On Aug 28, 12:28*am, jim beam wrote:
Frank Krygowski wrote:
On Aug 25, 11:38 pm, jim beam wrote:
Frank Krygowski wrote:

IOW, the entire
intake path is below atmospheric pressure.
er, that's because of the carburettor restriction.

No, "jim," it's because the pistons are moving downward while the
intake valve is open. *The expansion in the volume of the cylinder
plus intake path (manifold, carburetor/throttle body/etc, air cleaner,
etc.) causes a drop in pressure.

south of the valve restriction, that's true. *but since the manifold is
bigger than the valve hole restriction[s], the vacuum in the manifold is
due to throttle/carb venturi and the flow rate, not "vacuum from the
piston".

Wow. I thought you were a _little_ better at physics. Guess I was
wrong.


*Everything downstream from the air
cleaner's intake horn is below atmospheric pressure. *That's what
sucks the air into the engine.

you'll have a hard time measuring any pressure drop on the far side of
the throttle krygowski. *

Your sniping would be more understandable if you would define your
terms better. By "far side" did you mean "upstream"?

Not that it matters much, of course. Almost everything you're writing
shows tremendous ignorance of fluid mechanics.


First, if your venturi is venting directly to atmospheric pressure,
you will get the same effect, but only because the pressure
immediately downstream of the throat is almost exactly atmospheric.
If that pressure is sufficiently raised in any way (say, by a
restriction) the venturi throat will be above atmospheric pressure.
(See the robot gripper example below.)

Bernoulli's equation is a conservation of energy equation for fluid
traveling along a particular streamline. *For solid objects like
swinging pendulum bobs, conservation of energy makes potential energy
transform into kinetic energy and vice-versa. *For fluids in motion,
the same principle makes an additional change: pressure changes. *But
as with a swinging weight, the changes apply to the particular
particles of fluid that are flowing, not to the ones outside the pipe.

The reduced cross section area of a venturi's throat causes higher
speed (more kinetic energy) in that section of the fluid stream. *This
is balanced by having less "pressure energy" (so to speak) at the
throat than immediately upstream or downstream.

But is the throat's pressure guaranteed to be below atmospheric?
Definitely not.

but it /is/ guaranteed to be below input pressure.

Correct, but not pertinent.

If the pressure downstream of the venturi is, say,
100 psi, the venturi throat might be at 90 psi above atmospheric.
Drilling a hole into the throat would result in fluid spraying out
into the atmosphere.

This will never happen with a carburetor because the entire intake
passage is below atmosphric pressure.

no it's not - only the section south of the venturi is below atmosphere.

Absolutely, completely false.

The flow of air in the intake passages of an engine is caused by
pressure drop. The more restriction in a certain location, the larger
the pressure drop required across that restriction.

Air first enters the "air horn" portion of the air cleaner because
pressure within the air horn is lower than atmospheric. Yes, even in
the air horn, pressure is lower than atmospheric.

Air flows through the filter element of the air cleaner because the
pressure inside the air cleaner (downstream of the filter) is lower
than the pressure in the air horn.

This explanation continues all the way to the intake valve, where the
pressure inside the cylinder is lower than the pressure just before
the valve.

This is true even in a diesel engine. Its intake passages are below
atmospheric even though there is no venturi.

The only important exception is an engine which is supercharged or
turbocharged. But even there, pressure in the intake of the 'charger
is lower than atmospheric. The 'charger pumps it up in order to
increase the flow of air and fuel; but from there on, the pressure
gradient is downhill, except for the two-inch expansion of the venturi
(where fitted). In that section, flow is maintained by inertia, not
pressure drop.

So once again: Supercharging or turbocharging excepted, the entire
intake passage, going all the way upstream to the air horn, is below
atmospheric pressure. Otherwise flow would not occur.

Once again, "jim," if you restricted your posts to what you actually
know about, your posting volume would drop by over 90%.

- Frank Krygowski