here is What you need :
speed of the car(in meters per second)
(km/h divided by 3.6 = m/s)
(mph divided by 2.237 = m/s)
weight of the car in kilograms (write up half )
(punds divided by 2.205 equals kg)
stopwatch
calculator
fun at have: rev counter
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Here is the formula
endingspeed m/s * m/s * half the weight in kg
minus
starting speed m/s * m/s * half the weight in kg
divided by the number of seconds = average power in watts
here comes a few examples
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example 1
car weigh 1200kg accelerates 80-120km/h and it takes 7 seconds
33.3 m/s * 33.3 m/s * 600 (half the weight) = 665,334 joules
22.2 m/s * 22.2 m/s * 600 (half the weight) = 295,704 joules
665334joule - 295704 joules = 369630watt or 369.6Kw
369.6kw / 7 seconds = 53kw or average horsepower of 72hp
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example 2
car weigh 1600kg accelerates 75.6km/h-90km/h in 1.5 seconds (5000 - 6000rpm )
25 m/s * 25 m/s * 800 (half the weight) = 500 000joule
minus
21 m/s * 21 m/s * 800 (half the weight) = 352 800joule
500 000joule -352 800joule = 147 200watt ( 147kW )
147kw/1.5seconds = 116kw (or 157.76 HP)
( Now that we also have the RPM we can guess at this car has about 157hk on the drive wheels between 5 - 6000rpm )
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other useful formulas if you want to calculate hp or torque
Kw * 1:36 = hp
Hp * 7000/rpm = Nm
Nm * rpm/7000 = Hp
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If you want to be a bit more precise:
The formulas would be:
Hp * 7021.54/rpm = Nm
Nm * rpm/7021.54 = Hp
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and for those that use pounds and foot
hp * 5252/rpm = lb·ft
lb·ft * rpm/5252 = Hp
Remember that these formulas only have the accuracy of data you put in and it only calculates the AVERAGE horspower to the ground without any drivtrain or rolling resistance taken into the account
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you can actually use the above formula to calculate how much power is lost from air resistance and friction between the tires/road
if we use the same example cars as above
in Example 1 we start at 120 km/h and coast in neutral down to 80 km/h and take the time
in Example 2, we start at 90 and coast down to 75.6 km/h and take the time
then we use the above formula and pretend that we accelerate (allthough very slow)
Drive carefully and have fun
