Venturi effect on air intake

[RUSHY44]

This one is for all the clever people....

I have a basic understanding of the venturi effect. I liken it to squeezing the end of a hose pipe.

My query relates to is there a specific amount of "squeeze" that you put into an air intake pipe.

Also, the positioning of the "squeeze" - I feel it would make a difference (Not to BHP I shouldn't think) as to how close to the throttle body it is.

If it makes a difference the engine in question is a (And don't laugh!!) 1.4 105BHP K-Series.

Thanks in advance

[Zico]

I doubt you will recieve an exact reply, this is in the realms of cfd (computational fluid dynamics) I'd imagine.

You may find this wiki page on manifolds of interest though, it explains venturi effect, intake resonance, (hermholz) variable length intake manifold etc.

http://en.wikipedia.org/wiki/Manifold_%28automotive%29

If my understanding is correct you may find that increasing the velocity by the venturi effect will only serve to achieve a good resonance for a specific lower rpm range as explained in the volumetric efficiency and variable length intake manifold sections in the link above.

[tristancliffe]

A lot of cars use part venturi sections (diffuser sections) to get lots of high speed air into the airbox and radiator intakes, then slow it down so that the air is better used for cooling/breathing.

Obviously carbs use the venturi effect to control fuelling.

[JohnD]

Rushy,
Squeezing the hose pipe end is NOT a Venturi effect!
That places an obstruction in the flow, so that the pressure behind the obstruction rises.
A properly constructed venturi speeds the air (or water) in a way that causes the pressure in the restriction to FALL, as the fluid accelerates. If the fuel reservoir of a carburettor is at normal atmospheric pressure then the pressure difference between that and the low pressure, fast moving air in the venturi pushes fuel out of the 'jet'.

Why do you want a lower pressure in your intake? As tristan says, you want to slow the air down around the intake, so that its pressure is a high as possible, so as to aid induction.

If you are thinking of the 'trumpets' or so-called 'ram-pipes' that are fitted to the intakes of carburettors, these are more to do with:
a/ getting a smooth airflow down the intake - the rolled edge is most important, it should include 180 degrees. The slight narrowing is part of that airflow smoothing, not a venturi.
b/ using pipe lengtht to tune the standing wave that occurs in the intake to a favourable frequency to aid induction, by increasing the pressure at the valve. Shorter the pipe, the higher the revs at which this effect occurs

John

[Red Dog]

JohnD is 100% right - you want your intake system to have the least possible restriction in it. Venturi is only relevant with carbs as it's purpose is to deliver atomised fuel into the intake tract. With fuel injection no venturi effect is required as the injector nozzle does that for you.

If you tap into various points of your intake - before filter, after filter, without filter, manifold at WOT etc you can measure the pressure drop. The air intake tract needs to taper down towards the intake valve as this accelerates the flow. The intake valve should be the most restrictive part of the intake system (we're talking at WOT here).