Supercharger pulley ratio and boost calculator

Supercharger Pulley Ratio & Boost Calculator

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Understanding the different parameters

When calculating supercharger boost and pulley ratios, several key parameters determine how much additional air is being forced into the engine. Here’s a breakdown of what each input means and how it affects performance.

Crank Pulley Diameter (in/mm)

The crankshaft pulley drives the supercharger via a belt. A larger crank pulley increases supercharger speed, leading to higher boost levels. Conversely, a smaller crank pulley reduces supercharger RPM and decreases boost.

Supercharger Pulley Diameter (in/mm)

The supercharger pulley is mounted on the supercharger’s input shaft. A smaller supercharger pulley results in a higher pulley ratio, making the supercharger spin faster and generating more boost. A larger pulley slows the supercharger down, reducing boost.

Engine RPM

This is the engine speed in revolutions per minute (RPM). Since the supercharger spins relative to the engine’s RPM, higher engine speeds mean more air is forced into the intake, increasing power output.

Supercharger Efficiency (%)

Superchargers are not 100% efficient due to heat generation and internal losses. Most modern superchargers operate between 60% and 85% efficiency. A higher efficiency rating means the supercharger is delivering more usable boost with less wasted energy.

Supercharger Displacement (ci/cc)

Supercharger displacement refers to how much air the supercharger moves per revolution, usually measured in cubic inches (ci) or cubic centimeters (cc). Larger superchargers flow more air per rotation, generating more boost at lower speeds, while smaller units need to spin faster to produce the same effect.

Engine Displacement (ci/cc)

The engine’s displacement (size) is a major factor in determining how much boost pressure is created. A larger engine can absorb more air without increasing pressure dramatically, while a smaller engine will reach higher boost levels with the same amount of compressed air.

Ambient Pressure (PSI)

This is the natural atmospheric pressure that varies based on altitude. At sea level, ambient pressure is 14.7 PSI (1 Bar). Higher altitudes have lower atmospheric pressure, reducing the amount of available oxygen and slightly lowering boost levels.

How these parameters work together

Each of these inputs directly impacts the boost pressure and overall engine performance. The goal of optimizing a supercharger setup is to balance pulley sizing, efficiency, and engine displacement to achieve the desired boost without overloading the system.

By tweaking these values, you can see how changes affect pulley ratio, supercharger RPM, and boost pressure, helping you fine-tune your setup for maximum performance.