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The basic reason for replacing v6 and v8’s with smaller displacement boosted 4-cylinder engines is fuel economy. Aside from going to Hybrid powertrains, there is not other way to get such large improvements in fuel efficiency (more than 10%). The basic reason why this works well is that the average amount of power that has to be delivered to the driving wheels is very low compared to the peak power needed for heavy acceleration or hill-climbing. A large sedan only needs about 10-12 HP sent to the drive-shafts to maintain 65mph on a level road. When you throttle a big engine down to 10 HP, it runs very inefficiently. Throttled engines waste energy in the form of pumping work, and large heavily throttled engines also waste a lot of energy in the form of internal friction and heat loss. It is much more efficient to get the same 10-12 HP out of a small-displacement engine that can be boosted for peak performance. It may seem counter-intuitive, but opening the throttle increases efficiency!
Modern 3 and 4-cylinder turbocharged engines are designed and built for excellent durability at full-load. The other reason why many folks have a preference for a 6 or 8 cylinder engine is that they expect less noise and vibration, compared to a 4-cylinder. This is no longer true due to modern methods of isolating engine vibrations through sophisticated motor-mount and driveline technologies. Try any modern BMW, Lexus, or Mercedes 4-cylinder vehicle and you will see what I mean.
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