Power and Torque

What is horsepower and torque?
We hear a lot about horsepower and torque, but what is the difference between them ?

1. Horsepower is engine speed dependent and is a calculated number from the measured torque:

P(kW) = T*n* C1 P = power
T = Torque
n = RPM
C1= constant

Normally the unit is kilowatt (kW), but most of all we talk about Horsepower (hp)
      1 hp DIN = 1,36*KW DIN

2. Torque is a measured number of force delivered from an engine at a certain point and is RPM independent.

T(Nm) = V * p * C2 T = torque
p = pressure in the cylinder
V=cylinder volume

The unit here is Newton meter (Nm)

We talk about kW, hp or Nm but in different norms:
Bhp is British horsepower (1bhp = 1,013872 hp DIN)
SAE is in American norms
Here we measure the engine without dynamo, air filter etc...

DIN is in German norms (most used by manufactures)
Correction of the measured power and Torque in the norm with atmospheric pressure of 1013mbar and 20°Celsius
The conversion formula is

k = 1013 / atmp * ((273 + airt) / 293)^0.5 k = the DIN factor
atmp = atmospheric pressure in mbar
airt = air temperature in °Celsius

EWG is the new European norm
Correction of the measured power and torque in the norm with an atmospheric pressure of 1013 mbar but at 25° Celsius
The conversion formula is

k = 1013 / atmp * ((273 + airt) / 298)^0.5 k = the EWG factor
atmp = atmospheric pressure in mbar
airt = air temperature in °Celsius


Other important conversions to know are :

1kgm = 9,81 Nm
1 kgm = 7,23 Lbft
1 km = 0,6214 miles
1 gallon (American) = 3,78533 litre
1 gallon (English) = 4,54596 litre

These international standards, just as DIN and EWG, are very important to allow us to compare two measurements independent of the air temperature of air pressure of the day. We can read from the formula that higher air pressure and a higher air temperature will give us a bigger k factor and a lower air pressure and lower air temperature a smaller one. Important for you, is to know what this means in practice.
The best way to explain this, is that an engine with more horsepower will give you better time from 0-100 km/h and an engine with more torque will give you better time from 80-120km/h in the same gear. To go from 0-100 km/h you need to go high in the revolutions to reach at least the rpm of maximum hp's. For the 80-120km/h test in the same gear, you cannot make a choice of rpm, so the power (torque) at the RPM range between 80 and 120 km/h will give you the ability to accelerate from one speed to another speed.
Therefor, it is possible that an engine with the same hp's or even less but with more torque can let you down, if the car has to accelerate in the same gear. For example a VW TDI can accelerate better from lower RPM than a VW GTI. 

What to think about the promised power and torque?
First you have to look at the unit of the promised gain before you can compare. Is the increased power in kW , hp, bph, SAE, DIN or EWG (see above). The torque may be in Nm (also DIN or EWG) ,in kgm or in Lbft.

* Some tuners give directly the maximum achieved power and torque at a certain Rpm
* Some tuners give only the gain of power and torque, this can be done in different ways :
   - either the tuner gives the difference at the maximum power- and torque points on a certain RPM;
   - or the tuner gives you the maximum difference independent of the RPM

For example, it is possible that a tuning might give an extra 10 hp at the point of maximum power, but might give you 15 extra hp in the midrange of rpm. Consequently, for the same conversion, one tuner speaks about +10hp and another of +15hp.
This is a very important point to know before choosing a tuner.


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