Weber tall venturi question
 

Can someone please explain the effect these have versus the standard height?

Does it have any effect on mixtures? I.e can they be swapped with shorter ones without adversely affecting correctly set mixtures?

Thanks

Cam,

Great question,....:)

The effect of the tall auxiliary venturies depends largely on the bore size of the carbs, main venturi size and camshaft profile.

Porsche used them in high RPM racing applications where the main venturi was quite large and the camshafts had a lot of duration with narrow lobe centers. The low manifold vacuum negatively affected the ability to initiate fuel flow from the float bowls so the tall aux venturies provided a stronger vacuum to help fuel delivery.

Soooooo,...the answer to your question lies with the details about each individual's configuration. :) :)

Normally, they will not affect jetting too much but there are a lot of variables at work here.

Thanks Steve

I was thinking along the lines that cam technology has changed over the years and on a motor like mine (2.3 with your heads and 34mm venturis with the DC44 cams on 102 lobe centres) just how much difference they would make?

Is the effect felt right through the rev range? And what is the effect?

Would it help with low throttle surging or is this more an issue with the type of emulsion tubes ?

Respectfully

Your humble student :)

My pleasure, anytime!

Tall venturies are nice to have when one is using main venturies for high RPM power. These will improve low and mid RPM throttle response and torque and I'd recommend them for your 2.3.

I think your surging is more attributed to emulsion tubes/main jets but the tall venturies cannot hurt that at all.

For the record, exhaust system configuration plays a big role here and that means header size, primary lengths, secondary lengths, crossover position and muffler design. These factors really affect jetting and running below 6K with aggresive cams. :)

Your obedient mentor,

I remember in CanAm, looking at the top of the stacks on those engines, at certain RPM's the fog of fuel would appear.
It was almost certainly a reversion of the flow, but did not seem to affect the performance very much.
Also....the stacks on those engines were of different lengths (height) to make the intake runners the same over-all length.
Bob

Bob,

Indeed, that was intake reversion that you witnessed,....:)

We see that all the time on the dyno with race engines that have aggressive cams.

Fuel injection is not as affected by this as much as carburetors are, but its something to avoid or at least contain due to torque loss.

see here at 8:31. . . nice pic of reversion from the old Solexes! (See if you can pull yourself away from watching the rest of the restoration of 300037)

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Great video, Dr. Cramer,....thanks for posting that. :)

Thank you Herr Mentor Weiner :)

Another question.....

Are the tall venturis their length (just over 4 " or about 107mm) for a reason I wonder? I.e. I guess their length is reasonably optimal for most applications?

One would imagine differing engine configurations might respond to slightly different lengths. Not that I'm going to start playing around but it seems to be something that no-one really plays with.

Out of interest o they make tall venturis for PMO's?

At your service, anytime,....:)

They only made them in one length so I suppose that Eduardo Weber, SpA did the requisite R&D to arrive at those dimensions. That said, they were used on several models of larger IDA's :) :)

Richard at PMO never felt it as necessary due to major improvements in the idle and progression circuit that improved fuel atomization at part throttle.

On a related subject, I've seen big gains in airflow using full-radius intake bells on EFI engines but I've not tested them with Webers/PMO's on the flowbench. Thats an upcoming R&D project.