Zenith-2 modification

[Werner]

Hallo und guten Tag miteinander.

I'm looking for collective wisdom from anyone who has already done this. with success or failure.

When I look at the carburetor's intake port, I see that the venturi insert protrudes about 1.25 cm into the intake port before it rises at a 90° angle.

This is bad from a flow perspective. Does anyone have experience grinding down this crescent-shaped opening?

The cylinder filling should improve. But perhaps the mixture needs this swirl edge to vaporize more effectively.
The designers must have had some reason for doing it this way. Or perhaps they genuinely didn't foresee this disadvantage.

I don't want the engine to rev higher. But a little more power uphill would be helpful.


Dankeschoen voraus!


Note: The attached image is not my copyright; I've only borrowed it from an for an online display.

 

[nkaminar]

The venturi should not be modified. You are right that it is restricting flow but if you carve part of it away it would not function as designed. The venturi needs to have the full flow through the throat to draw in the proper amount of gasoline. The intake portion of the carburetor is a larger area than the venturi to allow the air to make the turn with minimum restriction.

To improve flow at higher rpm use a different carburetor or two of the Zenith carburetors. A down draft Weber works great.

 

[Bob Bidonde]

Try it and let us know the result. If you do not, I will try it this summer.

 

[Jim/GA]

For ~20% better flow area, switch to a Model B carburetor and open the intake manifold a bit to match it.

Before doing that, for hill climbing power, install a cylinder head with 5.5:1 Compression Ratio. This gives you the most performance improvement for your money. You will notice it when climbing a hill in High gear.

Then change to the Model B carb for a bit more performance, if you want to, but you will probably conclude you don't need it, that the head is enough.

 

[Marshall V. Daut]

I don't recall all the modifications you have made to your Model A since you first visited this discussion forum, but a simple carburetor mod will not magically turn your engine into a hill climbing champion, even adding a downdraft carb. Yes, a Model B or downdraft carb of some flavor will help boost power a little. However, the hills and mountains in Germany between Hamburg and Munich I remember driving my 1928 Roadster through in 1974 will require more than a carb upgrade to conquer. Jeez, you guys drive fast on the Autobahn!!! ZOOOOOOOOOOOOOOOM right on past my little Roadster struggling to maintain the minimum Autobahn speed!
Since more power is your goal, if you haven't installed at least a 5.5 or 6.0 cylinder head, you will never be happy with your Model A's hill climbing capability. Even with such an upgraded cylinder head, you're still somewhat limited by design with the so-so standard one-size-fits-all camshaft grind that was installed to placate the average driver back in The Day. Changing to a more aggressive design (Like Jim Brierly's C-grind) that will help the engine breathe better, teamed up with the higher compression head and either a Model B or downdraft carb (Stromberg 81, 97 or a Weber), WILL make all the difference you desire. It won't be cheap to do this, but you won't believe the difference those three upgrades will make in your Model A - yet still retain the Model A's character we all love.
By all means try your carb modification, if for no other reason just to see what happens. Just don't expect too much of an improvement. If that were possible by doing what you contemplate, someone in the last almost 100 years has most certainly done this - yet who has heard about it? That tells me any success has not become world famous or urban legend in the Model A engine hop-up crowd.
Marshall

 

[Marshall V. Daut]

Jim/GA types faster than I do, but our message is the same.
M.

 

[Big hammer]

Werner , if you have an extra venture go for it ! My guess is if you can get it to run it will run lean , if your carburetor is running rich it may work ? I know you will report back .

 

[Werner]

Schoenen Nachmittag!

Thanks for the quick responses. I had hoped that, in your vast Model A country, these experiments had already been undertaken; thereby sparing me the trouble of a failed attempt.
Presumably, there were no successful outcomes; the idea would have been too simple. The folks at Ford weren't fools,

It’s not that I’m looking to tune up my car; I want it to remain largely as it is. I don't consider it suitable for high-speed driving.

However, I recently had to re-bush the throttle shaft, and I thought that - while I was at it - I might be able to squeeze out a couple of one or two extra horsepower "for free."

I’m going to do a try; I already have a spare venturi ready.

Either way, I’ll let you know how it goes. I don't think, that the engine runs better in all RPMs. But I try it, it ist only little work with no costs.

Thanks again for your hints!

 

[philosofriend]

The shape of the inside of the Venturi is engineered to create a low pressure area at the tips of the two jets. The air needs to go smoothly up the Venturi with the cross-sectional area changing evenly around the circle as the air goes up. Cutting away part of the Venturi will interfere with the Venturi's job of getting a smooth airflow that gently gets up to maximum speed right where the gas gets sucked in.

The restriction of the Venturi is what speeds up the air and sucks the gas in. If it was just airflow, you could make the carb run better by leaving the Venturi out! This is exactly what makes fuel injection so good. EFI leaves out the Venturi and has a powerful pump force the gas into the air stream.

Don't do your experiment until you have another Venturi at hand.

 

[30ccpickup]

Nice! I created the image for exactly the way you are using it!

Hallo und guten Tag miteinander.

I'm looking for collective wisdom from anyone who has already done this. with success or failure.

When I look at the carburetor's intake port, I see that the venturi insert protrudes about 1.25 cm into the intake port before it rises at a 90° angle.

This is bad from a flow perspective. Does anyone have experience grinding down this crescent-shaped opening?

The cylinder filling should improve. But perhaps the mixture needs this swirl edge to vaporize more effectively.
The designers must have had some reason for doing it this way. Or perhaps they genuinely didn't foresee this disadvantage.

I don't want the engine to rev higher. But a little more power uphill would be helpful.


Dankeschoen voraus!


Note: The attached image is not my copyright; I've only borrowed it from an for an online display.

 

[Jim/GA]

The shape of the inside of the Venturi is engineered to create a low pressure area at the tips of the two jets. The air needs to go smoothly up the Venturi with the cross-sectional area changing evenly around the circle as the air goes up. Cutting away part of the Venturi will interfere with the Venturi's job of getting a smooth airflow that gently gets up to maximum speed right where the gas gets sucked in.

The restriction of the Venturi is what speeds up the air and sucks the gas in. If it was just airflow, you could make the carb run better by leaving the Venturi out! This is exactly what makes fuel injection so good. EFI leaves out the Venturi and has a powerful pump force the gas into the air stream.

Don't do your experiment until you have another Venturi at hand.

You are correct about the shape of the inside of the venturi. The narrowing of the neck makes the air have to flow faster. When it does, it creates a low pressure zone at the neck. See "Bernoulli's principle".

In fact, it is patterned after the shape of the top of an airplane wing. It is that shape rotated around a central axis. That axial symmetry is important. Cutting a wedge out will really mess up the symmetry and the effectiveness of the venturi.

Even though a replacement venturi does not cost much, I wouldn't waste my time or money with this experiment.

 

[Gene F]

Linear feet per minute vs cubic feet per minute. There is a lot more to this than what a person may think. Even the swirl of airflow can make a difference. However, this is getting into some fairly scientific engineering principles. Principles that would be tested on prototypes.

Best thing you can do is get a B carb, and a B intake. Not only will the B flow more air, but it has that power jet tube.

 

[nkaminar]

At the lower rpm, say below 1,300, there is no difference between a stock Zenith with the stock manifold and a high flow carburetor, like the Weber. The compression ratio will improve the torque throughout the rpm range but a larger carburetor will only shine at the higher rpm. You will notice the difference at the higher rpm, like 1,500 and higher.

Jim, post #11, and Philoso Friend, post #9 have the correct response.

By the way, Jim, the modern wing theory is all about momentum. The downward flow of the air is what makes the plane fly. Bernoulli's principle and the suction on top of the wing have been removed from wing design. Airplanes can fly upside down, although not as efficiently. High performance aerobatic planes have symmetric wings.

 

[RENNERS CORNER]

Do people really think that Ford/Zenith just changed this venturi and missed or didn't care that the single venturi blocked flow ?????

Airflow is notorious for acting in ways that defy common sense.

You are only guessing that the bit hanging down has negative impacts.

I have been testing carburetor flow with these carburetors for may years and very often things are not as they seem.

You need a SUPERFLOW bench to test something like this.

Even if you were to modify a venturi I really doubt that you could tell the difference by the seat of your pants, but with a Flow bench you could.

I will add this modification to my list of things to experiment with.

Like the double venturi you would have to pin the venturi in place to prevent it from being installed wrong or moving on its own.

Keep a look out for future postings on venturi's and carburetor air flow testing.

David Renner

 

[Werner]

Thanks to my background in aviation, I am versed in the subject of aerodynamics. I understand the critical importance of laminar flow—and how, upon its separation, the airflow abruptly devolves into turbulence—a phenomenon no plane pilot welcomes.

Furthermore, having worked extensively with carbureted engines in my professional life, I also appreciate just how beneficial vigorous turbulence can be in transforming a mixture of fuel and air into a homogeneous gaseous state.

The carburetor, in particular, is a remarkably complex device to fully comprehend—one whose designers clearly invested a great deal of ingenuity.
Crucial to its operation is the velocity of the airflow, which is necessary to generate the high degree of negative pressure required. The fact that scientific understanding is a process of continuous evolution is illustrated, for instance, by the realization that—contrary to earlier assumptions—a perfectly mirror-smooth surface is not actually the ideal condition for maintaining laminar flow; rather, a surface exhibiting a very fine degree of roughness is preferable.

A carburetor is difficult to fully grasp. Theoretical understanding is helpful when optimizing one; however, it is equally important to recognize that, ultimately, practical application determines the outcome tipping the scales between success and failure, which often lie in close proximity.

 

[Werner]

Guten Abend.

I can now provide an update. Modifying the intake funnel to improve airflow cannot onely be done without making additional changes.
With this modification, the engine certainly feels a little bit more powerful from mid-range RPMs upwards. (For safety reasons, I did not test the top speed; the car becomes rather uncomfortable to drive once it exceeds 75 km/h.)
However, throttle response during acceleration is characterized by extreme jerking. When the engine is cold, the car is barely drivable when pulling away from a standstill.
Fine-tuning the system—for instance, by using different jets—would likely be feasible. However, the effort required for such fine-tuning is extremely high. It simply isn't worth it, as the amount of time consumed by test drives and similar adjustments is substantial.
The reason for the intake's originally aerodynamically suboptimal design is to ensure that the air-fuel mixture is thoroughly swirled, thereby becoming homogeneous.