I didn't notice anywhere on the solder design, and attempt to remove the sharp corners. If you want to improve the design, that would be a good place to start. The sharp corners will produce turbulence, which will affect how the venturi functions.

As Tony said, the pressure drop (that is between the normal flow in the pipe, and in the neck of the venturi) is proportional to the velocity difference, which is proportional to the reduction in area across the venturi.

That said, there is a limit to how fast you can push a fluid through a pipe, thus there is a limit to how much you can reduce the area.

This page may be of some inspirational value: Venturi, eductor, ejector, solids conveying, vacuum system

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