The power band which refers to the range of operating speeds under which the engine is able to operate efficiently. A typical gasoline automotive engine is capable of operating at a speed of between around 750 and 6000 RPM, but the engine's power band would be more limited. The engine would typically not generate maximum torque until higher operating speeds of perhaps 2500 RPM, after such, the torque drops off. The peak power (horsepower) might be closer to 5000 RPM. Such an engine would be said to have a "power band" of 2500-5000 RPM (another example would be from torque peak to redline: 2500-6000 RPM).
This can be applied to any engine and establishes a reliable quantification of the above notion "the engine is able to operate efficiently".
The tuning of the power band is a great challenge. It is possible to create a peaky engine which generates more power from an engine if the manufacturer is willing to tune it for a very narrow power band. However, an engine with a narrow power band is more difficult to use. Such an engine must be coupled to a close-ratio transmission with many gears in order to remain in its power band while providing an acceptably wide range of output speeds. A flexible engine has a wide power band with less peak power, but could be tied to a less complex transmission with fewer gears and would not need to shift gears as often. Such an engine is also often called torquey because it maintains a more constant level of torque over a wider range of RPM. The vehicle's overall performance can be maximized by maximizing the "area under the graph" of either the horsepower or torque curve.
Sports cars and other performance vehicles are generally designed for peak power in a narrow power band. In these vehicles, the higher cost of a complex transmission would be more acceptable, and the driver could be assumed to be more willing to shift gears often to remain in the power band. These vehicles attempt to achieve the greatest possible power to weight ratio, and benefit greatly from using a smaller engine tuned for high peak power rather than a large engine with a wide power band. Trucks and full-size cars are more often tuned for a wide power band and use larger engines to achieve acceptable power over a wide range. These vehicles have the benefit of not having to shift as often as vehicles with a narrow power band.
Since automobile shoppers rely heavily on the peak power output figure (typically given in horsepower or kilowatts), some auto makers tend towards producing "peaky" engines. For example, Honda's 2006 Civic Si's K20Z3 engine generates 197 hp (147 kW) at 7800 RPM. Though it produces a fairly flat torque curve compared to many engines, it only produces 139 ft·lbf (188 N·m) and it has relatively sharp (or "peaky") power delivery, this requires the driver to keep the engine at high RPM to extract the best performance from the Civic. In contrast, Volkswagen's 2006 GTI's 2.0T engine produces about 200 hp (149 kW) from 5,100 RPM to 6,000 RPM and a relatively flat torque band of 207 ft·lbf (281 N·m) from 1,800 to 5,000 RPM. This wide power delivery makes it easier for the driver to extract the vehicles best performance.
Since the power band is in between the peak torque and peak power the distance between the peaks can change how the car/bike behaves when it reaches it. A two-stroke's power band is very short and the bike will seem to 'jump' when it hits it, whereas a four stroke typically has a greater distance between the peaks therefore giving the impression that it doesn't have one because it isn't jerky. However, a four stroke can be tuned to have a narrow power band and it will jerk much like a two stroke.
Because a CVT vehicle has the capability of keeping RPMs within the crest of the power band under acceleration, a peaky engine is optimal. Under full acceleration 100% of the available power can be extracted at all times. There is no shifting, and no moving out of the power band. This type of transmission is more efficient than others due to power band issues mentioned previously, but is not favored by many due to the lack of apparent power. Drivers are accustomed to the sudden lurch off the line and the shifting of the transmission. Though these are only actions of less efficient transmission, some vehicle manufacturers have computerized such events in to the transmission to add to the perception of power and torque.
Engines for ships and aircraft are also generally designed with a narrow power band as these vehicles do not have to operate over a wide speed range. They instead reach their optimal operating speed and remain there for the duration of their trips. As a result, they benefit from tuning for peak power and efficiency in a narrow power band.