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      11-17-2011, 11:15 PM   #1
MPBK
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Fuel consumption discrepancy?

Why is it that the official fuel consumption (gas mileage) for the 135i and 335i (using the same engine) are so different when you compare manual versus auto transmission?
From BMWUSA (http://www.bmwusa.com/default.aspx)
For 335i


For 135i



Notice that (on highway) for the 335i, the auto transmission is more efficient (better gas mileage) than the manual. So why in the 135i the auto transmission is so much worse than the manual (even worse than the 335i, which is a heavier car)?
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      11-18-2011, 12:17 AM   #2
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The DCT on 135s doesn't have an overdrive.
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      11-18-2011, 10:00 AM   #3
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Quote:
Originally Posted by Groundzero View Post
The DCT on 135s doesn't have an overdrive.
DCT is already 7 speeds... what does it need? 8?
Notice that the city gas mileage is also worse... doubt the overdrive affects city driving much.

Last edited by MPBK; 11-18-2011 at 10:17 AM.
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      11-18-2011, 10:26 AM   #4
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I just noticed the gear ratios for both cars... the auto transmission in the 335i has significantly taller ratios than the one in the 135i. Bummer that they had to do that. Crippled the 135i's fuel economy.
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      11-18-2011, 10:57 AM   #5
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Quote:
Originally Posted by MPBK View Post
I just noticed the gear ratios for both cars... the auto transmission in the 335i has significantly taller ratios than the one in the 135i. Bummer that they had to do that. Crippled the 135i's fuel economy.
I'm pretty happy that they didn't put taller grearing in my 135. I'd rather have the slightly better acceleration than a couple more miles per gallon. Didn't buy the car for fuel economy.
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      11-18-2011, 12:01 PM   #6
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I'm almost positive that the quoted 3.46 differential ratio for the 135i DCT is an error. If that were the case, the car would be turning something like 2,800 rpm @ 60 mph which is higher than than my 128i 6 speed MT in 5th!. IIRC, some DCT owners have reported rpm @ 60 as closer to 2,000 rpm which would mean a FD ratio of about 2.6, given the direct drive 7th gear. Lee123, what is your rpm in 7th at 60 mph?

Edit: The 335is coupe with DCT has a 2.56 FD ratio but the fuel milage is still not greart at 17/24.
http://www.bmwusa.com/Standard/Conte...fications.aspx

And since the OP used the the 335i sedan's Steptronic figures, apparently the DCT extracts a significant mpg penalty in both 1 and 3 series.

Tom

Last edited by Tom K.; 11-18-2011 at 12:07 PM.
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      11-18-2011, 12:27 PM   #7
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Quote:
Originally Posted by Tom K. View Post
I'm almost positive that the quoted 3.46 differential ratio for the 135i DCT is an error. If that were the case, the car would be turning something like 2,800 rpm @ 60 mph which is higher than than my 128i 6 speed MT in 5th!. IIRC, some DCT owners have reported rpm @ 60 as closer to 2,000 rpm which would mean a FD ratio of about 2.6, given the direct drive 7th gear. Lee123, what is your rpm in 7th at 60 mph?

Edit: The 335is coupe with DCT has a 2.56 FD ratio but the fuel milage is still not greart at 17/24.
http://www.bmwusa.com/Standard/Conte...fications.aspx

And since the OP used the the 335i sedan's Steptronic figures, apparently the DCT extracts a significant mpg penalty in both 1 and 3 series.

Tom
Thanks, Tom. That explains the discrepancy.
Yeah, the DCT is what sold the 135i to me, but I wish it were more efficient, as the Step, which can extract more miles than the manual.
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      11-18-2011, 12:32 PM   #8
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Quote:
Originally Posted by MPBK View Post
Thanks, Tom. That explains the discrepancy.
Yeah, the DCT is what sold the 135i to me, but I wish it were more efficient, as the Step, which can extract more miles than the manual.
I'd be curious as to the reason that the DCT cars seem to be 10~15% thirstier than the 6 speed Steptronic, given similar gearing. Especially since there is no torque converter in the DCT!

Personally, I'll stick with my MT with only 6 speeds...

Tom
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      11-18-2011, 12:35 PM   #9
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Quote:
Originally Posted by Tom K. View Post
I'm almost positive that the quoted 3.46 differential ratio for the 135i DCT is an error. If that were the case, the car would be turning something like 2,800 rpm @ 60 mph which is higher than than my 128i 6 speed MT in 5th!. IIRC, some DCT owners have reported rpm @ 60 as closer to 2,000 rpm which would mean a FD ratio of about 2.6, given the direct drive 7th gear. Lee123, what is your rpm in 7th at 60 mph?

Tom
I'd love to answer that question, Tom, but I don't remember exactly and car is still in the shop while they try to get the new radio working. If I get it back later today I'll check and let you know. I will say it's much closer to 2,000 than 2800.
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      11-18-2011, 05:08 PM   #10
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Got the car back and it's right at 2,000 rpm at 60 mph.
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      11-18-2011, 05:21 PM   #11
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Quote:
Originally Posted by Lee123 View Post
Got the car back and it's right at 2,000 rpm at 60 mph.
Thanks, Lee.
Looks like DCT technology has a lot of room to improve. There's no reason why it can't be more efficient than a manual, like the Steptronic is now.
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      11-18-2011, 07:40 PM   #12
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Quote:
Originally Posted by MPBK View Post
Thanks, Lee.
Looks like DCT technology has a lot of room to improve. There's no reason why it can't be more efficient than a manual, like the Steptronic is now.
So the DCT 135i has a 2.56 ratio and not the 3.46 posted on BMWUSA.com. BTW, the 335i with Steptronic has a 3.46 FD which makes sense as both 5th and 6th are overdrives.

But I disagree that the Steptronic is more efficient than the manual as they have identical mpg ratings for the 128i coupe (18/28) and the MT is significantly quicker from 0~60 - by more than a half second.

http://www.bmwusa.com/Standard/Conte...fications.aspx

But the question of why the DCT gets such poor gas milage when mated to the x35i motor remains...

And for what it's worth, the only M car available with the DCT will be the upcoming M5. The 1M, and M3 are 6sp manuals and the M-trucks (X5M and X6M) have only the 6 speed Steptronic.

Tom

Last edited by Tom K.; 11-18-2011 at 07:47 PM.
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      11-19-2011, 04:14 AM   #13
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We're really only talking about a difference of a few hundred RPMs here. I'd need to see some math before I'll join y'all on any bandwagons about this causing significant loss of fuel economy.
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      11-19-2011, 05:47 AM   #14
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Quote:
Originally Posted by Tom K. View Post
But I disagree that the Steptronic is more efficient than the manual as they have identical mpg ratings for the 128i coupe (18/28) and the MT is significantly quicker from 0~60 - by more than a half second.
Well, the Steptronic in the 128, 135 and 335 have completely different gear ratios. We don't know why BMW chose the gearing that it did. There may be factors other than gas mileage.
The fact that in one application, 335i, it results in better gas mileage than the manual shows that it can be more efficient, and that the inefficiencies of a torque converter can be overcome.
I'm just saying that the DCT has much to improve if it's not matching the manual yet, given that it doesn't even have a torque converter.

I agree that the question on gas mileage remains.
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      11-19-2011, 10:32 AM   #15
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The only logical explanation is gearing unless you want to theorize that the DCT clutch slips all the time (which is almost certainly not true because it would fail quickly if it did). Modern torque converter automatics lock up in top gear. So they do not have losses in top gear like the old non-lock up conventional automatics did. This means that the steptronic is as efficient in delivering power once in top gear as a manual or DCT transmission. But engines nearly always get better mileage turning slower. There is friction in the engine that is a function of how fast it turns. So if you spin the engine faster in top gear, you will get worse gas mileage.

It is not uncommon to have an automatic rated higher mileage than a manual. If you check out the final drive ratios you will find the automatic is spinning slower in top gear than the manual. My theory is that car designers do this because they are afraid manual drivers, like me, will complain if they have to downshift to pass somebody quickly. The automatic downshifts itself.

BMW has just set up the DCT for quicker acceleration than the steptronic. They may also worry that drivers will leave it in manual mode and complain about the acceleration in top gear.

If you eliminate gearing, a manual transmission will always get better mileage than a torque converter automatic. The torque converter has looses which are shed in the cooler ahead of the radiator. A manual, or a DCT, does not have these looses. But gearing is a bigger factor, especially in the highway rating.

A DCT will have some loss for the hydraulic system that shifts. But that should be pretty minor. If there is a cooler then I am wrong and it is significant. If there is no cooler on the DCT then the loss is not large enough to affect mileage more than something on the order of a tenth of a mile per gallon.

If it matters, yes, I am a mechanical engineer by education.

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      11-19-2011, 11:37 AM   #16
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Quote:
Originally Posted by JimD View Post
The only logical explanation is gearing unless you want to theorize that the DCT clutch slips all the time (which is almost certainly not true because it would fail quickly if it did).
Jim
Agreed, but Jim, the 3 trannies have the following overall ratios in top gear:

135i DCT: 2.56 (2.56 x 1.00)
Relying on the 2k revs 60mph Lee reports and this is BMW's spec for the 335is DCT

135 MT: 2.62 (3.08 x 0.85)

335i E92 Step: 2.39 (3.46 x 0.69)

So the 135i has essentially the same overall gearing in top gear and should return similar EPA highway results for both transmission options.

So does the EPA "highway cycle" spend more time in the intermediate gears? Or are both of the DCT clutches are slipping???

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      11-19-2011, 12:41 PM   #17
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I've had a DCT for a year and the clutch does not slip except perhaps slightly when starting. That's probably why the throttle is applied slowly and the clutch engages slowly when attmpting a full throttle standing start without the launch control. I drove a 1978 280z with slipping clutch for 2 years, being too broke at the time to get it fixed. Attempting a fast start from a light felt like it was doing a burnout without any tendancy to fishtail. RPMs would go way too high for the speed and the car would gradually catch up to it. It would not spin the tires on dry pavement, the clutch would slip. Same thing starting off on a steep hill. It was sort of like continually driving on slightly wet pavement. The slipping is not constant, it is related to load and very noticable for that reason. Anyone who thinks the DCT clutch slips all the time should try the launch control once. Even if you let the car go at 2500 rpm there is no doubt that the clutch takes hold quickly and firmly. I wish there was a way to get about half that effect at other times since the launch control is way too much for pulling into traffic, but that's another subject.

Last edited by Lee123; 11-21-2011 at 10:50 PM.
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      11-21-2011, 09:52 PM   #18
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Another problem with blaming the lack of torque lockup converter in the DCT is that that only matters for highway MPG. Around the city you're not going to be cruising at 70.
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      11-21-2011, 11:05 PM   #19
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Quote:
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Another problem with blaming the lack of torque lockup converter in the DCT is that that only matters for highway MPG. Around the city you're not going to be cruising at 70.
I'm not sure what you're referring to here...
DCT doesn't have a torque converter. It's a manual transmission, so it uses clutch plates. They always lockup, by design.
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      11-22-2011, 10:12 AM   #20
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Even more to the point, then.
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