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Old 03-21-2007, 05:23 PM   #41
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Brian, if you tap into the pipe that's between your airflow meter and compressor inlet you will not get vacuum. The only place on gasoline engine that has full time vacuum is the area behind the throttle plate. If it's in front of the throttle body/plate you will NOT see vacuum. If you don't believe me, hook up a vacuum gauge in the different spots and see what your readings are.

Any place you have vacuum at idle, WILL have boost or positive pressure from the turbo at higher RPMs.

Remember that vacuum is caused by the downward stroke of the piston and the throttle plate being partly closed. For a example, a diesel engine doesn't have vacuum because they don't have a throttle plate or butterfly. Their RPMs are controlled by the injection amount of the fuel.

Braineack, it was not my intention to belittle all that you have put forth in the forums here. I have read through them and found very good information.

Brian, same with you man, no disrespect meant.

Tony
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Old 03-21-2007, 05:43 PM   #42
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Very interesting discussion and I think I understand except for the whole catch can thing.
Is the catch can there just to insure that you do not somehow get oil or oily fumes into the turbo and thus get smoke, or is there some other reason??????

I could assume that as the engine ages more blow-by will result and under the correct conditions such as PCV closed (with a little boost) but some pressure in crankcase from the blow-by, it would go directly into the turbo and intake and produce some smoke.
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Old 03-21-2007, 06:02 PM   #43
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and please by all means disregard my FAQ and all the help I've ever given in the past, I have no idea what I'm talking about. I'm out to get you all.
Please Brian, don't take it so personal. Just because you have helped a lot of people (I've even read your FAQ, it's good) doesn't mean you are infallible. I've posted links supporting my position (the factory diagrams, not the Honda idiot speak). If you have material to support your idea, I'm all ears.
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Old 03-21-2007, 09:31 PM   #44
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Brian = Brian
Brain = Scott



So, somebody 'splain this to me: if there is boost or pressure present in the pre-compressor inlet tube, how could the AFM measure airflow and, for that matter, how would a turbocharger fundamentally be able to work?

While I agree that there can be a vacuum pulled just behind the throttle butterfly on a turbocharged car at low-load, small throttle openings (while there's also a vacuum in the intake manifold), how could there be vacuum there when the turbocharger is producing boost (in the intake manifold)?

Finally, Tony, why would you ever want to pull air into the crankcase to create pressure? This pressure is what causes the turbo to not drain effectively, causes excessive blow-by past the piston rings and robs power.
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Old 03-21-2007, 09:36 PM   #45
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Brian = Brian
Brain = Scott
and Scott = Subway Jared :gay:
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Old 03-21-2007, 10:01 PM   #46
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you guys are just silly
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Old 03-21-2007, 10:03 PM   #47
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Brian = Brian
Brain = Scott
I knew that somewhere in my head. From now on it will just be Briain.

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So, somebody 'splain this to me: if there is boost or pressure present in the pre-compressor inlet tube, how could the AFM measure airflow and, for that matter, how would a turbocharger fundamentally be able to work?
I'm not sure if anyone is supporting that idea. Well, I guess I do because without the king of all pressure (atmospheric pressure), none of this works anyway!

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While I agree that there can be a vacuum pulled just behind the throttle butterfly on a turbocharged car at low-load, small throttle openings (while there's also a vacuum in the intake manifold), how could there be vacuum there when the turbocharger is producing boost (in the intake manifold)?
There can't be. Who said that?

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Finally, Tony, why would you ever want to pull air into the crankcase to create pressure? This pressure is what causes the turbo to not drain effectively, causes excessive blow-by past the piston rings and robs power.
I'm not Tony, but no one said we want to build pressure in the crankcase! Just because fresh air is entering the crankcase doesn't mean any pressure is building.
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Old 03-21-2007, 11:19 PM   #48
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maybe mcshlang means you wont have MUCH vacuum before the turbo. It's not 20 inches at all locations pre-turbo and certainly not more than in the intake manifold.

case in point: what if you take off your intake and run an open turbo inlet? can you just leave your vent side of your pcv within a foot of it?

lets see... pressure differential is going to be such that you're always removing air from the crank case, right?
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Old 03-21-2007, 11:58 PM   #49
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Properly piped, you can theoretically reach atmospheric pressure only momentarily in the crankcase as the PCV closes and the venturi effect has not yet taken hold from the opposite side of the valve cover plate into the intake tract on a NA Miata.

On a turbocharged Miata, properly piped, you cannot reach atmospheric pressure in the crankcase since as boost is seen at the PCV there is already a below atmosphere condition between the AFM and turbo inlet caused by the increased flow of air into the turbo acting against the restriction of the air filter and AFM.

Sorry, but FWIW, Brian and Braineack have it right.
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Old 03-22-2007, 12:00 AM   #50
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Okay, I probably got a couple things Mike or Tony said mixed up with one another.

Tony said, "Braineack, if you are indeed using vacuum on the vent side, what do you think happens under boost? You would be pressurizing your crankcase...not the way to make power!!"
Tony: We're using vacuum on the vent side to make sure no pressure builds up in the crankcase and robs power/hurts turbo oil drainage. Some pressure in the combustion chamber will leak by pistons and, without a PCV vent, you would witness the two, aforementioned problems. Some people get away with merely a filter on that vent, and I would say they have good piston-ring-to-wall-seals and turbos that drain easily. Others (like, ya know, every builder of every aftermarket forced induction kit you see on the market for Miatas) make provisions for running that vent to a point in front of the compressor (be it supercharger or turbocharger) that will see a modicum of vacuum-ish flow.

Mike and Tony: You both seem to think there's no vacuum in front of a compressor inlet. While I'm not saying there's a full 30" of vacuum in the spot where the PCV vent would connect, there is certainly some, otherwise far more people with correctly-spliced PCV vents would be experienced smoking and blown turbo seals. I've never measured it, but it's gotta be less than one atmosphere...maybe in the ~1-15" range?
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Old 03-22-2007, 12:45 AM   #51
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Brian, there are two reasons for running a line from the vent or driver's side tube to the point between the compressor and the air flow meter. One is that you are using metered air that has gone through the airflow meter. And if running an airflow meter, the PCV function at idle can interrupt the proper fuel trims and cause lean running at idle.

The other and less important one is that you are using filtered air without using a second filter on the vent line since it has gone past your main air filter.

Trust me, or better yet measure the vacuum at the different points and you will see.

The only thing you will measure in front of the compressor is atmospheric pressure.


Brian said "Tony: We're using vacuum on the vent side to make sure no pressure builds up in the crankcase and robs power/hurts turbo oil drainage."

Well, yes of course, that's the job of the PCV valve. At idle the PCV valve sucks fumes in to the intake manifold to burn in the combustion chamber. That's your source of vacuum in the crankcase.

Anyways, I don't want to beat this thing further into the ground.

Thank you all very much for being very civil...I like this board!

Tony
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Old 03-22-2007, 12:49 AM   #52
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Brian, where in Florida do you live?


Tony
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Old 03-22-2007, 01:35 AM   #53
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Trust me, or better yet measure the vacuum at the different points and you will see.

The only thing you will measure in front of the compressor is atmospheric pressure.
Im confused by the vacuum at different points part. where on the intake manifold are you going to see any vacuum under boost?

Am I misunderstanding something obvious?
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Old 03-22-2007, 02:01 AM   #54
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"Im confused by the vacuum at different points part. where on the intake manifold are you going to see any vacuum under boost?"

Nowhere, you don't see vacuum under boost. You only see vacuum at idle and part throttle because that's when the throttle plates are closed or causing a restriction.

Boost pressure replaces manifold vacuum under wide-open throttle (WOT) in a force induction engine. If it's a normally aspirated engine then you would have zero to very little vacuum at WOT.

Tony
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Old 03-22-2007, 10:34 AM   #55
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Originally Posted by mrtonyg View Post

The only thing you will measure in front of the compressor is atmospheric pressure.

Tony
When plotting a compressor map there are provisions in the equations to account for the pressure drop caused by the filter pre-compressor. It modifies the value used for atmospheric pressure beyond the standard relationship to sea level number. "Corrected compressor inlet pressure" if I remember correctly.
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Old 03-22-2007, 11:12 AM   #56
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Originally Posted by mrtonyg View Post

Well, yes of course, that's the job of the PCV valve. At idle the PCV valve sucks fumes in to the intake manifold to burn in the combustion chamber. That's your source of vacuum in the crankcase.

Anyways, I don't want to beat this thing further into the ground.

let's beat it more.

the pcv valve is open at idle, since there is 20 ~hg. of vacuum in the intake manifold.

throw boost into the equation, the pcv fully closes and seals off the crankcase from that side, preventing boost from entering or vacuum escaping.

so we have the "breather" on the other side. Allowing a passage of air in/out of the crankcase while the engine is at or above atmospheric.

if the breather is left open to no source, what happens? nothing, there is no boost/vaccum in the crackcase.

hook that same breather line to your turbo intake, and you will pull air from both your intake filter and crankcase line.

this will create a small instance of vacuum in the crankcase, helping seal rings and pull the oil of the the turbo drain line. And yes, all at the same time keep all your air metered.
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Old 03-22-2007, 05:45 PM   #57
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"Im confused by the vacuum at different points part. where on the intake manifold are you going to see any vacuum under boost?"

Nowhere, you don't see vacuum under boost. You only see vacuum at idle and part throttle because that's when the throttle plates are closed or causing a restriction.

Boost pressure replaces manifold vacuum under wide-open throttle (WOT) in a force induction engine. If it's a normally aspirated engine then you would have zero to very little vacuum at WOT.

Tony
so wouldnt the pcv be closed under boost, then the other vent needs only to be lower pressure than the crankcase to vent. thus venting the crankcase vacuum or not. I dont think its a matter of amount of vacuum its a matter or pressure difference, correct?

highs flow to lows, crankcase flows into the intake (between afm/turbo) under boost and into the intake manifold at idle... sounds right to me.
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Old 03-22-2007, 06:44 PM   #58
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I'm glad someone sees the light
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Old 03-22-2007, 10:49 PM   #59
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If you have such a low pressure in front of the compressor housing as to actually contribute to evacuate the crankcase in the way of producing a vacuum, you have some seriously restrictive air filter.

Think about that.


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Old 03-22-2007, 11:31 PM   #60
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braineack is correct
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