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The Venturi effect

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David Vizard
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The Venturi effect

Post by David Vizard » Sat May 30, 2020 6:40 am

I've heard quite a few times of head porters claiming to be using a 'Venturi effect' to improve the flow capability of a port. And so they may be but for the most part you can bet it's just the use of fancy sounding terminology to impress a would-be customer . The reality is this - if you ask them what is the 'Venturi effect' they will more than likely be stumped. This doesn't mean that the Venturi effect doesn't exist it's just that the reality is it's little understood.

So let us just set the record straight here and define the Venturi effect. The Venturi effect is what we used to draw fuel out of the float bowl of our carburetors. As the air speeds up at the minor diameter of the Venturi the velocity goes up and the pressure at the minor diameter drops. It's this drop in air pressure that allows us to draw fuel from the float bowl of the carburetor. Now let's look at what happens in the Venturi as air departs from the minor diameter. As it does so the pressure increases and if we had 100% efficient Venturi the pressure would return right back to what it was just as it entered the Venturi. In the real world nothing is ever 100% although it Venturi can get really close to it. If we look at the geometry of the valve seat and port we find that it is possible to have a Venturi effect occur at three distinct points in the lift curve of the valve.

Because of the way we measure airflow are typical cylinder head flow bench we find that when Venturi type flow is in effect the efficiency with which air moves past the valve or through the port the flow is higher than would normally be the case. So why is this so? A gain let's move to a simple Venturi and perhaps all will be clear.

If we were to flow a simple unobstructed Venturi on our flow bench we would find that at a pressure drop of 28 inches across the Venturi that it is from the atmosphere to the inside of the flow bench the flow looks extraordinarily high. Far higher in fact than the hundred percent mark that we might expect to be the limit. So why is this so? Well in simple terms this is how it all comes about. When the air flows into the Venturi and speeds up at the minor diameter the pressure of the minor diameter drops. Although we may have 28 inches across the entire Venturi the air it it's coming into the Venturi doesn't see the 28 inches pressure drop what it sees is the pressure drop that occurs at the minor diameter of the Venturi. Because there is a pressure recovery after the minor diameter due to the velocity dropping, we find that the Venturi appears to be flowing about 180% of what it should do. So, is this an error in our science? The answer to that is – NO - if we were to put a pressure tapping at the minor diameter of the Venturi and step up the bench depression until we read 28 inches at the minor diameter we would find that the airflow would be considerably less and the Venturi would measure out with some efficiency figure of around 95%.

So, what's all this got to do with intake and exhaust ports as we can't really shape them to make Venturi's – or can we? Let's look at the gap between the seat of the valve and the seat in the head at low lift. Here we find that the shape of the gap is convergent, then parallel, and then divergent thus forming a mini Venturi. As such, although crude in form, it does nonetheless act as a Venturi. If we measure the airflow through a valve by the actual gap between the two seats, we find that the discharge coefficient can easily reach as much is 120%. However, if we measure it by the approved SAE method of using valve lift to generate a curtain area through which we assume the air must travel we find that it's only typically about 70% efficient. That said we need to chase this figure because we can just as well screw it up as we can enhance it.

By the time either the intake or exhaust valve has reached about 0.075 inches lift this Venturi effect tends to disappear (that is assuming it was there in the first place as it doesn't take that much to destroy whatever Venturi effect may have been possible). At somewhere around about 0.125 – 0.150 inches lift we can find that there is an opportunity to develop a port having a secondary Venturi effect which can, under ideal circumstances, persist up to as much is 0.250 inches lift. These ‘Venturi design’ ports benefit from having higher flow rates while the Venturi action is in effect. At this point one could ask if this is the end of the gravy train from Venturi ports – the answer to that is no. Good cup car exhaust ports that I've had across my bench have shown high lift SAE flow efficiency figures well over 100%. Now if that doesn't look like magic then nothing does. However, there is no such thing as magic. These superefficient exhaust ports have such a steep updraft angle and are formed such as to be able to generate a nozzling effect. Maybe it would be a bit more understandable if I said that the exhaust port is acting like a crummy Venturi nozzle in as much as it is good enough to exceed 100% efficiency by the means we measure it on our flow benches yet falls well short of a well-designed Venturi.

A few years ago there was quite a few posts on speed talk about the CFM per square inch that was possible in a typical V-8 engines intake port if you're flowing at 28 inches then 100% flow efficiency for a given area assuming no pressure recovery is 146 CFM. It seemed to me that anyone achieving over about 138 CFM per square inch of intake port had something to brag about – and not without good cause. But let me pose the question here – is 146 the absolute limit to the flow per square inch of the port and if it's not, under what conditions could we expect to see higher than 146 CFM per square inch?

I look forward to some of your thoughts here.

DV
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Re: The Venturi effect

Post by Rob R » Mon Jun 01, 2020 7:23 am

The way I see it on the exhaust side of things, and I may be and incorrect in this thinking.Since heat is energy you would want to utilized it as close as you can to the exhaust valve in the exhaust port at its hottest point in the exhaust track.
By changing the heat energy into kinetic energy (reference DeLaval nozzle) would be beneficial in making the exhaust port more efficient. In conjunction with changes in the valve timing events to complement the more efficient exhaust port could possibly be a way to making more horsepower,Rob.

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Re: The Venturi effect

Post by Joe-71 » Thu Jun 11, 2020 10:46 pm

On page 59 of your Theory Book you show a diagram of what at the time was considered an optimal venturi for a port and valve seat. For many years I tried to use that diagram to improve my port shape on almost all my work, and it did increase the airflow when testing on the flow bench. It works well with aluminum heads where you have a known thickness under the seat between intake and exhaust ports, but not so well on a vintage head where oversize valves have been installed. Case in point, is the 289 heads that have had an oversize valve job, and to create that venturi you most likely will break into the water jacket. On stock size valves, or small increase in size, it can be created effectively. Problem I run into now that I am retired, is that the machine shops have workers that have no imagination and can't think outside the box doing a multi-angle valve job, and don't leave any material for a venturi, or they use cutters that form the shape totally wrong. I just had two pair of 289 heads in the shop for rough valve job, and told the machinist I wanted multi-angle on intake seats, and a radius would be acceptable on exhaust seats. Came back opposite. Ruined the flow. Now I can't do the venturi, and I will need to start with a fresh set of heads since the valves have been cut, and the CCs established.

My question is: Do you still stand by the diagram? Thanks, Joe-71
Joe-JDC

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