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.
>
> > The action of the
> > piston going down with both exhaust valves closed
> > would normally cause
> > a vacuum in the cylinder if we were starting out
> > from zero, but we aren't
> > starting out from zero, we started from pressure,
> > and the combustion
> > event added a lot more pressure. As the piston
> gets
> > to the bottom of
> > its stroke, I must admit that I am not sure if the
> > condition inside
> > the cylinder is still pressure, neutral, or a
> > vacuum, but I can say
> > with a good degree of certainty that whatever the
> > pressure, it is
> > higher than it was at BDC on the intake stroke.
> The
> > combustion event
> > "destroyed" some or all of the vacuum. In any
> case,
> > now the exhaust
> > valve is open, and the piston rises and pushes the
> > spent mixture out
> > into the exhaust system. At this point, this
> > mixture is pressurized.
> > It must be, because if it wasn't at a higher
> > pressure than the exhaust
> > system, it would stay in the cylinder or the air
> > from the exhaust
> > system would try to enter the combustion chamber,
> > and neither of those
> > are going to work to run an engine. :-) So,
> > following this mixture
> > then, its shoved out into the exhaust system,
> which
> > if it is not
> > plugged, allows it to make its way out to the
> > atmosphere. There will
> > be a slight pressure reading in the exhaust system
> > of perhaps 1-2psi
> > due simply to the restrictions created by the
> pipes.
>
=== message truncated ===
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