Length of auxiliary venturi

[H-viken]

We are rebuilding a pair of Weber carbs and noticed the previous owner used one length of aux centuries on one bank and another on the other (by ignorance or on purpose will remain a mystery)

What does the length of the auxiliary venturi influence? When is a longer vs shorter auxiliary venturi needed and why?

Does the height of the trumpets play together with this as well? Understand there are two different heights: 50 and 56mm

Thanks in advance!

Johan

[1QuickS]

My take on this topic is this:

Tall auxiliary (booster) venturis are used on small displacement engines with very large main venturis. The reason for this is the low speed circuit will lose effectiveness before the main circuit is activated and the tall booster venturis help activate the main circuit earlier than the short ones will. The reason for this is the tall boosters are exposed to nearly atmospheric air pressure since they are nearly equal in height of the tall air horns (56mm tall). Since a booster venturi generates suction based upon differential air pressure from inlet to the minimum diameter, the taller venturi therefor has more differential air pressure and develops more suction. The short boosters are well within accelerated (low pressure) air in the air horns and do not have as much pressure differential from top opening to the waist.

The bottom of the booster venturis is located at the height of the waist of the main venturi which helps create air flow through the venturi. A large main venturi will generate less vacuum for a given airflow which is based upon displacement & RPM of the engine. So, the suction at the bottom of the booster is less with a large venturi at a given RPM.

These tall boosters were implemented for the 901/20 (906, 911R, 914/6 GT, etc.) engines that used 42mm main venturis for 2.0 liter engines. Race application engines in lightweight vehicles did not need too much driveability in the 3000 RPM region where transition from low speed circuit to high speed circuit is most noticeable but whatever could be mustered was needed for part throttle applications.

Tall air horns provide a little more low speed power and also protect the tops of the tall boosters.

[H-viken]

Thanks Paul!

You mention smaller engines with very large main venturis.

These Weber 46 carbs are going on a 3L engine with fairly aggressive cams, that have a bit of overlap.

In this case what would you consider to be a very large main venturi?

Johan

[1QuickS]

Small displacement engines with big venturis means 2 liters with 42mm main venturis, a 3 liter will want 38mm venturis or perhaps 40mm but this is not an application that would concern me as needing a tall booster venturi.

[H-viken]

Thank you Paul, very much appreciated!

Johan