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A lot of people think only about power, they want more and more power but they overlook one thing. The speed and acceleration of your car is directly related to the power-to-weight ratio.

A lot of people think only about power, they want more and more power but they overlook one thing. The speed and acceleration of your car is directly related to the power-to-weight ratio.

This is a measure of how powerful your engine is compared to the weight of the vehicle. So a massive V8 lump in a beefy 60’s American muscle car might seem like a good idea, but it might easily be outrun by a highly-tuned 2 litre 4-cylinder engine in a super lightweight Japanese car. The actual units of power-to-weight ratio don’t really matter, as long as you use the same units when comparing any two vehicles. So you can’t use bhp and weight in kilos to measure one vehicle and hp and weight in pounds for the other. Compare an Impreza 2.5i 173hp, 3067 lbs kerb weight, the Impreza WRX 224hp, 3296 lbs kerb weight and Impreza WRX STi 293hp, 3351 lbs kerb weight(Kerb weight is the total weight of a vehicle with standard equipment, all necessary operating consumables (such as motor oil and coolant), a full tank of fuel).

You can see as you go up the range, the weight of the vehicles increases, but so does the horsepower. Power-to-weight ratio is a two-sided equation. A vehicle will go faster with less weight, or more or a combination of the two. With the Subaru example, the power-to-weight ratios look like this: Impreza2.5i = 1:17.72,  Impreza WRX = 1:14.71 and Impreza WRX STi = 1:11.43.  The figures are easy to come by - divide the power by the weight to get the ratio. The ‘to’ in ‘power to weight’ is like the dividing line in a fraction. So 173hp / 3067lbs = 0.0564334, or in standard notation 1:17.72.

You can see that despite the higher-end cars getting heavier, the increase in engine power brings the power-to-weight ratio down so the car becomes quicker. This explains why motorbikes are so quick compared to cars. For example if you compare the 2007 Honda CBR600RR to the 2007 Subaru WRX STi, it becomes readily apparent why the bike will win every time.  Impreza WRX STi = 293hp, 3351 lbs kerb weight = 1:11.43 power-to-weight ratio and the CBR600RR = 118hp, 345 lbs kerb weight = 1:2.93 power-to-weight ratio.  So it’s not that the bike is more powerful - it’s not. The engine is only 600cc and it produces almost a third the horsepower of the car. But the bike weighs so much less that the weight side of the equation drops to the point where the ratio plummets.

So in a car, weight is everything. It can be expensive to start beefing up the engine to give you more power, but it can be really cheap to reduce the weight. As a rough guide, for every 100lbs (45kg) of weight you remove from the average car, you will drop 1/10 second from a timed quarter mile. For the ultimate sports car or street racer, beef up the engine and reduce the weight; increase the power side of the equation and decrease the weight side of the equation and the power-to-weight ratio becomes more favourable.

So how do you reduce the weight of your car. Well again it depends on how far you want to go. If you don’t care about carrying passengers, toss out the rear and passenger seats. Don’t mind getting a flat and calling a tow-truck? Get rid of the spare tyre and jack. If you’re going for a true drag-strip car, take out the glass windows and replace them with plastic ones. Remove the dashboard, carpet, headliner, etc. etc. etc. Beginning to get the idea? There’s really no limit to how far you can go.

One of the most popular weight-saving mods is a carbon fibre hood. If you’re interested enough in this topic to have reached this point on the page, then you’ll likely have seen cars with carbon hoods - they’re very obvious because the hood is almost always black.

 

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