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Old 05-01-2020, 12:08 AM
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Quote:
Originally Posted by Happy View Post
The 3.64’s on an M54 will require more energy (fuel).
Not true. Very broad understatement. By your logic, everyone should never shift out of first gear then. Which obviously doesn't make sense. You neglect the frictional losses of the engine that are inherent to speed as well as characteristics of a NA engine.

Quote:
Originally Posted by Clavurion View Post
This not that simple. You would have to know the engine's nominal consumption curve and it usually has a sweet spot RPM/load though on naturally aspirated petrol engine anything other than max revs is more or less restricted with pumping losses.
This is more/or less a correct statement. When calculating fuel mileage, it's fuel used per stroke per unit-distance...or miles per gallon inversely. So by this calculation, you would hold your distance value constant and road-speed constant when comparing different engine speeds between the two loads (two loads because two different "gearing" ratios we're comparing). I've done exactly this when sitting inside a vehicle with a laptop on a dynamometer in various gears. E.G. 55 mph in top gear vs 2nd from top gear. You'll see empirically, with data, that even tho the load is less in 2nd from top gear (due to gearing), the grams-fuel/per-stroke will actually show this, but not in a linear fashion AND ALSO not at the same rate due to engine speed, ergo, to travel the same distance at the same road-speed the engine is doing more revs from a lower gear and thus using more fuel EVEN THO the grams/per-stroke is slightyly less.

That's why Clavurion said there's a "sweet-spot" with this. Because conversely you can't get performance out of an engine by "lugging" it everywhere you go. At the same rate, you can't get efficiency near-redlining the engine everywhere you go.


Hopefully that makes sense.
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2003 BMW X5 3.0i -- MT5, 3.64s final gears, H&R lowering springs, K-Mac bushing kit
2007 BMW X3 3.0Si -- MT6
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