You have 11" manifold vacuum at idle or 11" MAP vacuum? 29" BARO is normal
though. What is the 20" you're talking about?
- Bernd
-----Original Message-----
From: Phil Jenkins [mailto:bugnik@yahoo.com]
Sent: Sunday, September 23, 2007 9:24 AM
To: dakota-truck@dakota-truck.net
Subject: RE: DML: RE: Re: Vacuum 101
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.
>
>
> Ok, given the above, now lets throw a clogged cat
> into the
> equation. Suddenly, this low pressure in the
> exhaust rises because
> the pistons are cramming all sorts of air (exhaust
> gasses) into the
> exhaust system, but it just can't flow through the
> cat fast enough and
> it is building up, just like putting a kink in a
> garden hose. This
> means that the pressure just outside the exhaust
> valve is higher than
> before, and less of the mixture inside the cylinder
> is going to get
> pushed out into the exhaust system. This means that
> compared to
> before, as the cylinder is ready to open the intake
> valve and suck in
> a fresh charge, the pressure in the cylinder is
> slightly higher than
> before. The intake valve opens, and the piston
> drops to create a
> vacuum and suck air and gas into the cylinder, but
> because it is
> starting from a slightly higher pressure than
> before, it cannot create
> as large a vacuum as before, and less fuel/air
> charge is sucked into
> the cylinder. This means that some of the air
> inside the intake
> manifold which would normally have been sucked into
> the cylinder has
> stayed inside the intake manifold, and this creates
> a
=== message truncated ===
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