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2013 BMW 7-Series engine (BMW)
2013 BMW 7-Series engine (BMW)

You & Your Car

Torque and horsepower: What’s the difference? Add to ...

The BMW 740i has a turbo-charged engine. I find it to be powerful and a better drive than the 730i. Am I imagining it or is there a difference? Also, in terms of breakdowns, would the turbo engine pose additional problems? – Sarjit

The seat of your pants is pretty accurate. The 740 is indeed a significant step forward on the performance front compared with the older 730.

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Both vehicles are/were available with a number of gasoline and diesel engines in various markets. To ensure an apples-to-apples comparison, let’s equip them both with the available 3.0-litre inline six-cylinder gasoline power plant. The engine in the 740i is turbocharged and uses direct injection. It produces 315-horsepower peaking at 5,800 rpm and more importantly, in relation to your question, peak torque of 332 lb-ft at 1,300 rpm.

The older six in the 730i produced 254-horsepower at 6,600 rpm and 221 lb-ft. of torque at 2,500 rpm. Published tests show the 740i able to accelerate from rest to 100 km/h in 5.7 second while the 730i required 7.8 seconds.

Much of the difference is a result of the more powerful engine, obviously. But the newer car has an eight-speed transmission and the older one six speeds. The extra gears help during acceleration.

What you feel and commented on is not the horsepower, but the result of the extra torque of the new engine and the fact is reaches its peak at a much lower engine speed.

To simplify things, think of torque as responsible for acceleration and horsepower for top speed. Torque is the measured power or output of an engine, horsepower is a number obtained through a calculation based on the torque. The 740i engine not only has more of the “power” you feel, it is at its maximum just above idle where you first sense it as you press the accelerator.

The turbocharger forces air into the combustion chamber under pressure. The direct injection maximizes the distribution of fuel within that “chamber.” The combination results in a more powerful explosion when the gas/air ignites, pushing down harder on the piston, creating more power or torque. Turbochargers are mounted in the exhaust stream. The exhaust gasses spin one side of an impeller on their way out of the engine.

The other side of that impeller is mounted in the intake side where its spinning motion forces more air into the engine. The turbo operates at extremely high temperatures and requires a steady flow of oil to carry some of that heat away. There are no reliability issues with modern turbocharged engines as long as the manufacturer’s maintenance schedule is followed.

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