07-15-2014, 02:44 PM | #1 |
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Transmission Gearing Comparison
Note: I've used the Motortrend dyno results as the basis for my data.
Noticed a few topics discussing the merits of the two transmission options. No one has really gone into any technical detail regarding changes in gearing so I'll take a stab at it. The graph below compares the effective torque at the wheels between the 6MT and DCT transmissions. Differences in the ratios are much larger in gears 1-3 than in the higher gears. Shorter overall gearing results in significantly higher wheel torque with the DCT for gears 1 through 3, but this is not the whole story. If you take a look at the areas of the graph highlighted in blue, I've pointed out speeds at which the 6MT results in higher wheel torque. This occurs due to a combination of the DCT car requiring an upshift due to the shorter gearing and the engine torque drop off at higher RPM. Since no one really uses first gear, I've excluded it from the next plot. Gears 5 and 6 are also omitted since they are effectively the same between the two transmissions. Obviously, gear 7 is gone as well. I think this is a more "real world" comparison. The advantage of the DCT becomes clearer and you can see that there are only a few spots between 20 and 110 mph where the 6MT will be putting down more torque. Specifically, 58-70 mph in second gear and 87-98 mph in third. It's nearly a draw in the 54-58 and 84-87 mph ranges. I'm not sure this means much in actual day to day driving, but the comparison is interesting. The DCT does weigh more than the 6MT, but faster shifts may offset this disadvantage. With the DCT, BMW has shortened gears 1-4 and added a seventh when compared to the 6MT. After charting the data, I noticed something interesting. With both transmissions, it makes sense to short shift in gears 3+. This is shown in the following graph. Optimal shift points appear to be as follows:
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07-15-2014, 02:52 PM | #2 |
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Damn, there are a lot of propeller heads on this board. Just go out and drive the damn thing and enjoy it.
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07-15-2014, 02:59 PM | #4 | |
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I think the moral of the story is that both transmissions are well suited to the car. The engine's torque curve will make it very easy to drive very fast without the need to nail a maximum RPM shift. If someone is trying to set a record at the track, they might want to shift a bit early in gears 3 and 4.
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07-15-2014, 03:01 PM | #6 | |
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07-15-2014, 03:15 PM | #7 | |
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07-15-2014, 03:44 PM | #9 |
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Can't make that conclusion. According to the data, the GEARING of the DCT results in very slightly more torque at the wheels over the usable operating range of the car. However, many factors are not accounted for. We don't know the true impact of the additional weight. We don't have a feel for the parasitic losses between the two transmissions. My data assumes that losses are equal.
I think the only thing that we can say with certainty is that that a DCT car will be easier to drive fast. Frankly, I think the optimal shift point data is the most useful outcome of the study.
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07-15-2014, 03:53 PM | #10 |
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07-15-2014, 04:54 PM | #11 | |
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07-15-2014, 05:20 PM | #12 |
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07-15-2014, 05:46 PM | #13 |
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I found a major flaw in the analysis just after reading the first sentence.
You cannot take the reading of a chassis dyno to establish optimal shift points. Even more error when using a roller dyno (the 4 wheel dyno used by MT makes it even worse). The shape of the power/torque curves is altered due to losses (mostly tires rolling losses) and inertial impacts. You should redo the exercise using the official power/torque chart published by BMW. |
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07-15-2014, 05:51 PM | #14 | |
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07-15-2014, 06:00 PM | #15 |
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07-15-2014, 06:52 PM | #16 |
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Agreed, and it is one of the reasons this board is so interresting
But I do, and at the rate of 18~20 track days a year, it is probably more than the majority here |
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07-15-2014, 07:07 PM | #17 | |
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Nice. My plans also being retired and all. Thunder Hill, Laguna Seca, and Infinion are my tracks of choice. |
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07-16-2014, 08:31 AM | #18 | |
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In reality, the optimal shift points will fall somewhere in the middle. Same three graphs as before
A quick review of the Sport Auto data suggests that the two gearboxes are about equal regarding max torque at the wheels. As you work your way up in speed, they trade max torque back and forth (blue boxes show speed ranges in which the 6MT gearing produces higher values).
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Last edited by Slimjim8201; 07-16-2014 at 02:02 PM.. |
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07-16-2014, 02:39 PM | #19 |
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Here are the plots using flywheel torque. Two items to point out when comparing these to the previous plots above. The optimal shift points appear to be at or very near redline in gears 3 and 4. Approximately 7350 RPM. Short shifting 5th to 6th at 7,000 RPM is predicted to be optimal. Also, these flywheel-based plots show a clear torque advantage for a DCT-equipped car in the 0-130mph range. Over that range, there exists a total window of 21 mph where the MT will put down more torque at the wheels, only 16% of the time between 0 and 130 mph.
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07-16-2014, 02:49 PM | #20 |
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Optimal Shift Points
Summing up the data, we have two sets of predicted optimal shift points. One set excludes drivetrain losses, the other includes drivetrain losses and dyno drag. Without knowing the exact impact of the dyno drag, we can only speculate that the optimal shift points will reside somewhere in the middle of the data range.
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07-16-2014, 04:08 PM | #21 | |
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I'm going to Montreal at the end of the month. Where do you track days? Tremblant? You ever go to james bay? |
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07-16-2014, 04:22 PM | #22 | |
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James Bay? Heck no. Way too cold and nothing to do up there. I hope you'll enjoy Montreal, cool town with plenty to do and see. |
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