Acceleration
Air Temperature and Tarmac: The Tenths Nobody Reports
Twenty degrees of air temperature costs a naturally aspirated engine 6.5 per cent of its power. The surface it launches from is worth more than that, and neither appears in the headline.
A performance figure is a measurement of a car and a day. Warm air is thin air: going from 15 °C to 35 °C at constant pressure cuts air density from 1.225 to 1.145 kg/m³, which costs a naturally aspirated engine about 6.5 per cent of its output. The surface matters more still, because tyre grip has a temperature window and a cold road can remove a fifth of the available traction.
Warm air is thin air
Air density follows the ideal gas relation ρ = p / (R · T), where R is 287.05 J/(kg·K) for dry air and T is absolute temperature. At the International Standard Atmosphere reference of 101.325 kPa and 15 °C, that gives 1.225 kg/m³. Raise the temperature and density falls in direct proportion to absolute temperature, which is why a hot day is worth so much.
| Air temperature | Absolute temperature | Density | Change against 15 °C | 500 hp engine becomes |
|---|---|---|---|---|
| 0 °C | 273.15 K | 1.2924 kg/m³ | +5.5 % | 528 hp |
| 5 °C | 278.15 K | 1.2692 kg/m³ | +3.6 % | 518 hp |
| 15 °C | 288.15 K | 1.2250 kg/m³ | reference | 500 hp |
| 25 °C | 298.15 K | 1.1839 kg/m³ | −3.4 % | 483 hp |
| 35 °C | 308.15 K | 1.1455 kg/m³ | −6.5 % | 468 hp |
The spread from a cold morning to a hot afternoon is therefore around 10 per cent of engine output on the same car, on the same day, a few hours apart. This is precisely why power measurement standards specify reference conditions rather than a temperature range: SAE J1349 corrects to 99 kPa and 25 °C, while DIN 70020 and UNECE Regulation 85 correct to 101.3 kPa and 20 °C. Without a correction factor, a dynamometer measures the weather.
Why turbocharged cars are less affected, until they are not
A turbocharged engine controls boost pressure rather than accepting whatever the atmosphere provides, so on a hot day it simply asks the compressor to work harder to hit the same manifold pressure. Within its authority the engine holds output while ambient conditions move, which is why forced induction cars appear immune.
They are not immune, the loss just moves. Compressing hotter air raises charge temperature after the compressor, which loads the intercooler, which raises intake temperature, which brings the knock limit closer. Engine management responds by retarding ignition or lowering boost, and the power arrives late rather than not at all. On a single run this is invisible. On the third consecutive run, with a heat soaked intercooler, it is very visible.
The surface is worth more than the air
Tyre grip is a chemical and mechanical property that has an operating window, and outside that window the rubber is either too stiff to conform to the road surface or too soft to resist shearing. The practical effect on a launch is larger than the engine effect.
- A cold surface, below about 10 °C, leaves a summer compound tyre well below its working range. Grip loss of 15 to 25 per cent against the same tyre warm is ordinary, and that translates straight into the traction limit.
- A prepared surface, as at a drag strip, is treated with adhesive compound and rubbered in by previous runs. Its friction coefficient can exceed a public road by a wide margin, which is why strip times and road times are not comparable.
- A polished or dusty surface can put a road tyre below 0.7. On a rear wheel drive car that alone is worth more than a second to 100 km/h.
- Track temperature is not air temperature. Asphalt in direct sun sits well above ambient, often by 15 to 20 °C, so the same air reading describes very different surfaces depending on cloud cover.
Altitude, which is the same effect with a bigger lever
Elevation reduces pressure as well as density. As a working figure, air density falls about 10 per cent per 1,000 m of altitude in the lower atmosphere, so a naturally aspirated engine gives up roughly 3 per cent per 300 m. A test at 1,500 m starts about 15 per cent down on power against sea level, before the temperature of the day is counted at all.
Drag falls with density too, which helps top speed, and that is why high altitude venues favour maximum speed runs and punish acceleration runs. The two effects genuinely pull in opposite directions.
What to ask of a published figure
Four conditions move an acceleration time more than most car to car differences: air temperature, surface temperature, surface preparation and elevation. A figure quoted without any of them is a figure quoted without its units. The sensible response is not to distrust the number, it is to treat a difference of a tenth or two between two cars tested on different days as noise rather than a result.
Questions readers ask
How much power does a hot day cost?
For a naturally aspirated engine, about 6.5 per cent going from 15 °C to 35 °C, because air density falls from 1.225 to 1.145 kg/m³. Across the realistic range from a cold morning at 0 °C to a hot afternoon at 35 °C the spread is close to 11 per cent.
Are turbocharged cars affected by temperature?
Less directly, because the turbocharger targets a boost pressure rather than accepting ambient density. The loss reappears as higher charge temperature, more intercooler load and a closer knock limit, so it shows up as ignition retard on repeated runs rather than as a lower peak figure on the first one.
Does cold weather make a car faster?
For the engine, yes: at 5 °C a naturally aspirated engine makes about 3.6 per cent more than at 15 °C. For the launch, usually no. Summer tyres below roughly 10 °C surface temperature lose 15 to 25 per cent of their grip, and on most cars that costs more than the engine gains.
Why do dynamometer standards specify a temperature?
Because otherwise the measurement records the weather. SAE J1349 corrects results to 99 kPa and 25 °C, DIN 70020 and UNECE Regulation 85 to 101.3 kPa and 20 °C. The different references are why the same engine carries different published figures in different markets.
How much does altitude matter?
Air density falls roughly 10 per cent per 1,000 m, so a naturally aspirated engine loses about 3 per cent per 300 m of elevation. Aerodynamic drag falls by the same proportion, which is why altitude hurts acceleration and helps top speed.
Sources
- UNECE Regulation No. 85, on net power measurement and the reference conditions applied to it.
- Engine power correction standards, on the reference pressure and temperature used by SAE J1349, DIN 70020 and ECE 85.
- Vehicle Physics Pro, tyre model documentation, on grip as a function of tyre condition and load.
Density values are calculated from ρ = p / (R · T) with p = 101.325 kPa and R = 287.05 J/(kg·K) for dry air. Naturally aspirated power is taken as proportional to density.