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Tuesday November 21st 2017


Boost ratio

A static test was conducted with and without the booster function to determine the increase of brake line pressure with the assistance of a booster.

Normal Booster function

Test is conducted with the engine idling and at normal running temperature. Driver’s seat was positioned to suit stature of driver so that force was applied perpendicularly to the brake pedal. Brake line pressure [MPa] and pedal force [N] were recorded for 50N increments of pedal force from 50N to 500N. For each reading, the pedal force was slowly increased until the intended value was reached. Pedal force was never reduced, even if the target pedal force was exceeded. Between each reading the pedal was released and the engine was allowed to idle for a few moments to rebuild vacuum in the booster.

The results of the Normal Booster Function can be seen in the following figure:


Failed Booster

The engine was stopped and the pedal was pressed a few times until the vacuum was exhausted. Then the procedure was repeated for the same range of pedal forces (this time, the line pressures were lower because there was no assistance).

The results of the Failed Booster can be seen in the following figure:



For the normal booster it can be seen that an increase in the Brake Pedal Force produces a considerable increase in the Brake Line Pressure up to 250N of Pedal Force. From that point onwards the increase is more subtle. The maximum Brake Line Pressure achieved for Normal Booster Function was 9.8MPa, however for the Failed Booster  the maximum pressure only reached 1.8MPa.

The following figure shows the relationship between the increase of Brake Line Pressure to the increase of Pedal Force. The knee point can be found for the Normal Booster Function plot. The knee point starts where the gradient of the trend line of the Normal Booster Function starts to become parallel to the gradient of the Failed Booster trend line. This occurs around 7.5MPa of Brake Line Pressure.


The Boost ratio was found dividing the change in gradient of the two cases:


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