Although it was introduced earlier this year, the Porsche 911 7-speed manual gearbox is still a feature that gets a lot of press. Although most journalists believe that whatever the engineers at Porsche develop is well thought out, the 7 speed transmission raised a few eyebrows when it was first introduced. Some frankly wondered if 7 gears are absolutely necessary.
To gain a further understanding of the whole “number of gears” thing, we need to discuss how the speed of an engine relates to the engine’s power. As the graph below illustrates, the turning speed of an engine is not directly related to its horsepower. Most engines have a sweet spot where the turning speed of the engine combines with the resultant torque to create a maximum horsepower. In the case of the hypothetical engine below, the peak horsepower occurs at some 2500 revolutions per minute (RPM). This is why cars have different gearing ratios in their transmissions. The object is to keep the engine rotating at the peak horsepower speed as much as possible for performance and efficiency.