would be a result of test equipment that lacks the sensitivity required to measure this slight pressure...


I think. I could be entirely wrong. What do you guys think? But keep in mind that a true vacuum is considered almost a philosophical concept and that according to quantum mechanics no volume of space has reached that state


In the 700s before and 1020 after.

mrtonyg

iTrader: (1)

Pat!, I am not sure what you mean, but:

Another great example of the vacuum vs. pressure differential is the diesel engine. Diesel engines don't produce vacuum because they don't have a throttle plate. Their RPMs are controlled by the injectors. The engine obviously injests air (low pressure created by the down stroke of the pistons) but again, doesn't produce any measurable vacuum.


Tony

PAT!

iTrader: (1)

vacuum = pressure differential lower than the reference point.

"Measurable" is the operative word. You move gases in two ways with a pump; you push and create pressure above atmospheric, or you pull and create pressure below atmospheric. This second is vacuum, regardless of how slight. The greater the restriction the greater the positive pressure or the vacuum.

Take a vacuum-cleaner. Measure the amount of vacuum produced at the nozzle. Now increase the diameter of the nozzle. What happens? The amount of vacuum drops. Now take the motor (pump) out of the cleaner and run it. What now? No vacuum... well actually there will be but you may have to sample inside of the motor to measure it. This is what is occurring with a diesel engine.

You have to remember when dealing with vacuum outside of a sealed container that it will be a gradient. With the highest point of vacuum nearest the pump. When dealing with positive pressure the situation is reversed, the highest point will be at the area of greatest restriction.


Basically it is physically impossible to suck a gas from one spot to another without producing vacuum. It just cannot happen. Any pressure lower than your referent, in this case atmospheric pressure, no matter how slight, even if it is immeasurably slight is still a vacuum. If you can't find it either your equipment isn't sensitive enough or you are looking in the wrong place.

bripab007

Hmmm...okay, all semantical debates aside, can we simply agree that, in 99% of Miata forced induction applications, you want the driver's-side PCV vent to be attached to a point which will certainly never see any boost, but also see a constant source of low-pressure and/or vacuum so as to help the oil drain out of the turbo cartridge, re-burn blow-by gases, create a better seal for piston rings and keep metered air metered?

Great, thanks, let's move on

mrtonyg

iTrader: (1)

Pat! stated: "Take a vacuum-cleaner. Measure the amount of vacuum produced at the nozzle."
Guaranteed you will NOT measure any vacuum at the nozzle unless you have a restriction. At that point all you have is a source of low pressure.

PAT!

iTrader: (1)

mrtonyg, it is you against the entire scientific community in the world.

Extending your logic there is no cold only sources of lower heat.

I'm just bored at work

mrtonyg

iTrader: (1)

"Extending your logic there is no cold only sources of lower heat"

Isn't that the scientific definition!?


Go ahead...turn on your vacuum cleaner and measure! Do the experiment.

Tony

magnamx-5

iTrader: (7)

Logic never asked to be well liked just respected tony some people will always be contrary. If this didnt hold true then we would never learn anything new.

y8s

iTrader: (8)

jesus you guys. you will measure a lower absolute pressure at the inlet of the vacuum cleaner.

Bernoulli. change in pressure <=> change in velocty.

as soon as the air starts to move into the vacuum cleaner, the pressure will drop.

PAT!

iTrader: (1)

You've just proved me right, mate. Lower pressure is the definition of vacuum.
Less heat than your referent is cold. Less pressure than your referent is vacuum.

Don't own a vacuum, I'm a bachelor with hardwood floors


Wait a minute, I've got a shopvac here at work...
Well what do you know? My HKS mechanical is showing -.05 kpa of vacuum measured at the rectangular edging attachment. Not a whole lot of vacuum but its there.

mrtonyg

iTrader: (1)

I redid the test and came up with just a hint of movement from the gauge, but it wasn't enough to get an accurate reading.

I was wrong on that point.


Tony

olderguy

iTrader: (19)

Now that we all seem to agree on what, but not why; an airplane is sitting on a magic conveyor on a runway that can match it's speed............