Power-to-weight ratio is the amount of horsepower a vehicle has in relation to its weight. It can be applied to any motor vehicle, but when factoring it on a wheeled vehicle, it should be stated whether flywheel or drive wheel power is used. Accurate power-to-weight ratios can be obtained by placing the vehicle on a chassis dynamometer. Understanding power-to-weight ratios is useful for tuning engines, suspension, and developing driveline packages for production vehicles. It is also important for commercial vehicles and military applications.
A power to weight ratio refers to the amount of horsepower a vehicle has in relation to its weight. When factoring a power to weight ratio, a vehicle’s weight must be divided by the amount of horsepower the engine produces. This measured power to weight ratio can be applied to any type of motor vehicle, from a motorcycle or car to a locomotive or airplane. When factoring power to weight ratio on a wheeled vehicle, the numbers can be misleading. In this scenario, it should be clearly stated whether the factorization is completed using flywheel power or drive wheel power, as the difference can be large.
When an engine’s horsepower is recorded, it is connected to an engine dyno and runs at wide open throttle for a measured period of time. The dynamometer reads the power produced at the engine flywheel. This is an accurate indicator of the amount of power the motor is producing without taking into account any static or parasitic losses. To better understand a vehicle’s true power to weight ratio, it must be placed on a chassis dynamometer. By placing the drive wheel or wheels on a chassis dynamometer and taking a reading of the power produced after passing through the transmission and drive axle, an accurate true power-to-weight ratio can be arrived at.
In performance or racing applications, finding out a vehicle’s power to weight ratio can be very useful information when tuning the engine or suspension. Tire size, shock selection and even braking systems can be better understood and engineered when power to weight ratio is accurately understood. In drag racing, fuel consumption and fuel consumption can be calculated using this power and weight information. In a production vehicle such as a pickup truck, engine size and payload can be taken into account using the power and weight information to develop a driveline package. Towing capabilities and capacities also benefit from understanding the power available versus the curb weight of the vehicle.
On commercial vehicles, such as tractor-trailer semi-trucks, both empty weight and loaded weight must be factored into the power-to-weight ratio equation. Aircraft are also subject to the laden and empty equations to properly determine the fuel needs to carry a payload of cargo or passengers. In military applications, the ability to carry bombs is directly related to the power available for the anticipated weight of the payload.
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