What factor determines a turbo-charger’s rotational speed?

The rotational speed of a turbocharger is primarily determined by the diversion of exhaust gas. This is because a turbocharger operates on the principle of using exhaust gas to spin its turbine, which then drives the compressor that forces additional air into the engine's intake.

When exhaust gases flow through the turbine section of the turbocharger, the velocity and volume of these gases determine how fast the turbine will rotate. A higher volume and velocity of exhaust gases will generate more power to spin the turbine faster, thereby increasing the boost pressure provided to the engine. This relationship is crucial because it means that the efficiency and performance of the turbocharger is directly tied to how the exhaust gas is channeled and utilized.

While other factors, such as the position of the throttle valve, can influence engine performance, they do not directly dictate the turbocharger’s rotational speed. The position of the throttle valve can affect the amount of exhaust gas produced and indirectly influence the turbo's performance, but the primary driver of rotational speed remains the exhaust gas flow through the turbine. Similarly, while air density at the compressor intake and bleeding off excess exhaust pressure can have effects on engine and turbo performance, they are not the primary determinants of the turbocharger's rotational speed.