03 March 2008

Diesel Torque vs. Petrol Horsepower... what car is faster?

If your thinking of buying a Diesel you will want to know how it performance compares to petrol car with typically less torque but more or the same bhp. I am not an expert on this topic but will try and keep this short and explain what is best without having the complex equation.

The most simple minded way of thinking about torque and horse power are to consider them as two completely different and unrelated values (Forget the equation!). If you have a high torque (Diesel/Drag racers) car then you use the torque to accelerate at low revs and foot to the floor if you want to drive it fast. If you have a high bhp (Petrol/Formula 1) car then typically you must use its horsepower found at higher revs at the red line.

Case for horsepower!

Because torque levels are so low in a petrol car (unless you have silly 3.o+ liter engine) the driver has to increase the amount they rev (rpm) to accelerate fast. When you increase the amount of revs closer to the red line you go faster and the car feels more sporty than a Diesel but you can't help but think your thrashing the engine! Formula One cars have less torque than a high performance Diesel but they achieve 800bhp because the engine revs as high as 16,000 rpm.

Case for high torque!

Turbo diesels often have 3 times the equivalent torque if a petrol car in the same class. For example we shall use the Civic Trpe-R with 202Nm of torque compared to a Toyota Auris T180 with a massive 400Nm of torque. If both cars are casually driving and both put the foot down the Auris would be considerably quicker however if the Civic driver dropped down a gear the Civic would be just as quick (maybe quicker because the car is lighter).

Having a high torque will make your car feel more powerful than a petrol because it is pulling away at only 2,200-3,500 revs and as a result uses less fuel (all good for the wallet). Often the problem with Diesel cars is that the torque runs out too quickly and you have to learn to keep the torque inside the power band and this means lots of quick gear changes. Diesel cars especially can be chipped to increase both turbo and fuel to a much greater degree than petrol cars.

Diesel is my choice of car and it is not just a fuel saving (Expect around 45mpg) but insurance groups are 3 or 4 times lower even for sporty models and the tax band rarely exceeds band D.

Ok the Maths Equation

This look complicated where P is power, τ is torque, and ω is rotations per minute. But it does not need to be and the equation can be simplified to:

Horsepower = (Torque x rpm) / 5252

You can more information here:

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