Final answer:
The APR system pertains to aviation, not automobile engines, and the reference material does not specify an increase percentage. Using physics calculations, the maximum grade an automobile can climb depends on power output, efficiency, velocity, and frictional force, but is not directly related to APR.
Step-by-step explanation:
The automatic performance reserve (APR) system is a feature in some aircraft engines, and the question appears to be related to aviation, not automobile engines. However, since the provided reference material is primarily about automobiles and engine efficiency, a possible interpretation could be a confusion in the question asking about additional power provided by car's engine management system under certain conditions. Modern cars with computerized engine controls can adjust the engine's operation for better performance and efficiency compared to cars from the 1970s. Nevertheless, the reference material provided does not give a specific percentage increase for the APR system to apply to an operating engine.
Regarding the automobile engine efficiency question, to calculate the maximum grade a car can climb, we first convert horsepower to watts (1 hp = 745.7 watts). The car's maximum power output is 140 hp, which equals 104,398 watts (140 * 745.7). Given the car's efficiency of 25%, the effective power is 25% of 104,398 watts, which is 26,099.5 watts.
At a constant velocity, power is the force times velocity, so effective power is equal to the force exerted against friction and gravity times the velocity of the car. The force exerted by the engine (F) can be calculated using the formula: Power = F * velocity. Hence, F = Power / velocity. Considering the frictional force, the net force (F_net) available to climb the slope is F - frictional force. If we rearrange the formula for slope (grade) as a percentage, we get grade = (F_net / weight) * 100. Plugging in the numbers and converting velocity to meters per second (50 km/h = 13.89 m/s), we can find the maximum grade the car can climb.