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Old 06-28-2012, 10:01 PM
bmw_or_audi bmw_or_audi is offline
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Originally Posted by Robert A View Post
How do you quantify the fuel economy bump with the 4 cylinder? The 528i when from 22 to 23 city mpg when the 240 hp 6 cyl. motor was replaced with the new turbo 4. To me, that's no improvement.
You are not going to see big improvements in city mpg. In city driving, you are accelerating often from low speed, meaning you are mostly fighting the car's weight. Turbo or not won't make a huge difference. But during freeway cruising, you are barely using 20-30Hp (guessing). At these low power levels, the internal resistance of the engine becomes relatively more important, and the small displacement gives you a clear advantage.

So basically, when you are generating a lot of power, the engine's internal resistance robs a relatively small percentage of power, so small displacement buys you very little. When you are generating low power, such as when cruising at legal speed limits, internal resistance becomes more important in terms of power, so small displacement gets you noticeably more efficiency.
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