On  2 Oct 97 at 8:21, Oyvind Kaurstad wrote:

> >> >These, as do most "economy gauges" use a pressure sensor on the
> >> >vacuum line of the engine. The harder the engine work the higher the
> >> >vacuum. Very imperic.
>
> > -Oyvind replied
> > >
> >> Do you mean "less pressure" when you say "higher vacuum"?
> >> If so, you are mistaken.
> >>
> >> If the engine works harder you get MORE pressure in the intake
> >> manifold.
> >>
> >> Just think of it, when you step on the loud pedal you open the air
> >> intake valve to allow more air into the engine. More air -> Higher
 pressure.
>
> >Faster airflow -> lower pressure ! In the manifold that is.
>
> Consider this situation:
>
> A regular carbureted engine is driving uphill at a moderate speed
> (constant) and with a medium throttle setting.
>
> In the intake manifold we have, say 0.7 bar.
>
> Then the driver open up the intake valve by applying full throttle.
> The pressure in the manifold now increase to, say 0.9 bar.
>
> Why?
>
> This happens because the pistons are moving with the same speed as
> before the throttle was applied, but the air intake is now wide
> open.
>
> If you have ever looked at a regular vacuum economy gauge, you
> must have noticed that it will show "good economy" when running at
> idle, but if you stop the engine it will show "bad economy"..
>
> There is no doubt that the pressure in the intake manifold is 1 bar
> with the engine stopped..?
>
> There is also no doubt that the pressure is less than 1 bar when
> running at idle?
>
> Still disagree?
>

The so called 'Economy Gauges' are a total waste of time - they flop
backwards and forwards crazily.  The tendency of car makers to omit
RPM gauges in favour of theses devices is idiotic - a car computer
might make sense of manifold pressure, but its useless info to a
person.
MikeS
<mikesmith_ozNOSP*M.relaymail.net>
(remove the you know what before replying)