What Is a Power to Weight Ratio?

What Is a 'Power to Weight' Ratio?

A “Power to Weight Ratio” is the term used to describe the relationship between a measurement of power [usually Kilowatt units] and its ability to move and accelerate a vehicle of a particular weight.

Put simply, it is a way of describing the ability of the engine's power to move the weight of the vehicle.

The formula is actually quite simple.
Using the specified ‘unladen weight’ (or kerb weight) of the particular vehicle, and dividing this by the number of Kilowatts that the motor is known to produce at peak power, a number can be reached.

The average two litre family sedan for example, weighs in the vicinity of 1380 kg.  The ‘power output’ of these vehicles is on average approximately 98 kilowatts - based on 2003 data obtained from NZAA ‘Directions’ Publication.

Therefore, the equation thus calculated, will provide a figure of 14.08 : 1 ...[1380 divided by 98  =  14.08].
This number represents the ratio of units of weight (14) that each respective (single) kW unit of power needs to act on, in order to move the vehicle.
PTW can also be calculated and expressed as kW/per tonne. To achieve this the engine power is divided by weight - as opposed to the weight being divided by power - and mutiplying the product of the division by 1000.  Thus, 98 -: 1380 = .071 X 1000 = 71kW/tonne.

The more power that can be added to a vehicle without increasing its weight, the lower will be this power to weight ratio.  By lower we mean that the "ratio" between the weight and vehicles power, is smaller by disproportion and/or separation.

This is why motorcycles, being relatively light, have ‘low’ power-to-weight ratios, particularly when their engine size is above 250cc.
Thus, they will often possess exceptional acceleration capability.

[NB. Motorcycles have much less mechanical structure, comparative to their engine size, than do motor cars.]   Refer also: "Egine Power Restrictions Examined..."

Confusion regarding 'ratio' designations: It is allways problematic to decide whether to describe a 'ratio comparison' as being "High" or "Low".  In terms of the descriptions above, such as for motorcycles, it is decided to adopt the 'low PTW ratio' description for instances where the vehicle in question has an effective or powerfull ratio for its engine to weight performance.
In the picture below of the F1 car, it's PTW ratio would be somewhere in the region of 932 kW/tonne

Therefore a heavy vehicle with a small (i.e. underpowerd) engine would similarly have a ‘high’ power to weight ratio.

Weight 'variability' factors with small or underpowered vehicles:

, power to weight is therefore easily changed by the addition of extra passengers - particularly in small-engined cars.  Because of this “variability” and other engineering factors, using ‘PTW’ ratios as guidance for licence restrictions for CARS is not practical.