09-27-2013, 10:56 PM | #1 |
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135i worse mpg than 335?
My Quick background:
2002 WRX (was great, but a bit small for a new family) 2004 Forester XT I recently drove a new 128i and it handled quite nicely, even nicer than my WRX, although it was somewhat slow. Didn't feel much faster than my forester XT, granted my XT is a stick and the 128i was an auto. Anyways I was surprised that the heavier 335 automatic gets 32mpg highway, a decent upgrade from my current 25mpg or so. But the smaller/lighter 135 automatic gets 25 mpg highway. 32mpg/25mpg = 28% better MPG from the same engine in a heavier car? The 135i seems unusually low. Anyone know why? |
09-28-2013, 12:09 AM | #2 |
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Hi sbike,
I believe it is due to the 335 being more aerodynamic. The 335 has a drag coefficient of 0.3 where as the M135 has 0.33 which will make a difference at highway speeds. Also have a look at this thread http://f20.1addicts.com/forums/showthread.php?t=885943, a few people seem to be doing better than what it listed. |
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09-28-2013, 02:00 AM | #3 |
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But the 3-series probably has a slightly larger frontal area, which is also a part of the aero drag equation.
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09-28-2013, 05:25 AM | #4 |
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I'm not sure this is posted in the right place, as I didn't think the F20 M135i was even available in the US. So perhaps this refers to the previous 135i
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09-28-2013, 07:54 AM | #5 |
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Since we are here already, so
cc x A [m^2] F20 (M135i Spec) 0.33 x 2.14 = 0.7062 E82 (128i) 0.32 x 2.10 = 0.6479 (http://www.carinf.com/en/d260415601.html) E92 (M3) 0.31 x 2.17 = 0.6727 (http://www.carinf.com/en/8430418707.html) ------------------------------------------------ I am no expert, and I am not trying (and cannot, sorry) to answer the fuel efficient difference. I find this aerodynamic discussion interesting, probably tons of leads out there. There is a link that gives quick calculation of the drag there: http://en.wikipedia.org/wiki/Drag_(physics), and one can actually calculate roughly power lost of gained by effects of aero. That said, just fyi for M135i which does not go to the states I get 35 mpg or so on highway travelling 70 mph (max. legal for us) cruise controlled. Cheers~ |
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09-28-2013, 09:18 AM | #6 |
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The F30 seems to have a drag coefficient of 0.27, and frontal area of 2.20, giving a total of 0.594, so significantly lower than the F20.
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09-29-2013, 01:21 AM | #7 |
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Just for completeness we can calculate the drag force acting on the vehicles as so
Fd = 0.5 * p * v^2 * Cd * A where p is the density of air or approx 1.293 So if we compare the F20 to F30 with the values provides at 110Km/h we get F20 0.5 * 1.293 * (110/3.6)^2 * 0.33 * 2.14 = 213.13 N F30 0.5 * 1.293 * (110/3.6)^2 * 0.27 * 2.2 = 179.26 N |
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09-29-2013, 01:46 AM | #8 |
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I see you have used air density at zero degrees C. Not so common in Canberra, is it?
To get the power needed to overcome aero drag, multiply the force with velocity. In this case, 9 and 7 horsepower. The difference in power will be a lot higher at high speeds. |
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09-29-2013, 02:04 AM | #9 |
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Not trying to steal the thread here but, if you wish the calculate the difference in percentage, most terms cancel out, and the number is indeed staggering.
The logical conclusion is to fold up one's rear view mirror at high speed (sorry bad joke, really dangerous) I did however ask BMW why they block off the fog lamp area where intake would have been if intakes are on both sides. My ***very crude*** guess is gaining 20 hp at 200 km/h if air is well transferred there.... But, people designing F20 must be thinking about something else and I really wonder what they're thinking, especially they got that unlimited highways in Germany. Surely they are experts in this aero stuff, but man, maybe now is hindsight, but the F30 and the new A class sure have better numbers.... |
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09-29-2013, 04:13 AM | #10 | ||
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Quote:
Quote:
I also didn't take altitude into effect, but we would be here forever it we added every variable. |
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09-29-2013, 06:17 AM | #11 |
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As has been said, most people are actually getting better consumption figures from the 135i than the stats suggest. I don't think anyone buying a 135i is getting it for the fuel consumption though so if economy is a factor, get a smaller engine or a diesel.
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