The evolution of performance

Many performance benchmarks are used for comparing the performance of cars in order to decide who is best (at something at least). Top speed and acceleration numbers (longitudinal and lateral), top speed are the most used ones. While these numbers are useful for comparing contemporary competitors, this time they will be used for comparing the evolution of record car performance.

The number with the largest halo-effect is still the top-speed. By using the fastest production cars as the selection criterion for this showing the evolution, the progress made can become clear. Turns out top speed itself has increased almost
linearly
over time, from 201km/h in 1949 (Jaguar XK120) to 435km/h in 2014 (Hennessey Venom GT). Since the power required to overcome the aerodynamic drag has a cubic relation (3) to the velocity, the increase in propulsion power required to achieve this would have to increase by a factor of 10,3. The maximum propulsion power increased from 162PS to 1244PS, which is a factor of only 7,7. Considering that new vehicles have higher output powertrains with absolutely (though most likely not relatively) bigger losses and also have downforce rather than lift, the aerodynamic drag coefficient must have gone down by at least 25% to make that possible.

Another standard figure used to compare the performance of cars is the 0-100km/h acceleration. Since power is the product of force and speed and force is the product of mass and acceleration, the power required for acceleration of a car is proportional to the mass, but inversely proportional to the acceleration time (assuming constant acceleration during this acceleration time). The power-to-mass ratio (which everyone somehow keeps calling power-to-weight) should therefore have a proportional relation to the acceleration time (if all other parameters remain constant).

The acceleration time has dropped from 9,8s (Jaguar XK120) to 2,8s (Hennessey Venom GT) for the same selection of vehicles as before. The power-to-mass ratio has increased to 818% between these vehicles (XK120 = 100%), while the acceleration time difference would suggest only 329% would be necessary. Somehow it seems that increasing the power-to-mass ratio does not help improving acceleration beyond a certain point. Drag racers will obviously disagree, but they tend to change the circumstances as well as the vehicle.

It would seem that with Rear-Wheel-Drive only the maximum achievable acceleration is 1,01g (and 0-100km/h in 2,8s), no matter how high the power-to-mass ratio is (0,85PS/kg is the estimated sweet spot). With All-Wheel-Drive 1,13g the limit seems (0-100km/h in 2,5s), since there is no real difference between the 1001PS (0,53PS/kg) and 1200PS versions. Time will no doubt proof these numbers wrong.

When comparing the power-to-mass of these record cars to their acceleration, it also becomes clear that the AWD cars achieve the same acceleration figures with up to 50% lower power-to-mass ratios as their contemporary RWD rivals. Traction therefore makes a bigger difference than power-to-mass. Drag racers would agree this time.

Traction has been improved by better tires and suspension systems, as well as optimised torque delivery via advanced transmissions and traction control systems for both RWD and AWD. The real improvement though is in the fact that mere mortals can achieve the record numbers and not just driving gods (or other people who consider themselves immortal).

The dry vehicle mass of the RWD record cars fluctuates between 1093kg and 1330kg. The AWDs remain between 1450kg and 1915kg. Progress can not be found in the mass of the vehicles, but in the power-to-mass which rises exponentially for every new record vehicle. Unless a revolution happens in the field of traction, not much progress should be expected in acceleration figures. Top speed figures on the other hand are still open. The sky is the limit. Especially if you mess up your aerodynamics. ¤

This article first appeared on my now defunct website www.autoweekly.info on 2014-05-09.

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