Weight and aerodynamic drag are two huge factors effecting how fast your car is, they are also two factors which we can improve while spending little or no money, here are my 12 tips. (Check the notes at the bottom for information about the numbers and percentages I use in the calculations)
A 100kg reduction in weight can improve fuel economy by 5%.
5% less power required to go the same speed means 5% more power available for faster acceleration (in a perfect world).
We know the difference of having one extra person in the car.
Don’t carry unnecessary things.
If you want to go to extremes you can start removing seats and pieces of interior.
2.Choose car without a sunroof
A basic tilt and slide sunroof can add at least 10kg to the weight of a car.
Remove spare wheel, remove jack, spanners. The spare wheel and it accessories can easily weigh 20kg.
Rolling resistance sucks power from your engine which could be used for speed!
4. Higher tyre pressures
Higher pressures equals less power used to overcome the rolling resistance, leaving more power available to accelerate the car faster, traction permitting.
NOTE: Reducing rolling resistance also reduces traction, stick within the recommend tolerances for your tyre and vehicle.
Reducing the amount of power your engine needs to overcome aerodynamic drag leaves more power available to accelerate the car faster.
Hard data is difficult to find but of the tests that are out there, winding the windows down is not quite as bad as having the AC.
Given the AC can take around 10bhp to run, if we assume open windows at 100km/h absorb about the same amount of power, 10bhp, winding the windows down allows a car to accelerate faster at higher speeds, 100km/h+
2% less power consumed by drag gives 2% more power to be used for acceleration.
Use this tool to find which narrower tyres you can use without effecting the rolling diameter (to keep speedometer and trip meter accurate)
Removing the roof rack when you don’t need it will reduce aerodynamic drag. Less power absorbed by overcoming drag leaves more power available to let the car accelerate faster.
There is a study that is quoting up to 25% reduction in fuel economy by fitting a roof rack. Even if the extra drag is 5%, it is still a big plus for acceleration at higher speeds.
Many cars are fitted with undertrays from the factory with the trend starting back in the mid nineties.
On new cars the undertrays will almost certainly still be fitted to the car but for older cars they could be damaged or missing completely.
Whatever the case may be, ensuring the car has an undertray reaching from the front of the front bumper and covering some part of the engine (back to the front edge of the front tyre for example) can reduce drag significantly.
In this test, fitting a custom undertray between the front of the front bumper back to the front edge of the front tyres reduced drag by 10%.
I’m not sure on the exact math but I would imagine reducing the fuel consumption by 10% is close to equaling freeing up 10% of the engine’s power.
I can imagine that the top speed of the car is increased significantly and the acceleration of the car at higher speeds is increased significantly by a 10% reduction in drag.
Same theme as the undertray. By reducing the amount of air going under the car there is less drag against the components under the car (the bottom of a car is nowhere near as smooth as the top!). Less drag increases acceleration especially at higher speeds.
One for the keen DIYer. If you don’t need the full cooling capability of the engine, sealing of some of the radiator inlet will reduce drag. Of the total drag of a vehicle, the cooling system alone can account for 5%-10%! Sealing the front of the car is a key tuning aid used by NASCAR teams.
Asking the engine to power more things reduces the power available for acceleration
An air condition compressor can take up to 10hp to run. This video shows an example.
Many cars have a wide open throttle (WOT) switch on the AC compressor. This disengages the compressor under WOT but less than WOT the air con is sucking power from the engine.
Stereos, sat nav screens, bluetooth, interior lights, sub woofers, extra electrical parts, inverters for mains voltage supply in car, fridges.
Anything that draws electrical power forces your alternator to charge the battery more often which sucks power from the engine. 1-14 hp according to this article
Bringing it all together
Bringing all these tips together, what can we expect on a car with 100bhp from the factory ie how much power can we expect to free up over 100km/h
Losing the spare wheel, not choosing car with a sunroof and removing all necessary clutter from the car. Lets say a 20kg weight reduction or 1% more power. (If you don’t carry unnecessary things around with you and/or you have a space saver spare, ignore this)
Increasing tyre pressures by 8PSI gives us another 2% (keep within tyre and car specs)
Keeping the windows closed, another 5% (if you keep your windows up, ignore this)
Reducing the tyre width by 5% gives us another 2%
Removing the roof rack another 5% (if you don’t use a roof rack, ignore this)
Fitting an undertray another 10% (if you car has an undertray already ignore this)
Lowering the car, 2% better (guess)
Seal part of the radiator inlet, 1%
Switch off air con, 10%
Accessories, only use bare essentials, no radio, sat nav, low energy lamps, 1%
Best Case Improvement
If all of these factors are applicable, this gives us a 40% decrease is losses (parasitical and aerodynamic) which can frees up power to be used to accelerate the car faster, above 100km/h (in this example example).
Me personally, I like have the security of a spare tyre, I keep my windows closed above 100km/h, I’m not going to fit narrower tyres, I don’t use a roof rack, my car is already lowered, I’m going to use the air con on a hot day, I’m not going to seal of part of my radiator. In this case I reduce my losses by 12% (10% coming from fitting a full undertray).
Wrapping it up
I think everyone is going to be different, somethings you will be able to do, some thing you won’t be able to do and some things you won’t want to do.
Assumptions and Calculations
There are a lot of assumptions and extrapolations/guesstimations in these percentage calculations, especially when it comes to converting fuel economy savings into power gains, even if the exact numbers are inaccurate all the tips listed will be make a car faster.
If you have any comments, let me know below!