The power to weight ratio is simply that; the weight of the vehicle divided by its peak power output, expressed as a decimal fraction.
This indicates how much weight is to be accelerated by each unit of power from the engine (i.e. lbs weight per bhp)
This ratio serves very well as a comparative figure for vehicles of different types, to generate an idea of the performance levels
that may be available for those cars. The better the ratio (i.e. more power, or less weight) then the better the performance
that can be expected from a car - especially when looking at straight-line performance, at least.
The form below allows you to enter figures relating to a particular car/engine combination and for those figures to be used to generate the power-to-weight ratio for comparison purposes with other combinations.
The ratio calculated above will use whatever units you have entered - so if you enter Kg weight and Bhp power, it will show Kg/Bhp.
But if you enter Lbs weight and Bhp power, it will show Lbs/Bhp. No conversion of units is performed.
Following on from related discussions of power outputs and weight distributions, a few points on power-to-weight ratios follow.
In way of explanation, consider a vehicle that may be available with two different power outputs, from engines of essentially the same weight.
It seems quite obvious that the car with more power will perform better than the lower-powered equivalent, also assuming that gearbox and final drive
ratios are the same. Also consider the same, more powerful engine being fitted to a heavier car; in this case performance would not be so good,
perhaps worse than the lower powered engine in the lighter car. All quite straightforward and acceptable when viewed in that way, I would hope.
Now view a different car, which is noticeably lighter than all of these vehicles, but with a powerplant that also produces much less outright power.
How can you tell which is likely to be the quickest?
This is where the power-to-weight ratio can be used as a comparative figure, by working out the ratio for each vehicle and powerplant combination,
then ranking them in order.
For simple maths, some fictional examples are shown in the table below, just to explain the theory.
| Vehicle | Weight (kg) | Power (bhp) | Ratio (kg/bhp) | Rank |
|---|---|---|---|---|
| A | 1100 | 120 | 9.17 | 3rd |
| A | 1100 | 140 | 7.85 | 2nd |
| B | 1500 | 140 | 10.071 | 4th |
| C | 600 | 80 | 7.50 | 1st |
Note that this calculator requires the use of JavaScript, so may generate warnings in your browser.
Note that measurements used here mix Imperial terms (bhp, lb, lb/ft) and metric measures (Nm, Kg) as these are more commonly encountered.