As I sit here and monitor the DML,I also am watching a
show about Alexander The Great,and there are distinct
parallels between the two. The fighting between The
Vacuumists and The Pressurists is fierce,yet you,as a
great commander,have stopped during the heat of battle
to assist one of your comrades who has lost his way
and knows little regarding the terrific stakes in this
conflict. To say it is appreciated is a small
token,yet it is all I have to give. Thank you
Bernd,and again I say:
Thank you,
Steve P.
--- "Bernd D. Ratsch" <bernd@dodgetrucks.org> wrote:
>
> MAP vacuum reading will be lower than actual
> manifold vacuum (normally about
> 3-5" lower).
>
> - Bernd
>
> -----Original Message-----
> From: Steve Preston [mailto:steveophonic@yahoo.com]
> Sent: Sunday, September 23, 2007 9:55 AM
> To: dakota-truck@dakota-truck.net
> Subject: RE: DML: RE: Re: Vacuum 101
>
>
> This helps me a little bit. But it creates a new
> question in my mind: should a person have the same
> reading from their MAP sensor via scanner that they
> would have from a vacuum gauge? Because I,in
> fact,did
> not get the same readings. The scanner said 10 in
> HG,and my gauge said about 19. But your subtraction
> that you came up with is making me wonder whether
> there should be a disparity?
>
> Thanks!
>
> Steve P.
> --- Phil Jenkins <bugnik@yahoo.com> wrote:
>
> >
> > Ok, I hook up my scanner with engine off. I have
> 29"
> > of barometric pressure from MAP sensor. Start
> truck,
> > at idle I have about 11" showing on the scanner.
> > 29-11=18" of vacuum. I plug the exhaust and have
> 20"
> > on the scanner. 29-20=9" of vacuum. Less vacuum
> with
> > exhaust plugged, higher reading on scanner. Does
> > that
> > sound right? (Pouring some gasoline on the fire.)
> >
> > Phil
> >
> >
> > --- "Bernd D. Ratsch" <bernd@dodgetrucks.org>
> wrote:
> >
> > >
> > > Proper wording may not be quite right and we're
> > all
> > > over thinking it now.
> > > To make it easy - grab a scanner, plug your
> > exhaust,
> > > and put a load on the
> > > engine under light throttle - watch the MAP
> > reading.
> > > Does the same thing
> > > every time.
> > >
> > > - Bernd
> > >
> > > -----Original Message-----
> > > From: jon@dakota-truck.net
> > > [mailto:jon@dakota-truck.net]
> > > Sent: Sunday, September 23, 2007 3:15 AM
> > > To: dakota-truck-moderator@bent.twistedbits.net
> > > Subject: Re: DML: RE: Re: Vacuum 101
> > >
> > >
> > > "Bernd D. Ratsch" <bernd@dodgetrucks.org> wrote:
> > >
> > > > Again, refer to the plastic bottle post.
> There
> > > are things such as
> > > positive
> > > > and negative pressure.
> > >
> > >
> > > I don't know that the bottle analogy holds in
> > > this case though
> > > since it concentrates only on the vacuum side of
> > the
> > > engine and
> > > ignores the pressure events. That is, we're not
> > > trying to suck
> > > through the exhaust, we're trying to blow
> through
> > > it. (Read on below
> > > for my more excruciatingly detailed thoughts on
> > > that...) ;-)
> > >
> > >
> > >
> > >
> > > > The pistons pulling the air into the cylinders
> > > (via the intake) does keep
> > > > vacuum in the intake stream.
> > >
> > >
> > > Yep - that's the *only* reason there is any
> > > vacuum in the engine; the
> > > pistons are pulling against the back of the
> closed
> > > butterflies at idle
> > > with only a tiny IAC passage through which air
> can
> > > pass through; its
> > > basically the same as trying to suck air through
> a
> > > straw - there is a
> > > vacuum created in your mouth. As the throttle
> > > blades open, the straw
> > > gets bigger, but there is still some vacuum in
> > your
> > > mouth until the
> > > straw gets so big that the restriction becomes
> > less
> > > than the power of
> > > your lungs.
> > >
> > >
> > >
> > >
> > > > The opening of the exhaust valves releases
> > > > that though the exhaust piping. Plug the pipe
> > and
> > > you can't release the
> > > > vacuum.
> > >
> > >
> > > I'm not sure that I buy that; or, more
> > correctly,
> > > I guess my
> > > problem with it is your considering vacuum to be
> > > something that can be
> > > "released". I'm more comfortable with the
> notion
> > > that a vacuum is a
> > > *lack* of that something; its a void. You can
> > > release pressure, but
> > > vacuum is something that you relase *into*, not
> > > something that is
> > > released itself. (Well, I guess except for
> > exposing
> > > a weak vacuum to
> > > a strong vacuum, you might think of the strong
> > > vacuum being "released"
> > > into the weak vacuum, but I'd prefer to just
> think
> > > about it as a
> > > pressure equalization. Whenever I try to think
> > > about vacuum as some
> > > entity moving from place to place, my brain
> flips
> > > out on me.) :-)
> > >
> > >
> > > Anyway, back to your point. Regardless of
> how
> > > one thinks about
> > > vacuum or pressure, I still don't think your
> > > description is correct
> > > because of the combustion event between the
> > opening
> > > of the intake and
> > > exhaust valves. Let me try to explain what my
> > > thinking is by an
> > > example. Lets consider an engine which has a
> > > certain vacuum level in
> > > the intake manifold. The intake valve opens and
> > the
> > > piston drops,
> > > which creates a vacuum in the cylinder which is
> > > greater than the vacuum
> > > in the intake manifold. This allows air (and
> > fuel)
> > > inside the intake
> > > manifold to be sucked into the cylinder (and
> > > likewise, air is being
> > > sucked through the throttle body into the intake
> > > manifold, and if the
> > > throttle body opening is not sufficiently large,
> > > this will cause a
> > > vacuum in the intake manifold). So, at this
> > point,
> > > the piston has
> > > moved to the bottom of the stroke, the intake
> > valve
> > > is closed, and we
> > > have a vacuum in the cylinder. Now, the piston
> > > moves up, and
> > > compression begins. Suddenly, this vacuum that
> we
> > > had has
> > > dissapeared, and we have pressure instead. The
> > > spark plug fires, and
> > > now there is a LOT of pressure. This pressure
> > > forces the piston back
> > > down, and as it does so, this pressure is
> reduced.
>
=== message truncated ===
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