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Horsepower calculators can help you figure out how much horsepower you need to reach a certain top speed with your car. However, before we start calculating how much horsepower you need to hit a certain top speed goal, let’s first look at some of the factors that affect your car’s or vehicle’s top speed.

In general, top speed is physically a balance point between all the forces acting on your car. When the sum total of the forces driving the car forward exactly equals the sum total of the forces holding it back, the car can no longer increase its speed and reaches a maximum steady-state speed.

In doing so we have broken down the top speed equation into two main factors which are:

Strength: Represented in horsepower

Resistance: Represented in drag

Breaking it down further:

Factors that affect the amount of force your car has behind you are related to:

  • The total amount of raw power you have to work with
  • Your vehicle’s final drive gear that combines the gear ratio of the transmission, the final drive ratio of the differential, and the diameter of the wheel and tire package

And the factors that affect the resistance your car has against it are related to:

  • The aerodynamic profile of the car that is summarized with a single number called drag coefficient that summarizes different parameters such as:
    • The frontal area of ​​the car (which determines how well the car penetrates the wall of air in front of it)
    • The car’s height or ground clearance determines the portions of the airflow that is divided and forced both over the roofline of the car and under the underside of the vehicle.
    • The car’s side profile which determines how air is expelled after passing over, under and around the car and determines the characteristics of the low pressure zone behind the car’s rear window or rear bumper. This zone always ‘sucks’ the car back effectively and should be minimised.
  • In addition to aerodynamic drag factors, there are also mechanical drags that come from the high-rev rotation of the internal components of the engine, transmission, wheel and tire package, heavy driveshaft, and axle shafts (especially in a car). with four-wheel drive, for example). ) And so on.

However, research has shown (and certain racing classes such as Formula 1 have pragmatically confirmed this) that after the point of about 100 miles per hour, mechanical drag factors become less significant in affecting a vehicle’s top speed. .

At those speeds, aerodynamic drag is the main drag force determining a car’s performance, which is why in sports like Formula 1, vehicles of similar power vary significantly in performance depending on which car has the right aero setup. for the best combination of top speed figures as well as assisted aerodynamic traction (downforce) during high speed cornering. By comparison, in a lower speed racing category, like auto crossover for example (which is limited by track design to around 80mph for the fastest cars), you’ll find that typically the The best performing cars are the ones configured with the best (next) mechanical drive. of proper suspension settings and good tire traction) with no real dynamic effects coming from the aerodynamic design.

Having said that, gaining a top speed advantage by altering your car’s drag coefficient can be a costly process with ever diminishing returns. After exhausting the first set of basic mods, like…

  • Lower the ride height of the car to reduce turbulence under the vehicle
  • Install an aftermarket front bumper with built-in air splitter or extend the factory bumper with an additional air splitter to promote a higher percentage of airflow over the car
  • Using lower profile ends on the car, such as lower profile mirrors or less aggressively angled rear wings (to provide a better balance between drag and downforce)
  • Panel coating on the underside of the car to give a smooth underbelly which helps speed up the air under the vehicle and reduces turbulence underneath. (You’ll find that manufacturers like Mercedes do this even in their lowest-tier compact cars to improve high-speed stability and highway mileage.)
  • Using a fitted or custom rear bumper with a built-in rear diffuser to improve the transition of the two air streams from above and below the car that merge behind the rear bumper and prevent the low pressure area behind the bumper from sucking the car backwards.
  • Cutting off the top of the car and lowering the height of the roof line relative to the hood and trunk (think of the roof height on a Corvette compared to a Jeep to better understand why this works)
  • Using strategically placed vents on the hood and front and rear fenders to promote airflow through certain high-pressure areas (such as under the hood or in the wheel wells) to reduce pressure in these areas and help increase airflow through the vehicle

…once this list of mods runs out, you’ll find that your drag coefficient may have realistically decreased by 30%. However, top speed is related to drag as follows:

Power to overcome aerodynamic drag = fA x Cd x 0.00256 x mph cubed / 375

Note in this equation that the cube of top speed is related to the drag coefficient Cd and therefore altering your drag coefficient from a typical 0.45 to a sportier 0.30 (a 30% reduction) only results in a 12% increase in your actual top speed (i.e. going from a top speed of 100 to 112 mph)

Yes, this is a noticeable gain, but in order to do something as significant as double your top speed, you’ll eventually have to start increasing your overall power level. Volkswagen designers working on the 1,100hp Bugatti Veryron quickly realized this and it’s precisely why they had to use so much power to hit their 400kph top speed target.

So going back to this equation mentioned above, we know that if a car is power limited in its top speed sprint (where we have more gears to use to accelerate or where we hit our top speed in top gear long before the redline ) then we know that increasing the power of the engine to take advantage of the remaining range of rpm (or gear ratios) is a very practical way to increase the top speed of the car.

In a practical sense, even if the drag coefficient of the car in question is unknown, it is possible to work out how much power is required to reach a given top speed target by comparing its current power and top speed levels to its maximum target. speed level By doing so, and using the following equation (derived from the general equation above), we get:

New horsepower = old horsepower * (new top speed / old top speed)^cubed

A practical example that is familiar to me is the 320 horsepower 3000GT VR4. This twin turbocharged car comes with a great aero shape and is capable of a top speed of 160 miles in 5th gear at 6000rpm with 1000rpm to go in that gear and an unused 6th gear.

By having the car so obviously power-limited (instead of gear-limited or drag-limited) at its top speed, some of the enthusiasts have modified this car and broken the 200 mph barrier.

Applying our previous formula:

New power = 320 hp * (200 mph / 160 mph) ^3

New horsepower = 625 horsepower

So what this says is that to hit a top speed of 200mph in a 3000GT VR4, we know we’ll need at least 625 horsepower (assuming we have enough gears and rpm to increase our wheel spin speed by 200/ 160 or 25% while still running below the car’s redline rpm).

As a final note, it might seem crazy trying to double the power of your car and reach a top speed that far exceeds any speed limit you’ll come across on a normal road. Nevertheless:

1- There are certain platforms such as the Chevy Corvette, the Mitsubishi 3000GT, the Toyota Supra… etc. where doubling or tripling the power levels of these iconic sports cars is not only common practice, but also relatively inexpensive ($7000 for the 3000GT using Dynamic Racing’s “Diablo Killer” upgrade package)

2- There are also sanctioned racing classes that allow enthusiasts to race their cars in top speed tests as well as standing mile acceleration tests. These are very addictive and difficult racing classes that attract only the most dedicated enthusiasts to extract every ounce of aerodynamic design, power, traction, gearing, stability and longevity from their vehicles and have become more of a racing cult or addiction which is difficult to break.

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