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      12-13-2011, 10:34 PM   #1
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E9X - Start Stop or Taller 7th Gear (DCT)

I was wondering if BMW should have used a taller 7th gear (DCT) instead of the start stop feature for better fuel economy.

Higher RPM's = Higher fuel consumption
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      12-13-2011, 10:53 PM   #2
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I think the start/stop was just done to pass whatever certifications were needed for the new model years. Anything that isn't automatically on isn't very productive.
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      12-14-2011, 09:46 AM   #3
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I was wondering if BMW should have used a taller 7th gear (DCT) instead of the start stop feature for better fuel economy.
The two aren't mutually exclusive, of course, so it's unlikely they'd trade one for another.

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Higher RPM's = Higher fuel consumption
Yes and no. There's an optimal RPM that trades consumption at a steady speed vs. consumption while accelerating or while under load changes. We could look at the extreme as an example - if you put in a gear to let the car go 70 mph at idle RPM the car would bog terribly on the slightest elevation change, grossly decreasing efficiency. In ther words the consumption vs. RPM curve is not linear. Did BMW hit the sweet spot? Maybe, maybe not. I suppose that depends on many factors, some of which will vary between driver, environment, etc. My guess is that they spend a lot of time on this stuff and probably arrived at the current gearing after much experimentation, data collection, and deliberation.

An 8 speed DCT might be part of the answer - those will probably be here sometime later this decade if DCT tech is not killed off in favor of a traditional planetary gearbox.
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      12-14-2011, 09:54 AM   #4
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so whats the optimal cruising RPM for our car?
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      12-14-2011, 09:55 AM   #5
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A different axle gear would change things as well.
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      12-14-2011, 10:09 AM   #6
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so whats the optimal cruising RPM for our car?
Popular opinion seems to be to keep the car at or above 2000 RPM to avoid bogging. But this is probably based more on maintaining non-stressed feedback from the car rather than how efficient it is operating.

Also, of course, there are only a couple reasonable cruising gears to select from at any given speed. So, to some degree, speed limits and traffic are going to decide the RPM of the engine for you. But there are still some choices. For example, in a 45mph zone, I might go 48 or 49 mph, and I would tend to choose 7th gear at about 1900 RPM (or whatever the exact RPM works out to be). Some people, however, might feel 7th is too stressed at that speed and choose 6th (some people only use 7th on the expressway, for that matter). Which gear is more efficient at that speed? I don't know. I suspect it is 7th but have no evidence for that.
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      12-14-2011, 10:14 AM   #7
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As an aside, I do recall folks reporting that they got better gas mileage while using S mode than D mode. I don't remember the details (like which Drivelogic settings they were using, for example), but it suggests that the computer logic for D might not be as good as a human can achieve. Whether the driver was choosing a higher gear, a lower gear, or some of each vs. what the D mode would have chosen - I don't know.
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      12-14-2011, 10:31 AM   #8
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Quote:
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so whats the optimal cruising RPM for our car?
I've seen 22mpg on freeway, cc @ 75mph. avg cruising mpg is about 20 (give and take) for me
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      12-14-2011, 11:54 AM   #9
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I'm guessing it was cheaper and easier for them to implement Auto Start/Stop (which they already had on lots of other cars around the world) since it's an almost purely software tweak with just a heavier duty battery and starter motor. Changing transmissions would have been added engineering, build changes, and then more parts to stock, and it probably would have annoyed more owners of earlier DCT cars too. And Auto Start/Stop gives any mileage improvement to 6MT drivers as well.
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      12-14-2011, 01:22 PM   #10
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A different axle gear would change things as well.
That would change the gearing for all gears though
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      12-14-2011, 04:18 PM   #11
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I believe the new EPA tests take Start/Stop into account so cars with this feature will show an improvement in MPG.

I don't think a taller gear will help with highway MPG. The issue becomes aero which is usually optimized around 60mph.

If we all really wanted to improve MPGs...eliminate ethanol in fuel.
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      12-14-2011, 05:44 PM   #12
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Quote:
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I believe the new EPA tests take Start/Stop into account so cars with this feature will show an improvement in MPG.

I don't think a taller gear will help with highway MPG. The issue becomes aero which is usually optimized around 60mph.

If we all really wanted to improve MPGs...eliminate ethanol in fuel.
Of course a taller gear helps in 7th as it keeps the rpms down and when there is no acceleration there is no such thing as "bogging" an engine down. It takes very minimal rpms to keep an engine going in a constant velocity such as highway cruising, however it will make 7th pretty much useless for actually accelerating which would be ok with me. Doesnt bother me either way but it would help. And yes staying at 65-70 does a lot for milage cmopared to 75 and above
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      12-14-2011, 07:33 PM   #13
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Of course a taller gear helps in 7th as it keeps the rpms down and when there is no acceleration there is no such thing as "bogging" an engine down. It takes very minimal rpms to keep an engine going in a constant velocity such as highway cruising, however it will make 7th pretty much useless for actually accelerating which would be ok with me. Doesnt bother me either way but it would help. And yes staying at 65-70 does a lot for milage cmopared to 75 and above
With this, I will put my trust in the BMW engineers. I hear you but if you get a gear that is too tall...you'll have to downshift for every little hill...not a problem for the DCT in D mode though.

I have considered this for my 330 6sp. I want to run a shorter rear and replace my 6th gear with a taller gear. So I get some extra low end grunt without screaming down the highway at 4000rpms or so.

I like the option of start/stop, although its mostly useless where I am at.

I would like the option to shut off 4 cylinders to save some fuel...wonder what the MPG gain would be?
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      12-14-2011, 10:11 PM   #14
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Some of the new chargers have that feature and it does not seem to bring down the mpg as much as I would have thought.

I don't know I really don't give a shit about mpg so I could care less. I would not be in an m3, which arguably is the worst mpg car available today save a lambo. Even a ferrari is getting similar mileage.

WIth that said, I had a 6 cylinder bmw for 4 years and averaged 15.9 mpg averaged over 4 years with mostly city and some highway driving. That is worse than I have gotten in my m3 so far but I drive in less stop and go. If you drive the 6 cylinders hard than they suck at milage too.

Really about how you drive it and if you lay off the accelerator than you get much better mpg, however what is the point? I redline it and hang out in 6k rpms and above almost 90 percent of the time so the powerband is instantly right there I did this on my 6 cylinder (went up to 7k rpms) and constantly rode the redline and the engine was as clean as a whistle! They did a completely cylinder head swap at 49k and the thing was spotless the techs said!
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      12-15-2011, 07:05 AM   #15
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Some of the new chargers have that feature and it does not seem to bring down the mpg as much as I would have thought.

I don't know I really don't give a shit about mpg so I could care less. I would not be in an m3, which arguably is the worst mpg car available today save a lambo. Even a ferrari is getting similar mileage.

WIth that said, I had a 6 cylinder bmw for 4 years and averaged 15.9 mpg averaged over 4 years with mostly city and some highway driving. That is worse than I have gotten in my m3 so far but I drive in less stop and go. If you drive the 6 cylinders hard than they suck at milage too.

Really about how you drive it and if you lay off the accelerator than you get much better mpg, however what is the point? I redline it and hang out in 6k rpms and above almost 90 percent of the time so the powerband is instantly right there I did this on my 6 cylinder (went up to 7k rpms) and constantly rode the redline and the engine was as clean as a whistle! They did a completely cylinder head swap at 49k and the thing was spotless the techs said!
IMO, MPG is one of the few weaknesses of the M3 as a daily driver. Not so bad here but I definitely felt the pain during euro delivery...$160 to fill up from a 1/4 tank.

That's the one area where the Vette and Caymna/Boxter have the M3 beat.
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      12-15-2011, 10:12 AM   #16
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...and when there is no acceleration there is no such thing as "bogging" an engine down.
That's not true at all. Bogging will be a direct effect of the load on the engine, which in turn is dictated by a number of factors including road surface, road condition, elevation change, etc.

If you disbelieve, put the car in a gear that yields very low engine RPM, set your cruise, and drive up a freeway onramp. The car will apply plenty of throttle, yet there will be no change in speed (no acceleration). Now, if you chosen a low enough RPM, prepare to experience *major* bogging.

That's one quick example off the top of my head. There are others that will give a similar experience.
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      12-15-2011, 10:16 AM   #17
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As an aside, I do recall folks reporting that they got better gas mileage while using S mode than D mode. I don't remember the details (like which Drivelogic settings they were using, for example), but it suggests that the computer logic for D might not be as good as a human can achieve. Whether the driver was choosing a higher gear, a lower gear, or some of each vs. what the D mode would have chosen - I don't know.
Anecdotally, I would agree with this. The automatic mode tends to be more aggressive with the downshifts, even in the lowest settings, than a driver would be while cruising.
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      12-15-2011, 10:21 AM   #18
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With this, I will put my trust in the BMW engineers. I hear you but if you get a gear that is too tall...you'll have to downshift for every little hill...not a problem for the DCT in D mode though.

I have considered this for my 330 6sp. I want to run a shorter rear and replace my 6th gear with a taller gear. So I get some extra low end grunt without screaming down the highway at 4000rpms or so.

I like the option of start/stop, although its mostly useless where I am at.

I would like the option to shut off 4 cylinders to save some fuel...wonder what the MPG gain would be?
No offense, but all these elements are really unnecessary in a car that should have performance in mind above all else. All of these technologies add weight and complexity to a car that is already very heavy and complicated.

The M3 is an all-round sports car, but if you try to make it TOO 'well-rounded', you will end up with a vehicle that is not really good at anything (much like the X6M).
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      12-15-2011, 10:22 AM   #19
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so whats the optimal cruising RPM for our car?
Between 8,000 and 8,300 RPM, of course.
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      12-15-2011, 10:41 AM   #20
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so whats the optimal cruising RPM for our car?
I run at 2,900 to 3,200 when cruising (just about 10% above posted speed limits - overtaking 18 wheelers etc). Milage under those driving conditions is around 10.8L/100 KM to 11.2L/100 KM on my car
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      12-15-2011, 10:42 AM   #21
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Between 8,000 and 8,300 RPM, of course.
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you are quoting the enterprise cooling system here yes
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      12-15-2011, 10:44 AM   #22
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I run at 2,900 to 3,200 when cruising (just about 10% above posted speed limits - overtaking 18 wheelers etc). Milage under those driving conditions is around 10.8L/100 KM to 11.2L/100 KM on my car
You run at that RPM even on lower speed surface streets or rural highways (like 45mhp/55mph limits)?

Or just on the expressway where the minimum achievable RPM is dictated directly by your speed?
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