How rapidly a turbocharger can develop positive pressure, boost, and at what engine speed, are important performance parameters for operators of turbocharged vehicles.
Measurements
To evaluate boost onset a measurement can be made to compare how the turbocharger performs when paired with different components. The measurement that will be used for making comparisons will consist of the time that it takes for boost pressure to build from one pound per square inch (1 psi) to twenty psi (20 psi). The engine speed at which 1 psi of boost pressure is observed will be recorded and used for plotting the measurements.
Several readings of boost onset using this measurement method were taken and plotted on the chart below.
This method of measuring and displaying boost onset data will used for future comparisons and thus an explanation for the reason for selecting this method is worth discussing.
Reasoning
Time
The development of boost pressure is closely related to the ability of the engine to develop torque, which the driver senses by the vehicle’s acceleration. The driver perceives acceleration as time passes, and boost building within a short time passage can be described as responsive. If the driver senses acceleration increasing slowly, over a longer period of time, that turbocharger is described as being laggy – building boost slowly. Because the driver is sensing changes with time, time is one of the measurements being used for this method of assessment.
Engine Speed
The turbocharger is driven by exhaust gasses and the greater the volume of exhaust gas that there is the greater available energy there is to drive the turbocharger. As engine speed increases the available energy in the exhaust system goes up. Because the engine is operated across a wide range of engine speeds the engine speed will be recorded and become the independent variable for these comparisons.
Boost Pressure Range
One psi of boost pressure is selected as the starting boost level to facilitate recording data. It is also chosen to keep the lower end of the range at a point where the turbocharger should be spinning at a relatively low rpm.
Twenty psi is chosen as the upper boost pressure to allow measurement of turbochargers that may not be designed for high boost levels. It also represents a point on the boost development curve where pressure should be increasing linearly with engine speed for the majority of turbochargers anticipated to be measured.
The chart above shows for two example turbochargers that the boost increase rate has begun stabilizing after 20 psi of boost pressure is met.
Chart Explained
The first chart above shows that with the IS20 turbocharger as engine speed increases the amount of time needed for boost pressure to build from one psi to twenty psi decreases. It also shows that the decrease in time becomes smaller for an equivalent increase in engine speed the higher the engine speed becomes.