46mm Webers with modified progression

[1QuickS]

I would like some input regarding experience using 46mm Webers on the street and that have been modified to have the "slit" progression circuit vs. the traditional drilled progression ports. The slit has been used in race applications however I need info regarding suitability for street applications. I provide a picture of the "slit" progression and of a 46mm throttle body that had the "slit" removed and four hole progression retrofitted in its place. The throttle plate with the slit progression has been drilled to help allow more air at idle to overcome the richness of the progression at small throttle openings...my belief.

Slit progression showing large air bleed in the throttle plate:




Four hole progression retrofitted into throttle body:

[jameshtaylor]

This is not an answer to your question. But I find it interesting that PMO originally used four holes arranged in a vertical line, not unlike your slot. They have since reported (and I experienced) improved performance with one of the holes blocked off, leaving three.

[gled49]

Jerry Woods kinda invented that with his friend Bill Beekhus (sp) and I believe it was done for street use. I watched lots of carbs being modified and believe it worked. gled

[Joe901]

Is it reasonable to assume that the number of drillings or the use of a slot for progression may vary on a case by case basis.

If you take the Weber 40DCOE as an example there are many different variations that are used depending on engine type and capacity.

The current 40DCOE151 for example has 2 progression holes but early 490DCOE18's had 3 of differing diameters and in a triangular layout.

The Dell'Orto 40 and 45DHLA also show differences. Early 45DHLA carbs had 4 holes and some of the later Lotus 2.2 variants had 6 holes.

It was generally accepted that DelOrtos had better progression.

I am not sure that one size will fit all as emulsion tube type and basic engine characteristics will potentially have a significant influence.