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P/W Ratio



P/W Ratio Rating  :      

You love cars and you are in the market for a new car. Chances are that you are looking for drivability & performance. You might think you know that knowledge about engine capacity and power is sufficient to know how fast the car would go. You simply take a view of spec sheet of potential contenders. Engine displacement, power figures & torque rating has been checked to find out suitable choice. But is it as simple as it seems?   

The answer is an emphatic ‘No’. Think about 100 horses pulling a car & then same number pulling the light commercial vehicle & then heavy commercial vehicle. Will the response be the same? It is seldom as simple as to compare the cc or power figures. Weight of the car (Kerb – without passengers. Gross with Passengers) has a huge impact on vehicle performance.     

 Ideally an enthusiasts or even a slightly more mature car user need to weigh factors beyond rated power and torque figures to buy a car which is most suitable for him. This in car terminology is referred to as Power to Weight Ratio OR P/W Ratio.    

For a start the technical definition is; Power-to-weight ratio is used as a measurement of performance of a vehicle as a whole, with the engine's power output being divided by the weight of the vehicle. 

P-to-W Ratio = Power generated by Engine (bhp) / Weight of vehicle (ton)  

In simple terms it represents the capacity of engine to pull the weight of one ton from point A to B.    

For example: Maruti- Suzuki Swift Vdi has Power Output of 75 bhp & Kerb-weight of the car is 1075 Kg (1.075 Ton). It's P/w is calculated as follows:  

 

Power-to-Weight ratio, P/W = 75 / 1.075 = 69.76 bhp/ton.   

Extending this logic engines with 50 bhp weighing 500 kgs (P/W = 100 Bhp/Ton) is far better to drive than 100 bhp car with weight 1200 kgs (P/W = 83.3 Bhp/Ton) . Simply speaking the P/W of 50 Bhp engine is higher that one with 100 Bhp power rating. In other words 50 Bhp car will pull better that 100 Bhp as the weight being pulled by teh former is lower than teh latter engine.  

Some examples of P/W ratio's which are self explainatory see how it the P/W ratio affects the drivability, keeping other factors constant like rpm at which torque is generated, aerodynamics & drag coefficient. Attempt is to correlate only P/W ratio with drivability.

Heavier the car, more power is needed to move it, simple logic, more weight requires more power to pull it. There is no counter argument to this basic law. Not only does weight affects drivability  but it impacts fuel efficiency (matter for another day).

 

POWER-TO-WEIGHT RATIO
  Engine Capacity (CC) Power rating (bhp) Kerb weight (Ton= Kgs/1000) P/W RATIO
SEDAN SEGMENT        
MARUTI - DZIRE (ZDI) 1248 74.2 1.115 66.5
MARUTI- SX4 (ZDI) 1248 89.1 1.245 71.5
TATA MANZA (AURA) 1248 89.1 1.21 73.6
FIAT LINEA (EMOT.) 1248 92 1.21 76
VW VENTO(HIGHLINE) 1598 103.9 1.22 85
HYUNDAI VERNA 1582 126.7 1.191 106.3
HATCHBACK SEGMENT        
MARUTI-SWIFT (VDI) 1248 74.25 1.075 69
MARUTI-RITZ (VDI) 1248 74.25 1.11 66.8
VISTA QUADR. (AURA) 1248 74.25 1.14 65.1
FORD FIGO (TITAN.) 1399 68.3 1.09 62.66
NISSAN MICRA XV 1461 63.3 1.008 62.7
FIAT PUNTO (EMOT.) 1248 74.25 1.19 62.3

 Basis P/W figures Hyundai Verna tops the chart followed by Vento, Linea and Maruti-Dzire. What it simply means is that Verna should offer the best drivability (ability to move the weight without groaning and grunting) with Dzire being at the other end in terms of drivability.   

Similarly in Hatchback segment, Maruti-Swift leads with highest ratio of 69 bhp/ton followed by its sibling Ritz. Fiat Punto, being heaviest, has lowest ratio figure indicating that weight has blunted the performance.   


 

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