Here's a few observations I've made reading these posts:
The analogy of putting a vacuum on a container isn't accurate because (as
Jon also mentioned) it would be like putting a vacuum on your exhaust pipe
and plugging the intake to create a higher vacuum (the source of the vacuum
is at the end of the flow). Yes the clog would create a higher vacuum in
that situation however that's not recreating a clog cat accurately. Using
the analogy of a water pump sucking from a pond and kinking the hose is
exactly how a clogged cat would work where the source of the vacuum is
between the intake and exhaust.
Does a higher MAP sensor reading equate to a higher vacuum? It seems to me
that a higher MAP reading (higher pressure) should lower the vacuum. Maybe.
I don't know.
James
-----Original Message-----
From: owner-dakota-truck@bent.twistedbits.net
[mailto:owner-dakota-truck@bent.twistedbits.net] On Behalf Of Phil Jenkins
Sent: Sunday, September 23, 2007 10:24 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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