How much power is enough?. Review on Petrol Vs. Diesel car

Updated Oct 07 to reflect the current cars in the market:

Auto reviews frequently tell you that a car is ‘adequately’ powered and give you the horsepower or torque figures for a car. But how does one make a like-to-like comparison of these figures among different cars?

Figures like 12.5 seconds for 0-100 kph acceleration that you typically see in a review don’t make much sense to me since I don’t use my car for racing—like most people, I use it for work and leisure in the city, and occasionally pile on some luggage and drive off with the family on a holiday.

In that context, ‘adequate’ power for me means that I should be able to accomplish all of these tasks in reasonable comfort. I need decent acceleration so that I don’t straggle at traffic lights or when the car is loaded, and I want to drive without undue gear changes. And of course, I want to be able to do all this at reasonable fuel economy.

Okay, now that we have set expectations, what is ‘adequate’ power? One good way to compare power across cars is to calculate the power-to-weight ratio. For example, the Maruti Swift has a power of 87 bhp and a Maruti Esteem 85 bhp. But the Swift has a kerb weight of 1000 kg and an Esteem has a kerb weight of 875 kg. Which means that a Swift has a power-to-weight ratio of 87 bhp/te(horsepower/ tonne), which is fully 11% less than the 97 bhp/te rating of Esteem.

A power-to-weight ratio of 75-100 bhp/te is considered ‘adequate’ for most situations.

Here’s a comparison of power-to-weight ratios of some popular petrol cars:

Maruti 800= 56 bhp/ te

Maruti Alto LXi = 64

Maruti Esteem = 97

Maruti Swift = 87

Maruti SX4= 87

Hyundai Santro= 74

Hyundai Getz = 83

Honda City GXi= 72

Toyota Corolla= 111

Toyota Camry= 97

The Corolla and Camry ratios can be considered as benchmarks since they are normally rated among the best family sedans in international surveys.

From the above, it does seem that most of the petrol cars(except Maruti 800 and Alto) are ‘adequately’ powered.

But what about diesel vehicles? The bhp/te figures for diesel vehicles look very different:

Tata Indica= 53

Tata Indigo= 64

Maruti Swift Vdi = 70

Ford Fiesta = 57

Hyundai Verna CRDi = 89

Toyota Innova = 44

Mahindra Scorpio = 48

Tata Safari Dicor = 52

Except for the Swift and the Verna, the diesel vehicles look underpowered. So should we have another criterion for diesels?

Well, sort of. Diesel engines are built differently from petrol engines and do not deliver the same horsepower as a comparable petrol engine because they do not attain the same high rpms(revolutions per minute) as petrol engines.

But diesel is a more efficient fuel than petrol and releases more energy. Therefore they produce more TORQUE, (or turning force) at a lesser rpm compared to petrol engines.  So maybe we should compare the torque-to-weight ratios for diesel engines.

Here are the torque-to-weight ratios of the above diesels in Newton-meter/ Tonne:

Tata Indica = 116 Nm/ te

Tata Indigo = 123

Maruti Swift Vdi = 177

Ford Fiesta = 139

Hyundai Verna CRDi = 196

Toyota Innova = 87

Mahindra Scorpio = 116

Safari Dicor= 121

Another factor that is important is the'torque curve'. This is the behavior of torque vs engine rpm. If the torque is constant or'flat' over a wide range of engine rpm, it indicates that the engine is delivering maximum torque over a wide range of engine speeds. This is also the range where the engine is operating in the optimum or most fuel efficient zone. The Innova has a very flat torque curve from 1400 - 3400 rpm. This ranges from just above idling all the way up to very high road speeds.

Clearly, the Verna and Swift lead the pack significantly. But torque only indicates the'pulling power' of an engine. A high torque-to-weight ratio means that you have great ability to start but to sustain that mommentum and accelerate, you need horsepower too.

So maybe we need a combination of good torque and horsepower to arrive at a criteria for'adequately' powered diesel cars.

For comparison, I looked at a few international favorites among diesel family cars. Here are the bhp/ te and Nm/Te figures for:

Vehicle                           bhp/ te                     Nm/ te

-

VW Polo 1.4 TDI           66                           110

Fiat Punto 1.3               85                            192

VW Passat 1.9 TDi      65                            154

Clearly, the Swift, Verna, and the Fiesta among cars, and the Safari among MUVs compare favorably with the international benchmarks. Obviously, other factors such as fit and finish, servicing and price all play a significant part in the buying process, but what good are these without an adequate power plant?

My list is by no means complete. But the methodolgy can help you check if the vehicle you're planning to buy is'adequately' powered. All you need to do is to get the technical specifications of the vehicle from the company website and calculate the power-to-weight and torque-to-weight ratios.

Here's wishing you a more informed buying. I look forward to your comments.