The original carburetor air box on the Vespa 100 Sport is a convoluted path for air to travel. The thought was to increase efficiency of the airflow and increase engine performance slightly. Air entering a standard straight-edged intake pipe or around corners creates chaotic swirling of the airflow. These vortices effectively narrow the opening, making the air "fight itself" and slowing down the overall flow.
A velocity stack's curved, radius opening allows air from all angles to transition smoothly into the intake. This creates laminar flow, where air stays attached to the pipe walls, maximizing the volume of air reaching the combustion chamber. As air moves from the wide, flared opening into the narrower intake runner, its pressure drops and its velocity increases. Faster-moving air has more inertia, which helps it continue rushing into the cylinder even as the intake valve begins to close, leading to better cylinder filling.
The design was simple enough that I could easily build it in TinkererCAD using its standard library of primitive shapes. The geometry didn't require too much intricate sculpting.
The updated design was 3D printed using PETG plastic, a material specifically chosen for its fuel-resistant properties and durability in automotive environments. PETG is naturally resistant to gasoline, oils, and greases, making it ideal for parts that come into contact with fuel. To ensure a secure and stable installation, the velocity stack was designed to mount directly onto the carburetor’s existing threaded studs, eliminating the need for additional specialized hardware.
For the initial testing phase, a temporary protective mesh was fitted over the intake opening to serve as a primary debris shield. This screen was essential for preventing large contaminants or foreign objects from being vacuumed into the engine’s internal components, which could cause catastrophic failure during high-RPM operation. While functional for safety, the current screen is a rudimentary fix. It provides the necessary protection for testing but a filter for fine-particle filtration will have to be employed.
The initial tests were very good. The engine was running just a little rich prior to installing the velocity stack. The first test ride provided a bit of a lean idle which is easily remedied. The wide open throttle response was much improved. There was no hesitation through the range of throttle opening. The cylinder head temperature was slightly higher with the additional air being added. When a suitable air filter is fitted, that may balance the mixture better, or main jet increase may be needed.
Overall the project was a complete success.
