Venturi size

[roy]

Why does every car I have or have had with 40dcoe carbs ( bunch of Alfas & Triumphs , street cars & race cars) always seem to run better with small ( 30's & 28's) venturis ? What sizes do people really use ? The Weber books seem to suggest enormously big ones .

[Gordon Raymond]

Depends on if you want low end pull or wide open power. There are many books and posts on the subject. There is even a formula for a happy medium!
Lots depends on engine displacement, cams and (REAL) useable engine speeds.
Read some of the older threads in the Carburetor section, it should get you pointed in the correct direction. Gordon Raymond

[1,6 HF]

Carb performance depends on both the volume and the velocity of the air moving through the barrels; too little air volume, and top-end power is limited, but too little air velocity, and low-speed response is sluggish.

Minimum diameter venturis maintain greater air velocity at lower revs, thus increasing low-end torque and throttle response. But small venturis restrict air volume at higher revs, thus limiting maximum power. Maximum diameter venturis do just the opposite; they provide plenty of air volume for top-end power, but the air moves slower at lower revs, thus tending to give more sluggish throttle response. Not surprisingly, venturis in between the two extremes achieve some balance between the two. Much depends on your driving style & performance preference and, obviously, the size of the engine you’re trying to feed.

Selecting the proper venturi for your use always involves some guesswork and some trial and error (unless you’re racing, in which case more is almost always better). But there are charts and guides that can help with the selection; I’ve found the attached PDF to be a useful guide to DCOE set-up.

Weber DCOE setup.pdf

[RossoGTV4Me]

That was a very useful .pdf Ed. Thank you very much for sharing that with us!

Quote:

Originally Posted by 1,6 HF Carb performance depends on both the volume and the velocity of the air moving through the barrels; too little air volume, and top-end power is limited, but too little air velocity, and low-speed response is sluggish. Minimum diameter venturis maintain greater air velocity at lower revs, thus increasing low-end torque and throttle response. But small venturis restrict air volume at higher revs, thus limiting maximum power. Maximum diameter venturis do just the opposite; they provide plenty of air volume for top-end power, but the air moves slower at lower revs, thus tending to give more sluggish throttle response. Not surprisingly, venturis in between the two extremes achieve some balance between the two. Much depends on your driving style & performance preference and, obviously, the size of the engine you’re trying to feed. Selecting the proper venturi for your use always involves some guesswork and some trial and error (unless you’re racing, in which case more is almost always better). But there are charts and guides that can help with the selection; I’ve found the attached PDF to be a useful guide to DCOE set-up. Attachment 94776

[Tifosi]

When working with a single carb throat per single intake valve, (like the Webers are set up), a venturi that's 87% in size of the diameter of the intake valve will allow for the highest volumetric efficiency under normally asperated conditions. (potentially around 132% VE which is about the limit for NA engines)

Smaller and you're restricting, bigger and it's overkill.

[1,6 HF]

Quote:

Originally Posted by Tifosi When working with a single carb throat per single intake valve, (like the Webers are set up), a venturi that's 87% in size of the diameter of the intake valve will allow for the highest volumetric efficiency under normally asperated conditions. (potentially around 132% VE which is about the limit for NA engines) Smaller and you're restricting, bigger and it's overkill.

[Tifosi]

Quote:

Originally Posted by 1,6 HF If you really want to optimize for low-end tractability and instantaneous throttle response from low revs, you'll have to trade off some VE by dropping the venturi below 87% of intake valve diameter.

[Alfar7]

I knew it would be better to wait!
Now that the Mathematicians & Enginners have expounded the theories, & formulas, here`s some real info on why drivability is reduced with larger "venturis aka choke tubes".
Fuel is delivered from the "Auxillary venturis" as a result of lower pressure in the inside of the carburettor as a result of both engine vaccuum (less) and Bernoulli effect (more, as air speed through the carb is a result of vaccuum or more accurately absence of atmospheric pressure.Atmospheric pressure in the carb fuel bowl pushes the fuel into the "AV".).
With Weber carbs, the auxiliarry venturi is a poorly shaped small "venturi"(production compromise) in the center of the "bore" which delivers the fuel slurry created in the emulsion tubes and initiates atomization .
The choke tube ("venturi") as "designed" is intended to "improve" the atomization by reducing the size of the bore which speeds up the air fuel mix again reducing it`s "pressure" further atomizing the fuel.
So by design a smaller choke tube will deliver a better fuel mix but it does limit volumn.

But there are other considerations that must be considered!

FYI, the internals, (slide in airhorns, auxillary venturies and venturis (choke tubes)) used in the 45 and 48 DCOE`s are the same parts. The body of the 45mm is not bored as large in the area past the end of the choke tube and uses 45mm butterflies. So for comparison the 45 & 48`s can be considered fuel mixers of similar nature.

Given the same size choke tube a factory stock 40mm DCOE will deliver marginally better drivability because of the smaller inside bore(36mm) of the thick walled "auxillary venturi housing" than a stock 45/48 DCOE which has only a thin wall ring (46mmID, yes 46mm) surrounding the "AV". This is a result of the air speed around the "AV", and effect on fuel delivery with throttle change.
However, the "45/48" will atomize the fuel delivered ( less accurately) at lower speeds more effectively with the same size choke tube. As well the "45/48" will allow a higher total CFM with the same choke tube size because of higher flow (less flow disruption actually) around the area of the butterfly & throttle shaft. The 48 being better.

So, the simple solution to having more flow (cfm`s) and drivability becomes the true quest. (not intended to relegate "where`s the beer?" to a lesser status)

Well, it`s fairly easy. So easy in fact I`ll let the mathematicians & engineers
have an opportunity to share it with you!

[1,6 HF]

Quote:

Originally Posted by Alfar7 Given the same size choke tube a factory stock 40mm DCOE will deliver marginally better drivability because of the smaller inside bore(36mm) of the thick walled "auxillary venturi housing" than a stock 45/48 DCOE which has only a thin wall ring (46mmID, yes 46mm) surrounding the "AV". This is a result of the air speed around the "AV", and effect on fuel delivery with throttle change. However, the "45/48" will atomize the fuel delivered ( less accurately) at lower speeds more effectively with the same size choke tube. As well the "45/48" will allow a higher total CFM with the same choke tube size because of higher flow (less flow disruption actually) around the area of the butterfly & throttle shaft. The 48 being better.

[Alfar7]

? Is that your final answer?

[alfaparticle]

For another, because of the issue of the step between the choke and the throttle plate, the manufacturer doesn't recommend going with a choke over about 80% of the barrel diameter (but, hey, they're just engineers; what do they know?). That means that a 32mm/33mm is about the practical limit for 40s, even though they’ll take 36mm. And even though 33mm are available for 45s, the larger step between the choke and the throttle plate is probably going to lose in increased turbulence anything that it might gain in increased volume.

If Weber don't recommend going over 33mm venturis in 40DCOE's, how come they manufacture and sell 34's and 36's?

[1,6 HF]

Quote:

Originally Posted by Alfar7 ? Is that your final answer?

[Alfar7]

Times Up. Approching bedtime. (and I might forget....
At least an Engineer was heard from!
(Alfaparticle be an "E")

And I agree with Ed.You can buy Choke Tubes from 30 to 42mm for the series. From where are all these "Weber fittment statements originating??
As for the step it can be converted to a taper or reduced to match the back of a reshaped and more productive venturi design..
(Check out this site on venturi shapes & pressures;
Ideal Fluid Flow and Pressure Distribution )
Now to the issue of making large bore Webers (45/48mmDCOEs) a more drivable carb.
First the auxillary venturi, which is a poor shapped part.
To start with these carbs generally come with 4.5 AVs. as they have 4.5mm long fuel delivery slots (5.0mm are available from importers for the race carbs not using slide in airhorns. But any of them can be built with thes "bandless" AVs.Then your bore around the AV is 48mm for more air flow volumn.)
The larger bore gets the larger AV due to reduced venturi effect of the large bore and this eases the flow of