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      07-15-2014, 02:44 PM   #1
Slimjim8201
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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, 02:55 PM   #3
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Optimal shift point is definitely big improvement over n54.
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      07-15-2014, 02:59 PM   #4
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Quote:
Originally Posted by GBPackerfan1963 View Post
Damn, there are a lot of propeller heads on this board. Just go out and drive the damn thing and enjoy it.
Guilty as charged.

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   #5
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Great post. Is there anyway to show graph RPMs, so we can see how you arrived at the optimal shift points? I'm not questioning your results, just curious to see reason.
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      07-15-2014, 03:01 PM   #6
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Quote:
Originally Posted by Slimjim8201 View Post
Guilty as charged.

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.
It's all good. Every board needs their technically inclined, engineer based evaluations of a car that just hauls ass.
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      07-15-2014, 03:15 PM   #7
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Quote:
Originally Posted by Audio Fan View Post
Great post. Is there anyway to show graph RPMs, so we can see how you arrived at the optimal shift points? I'm not questioning your results, just curious to see reason.
We can see the crossover points graphically in the plots, particularly the third graph. Once you know that speed, you can find out the engine RPM for a given gear. (Column A)

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      07-15-2014, 03:34 PM   #8
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So according to these charts, DCT wins.
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      07-15-2014, 03:44 PM   #9
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Quote:
Originally Posted by SOM3 View Post
So according to these charts, DCT wins.
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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Quote:
Originally Posted by GBPackerfan1963 View Post
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, 04:54 PM   #11
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Quote:
Originally Posted by Slimjim8201 View Post
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.
Thanks for taking the time to post. I would be interested to know what the optimal starting revs would be for DCT in gears 2-5 for optimum acceleration, I would take a guess at around 4000? If you can apply your scientific approach and give the values it would be appreciated!!
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      07-15-2014, 05:20 PM   #12
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Quote:
Originally Posted by Slimjim8201 View Post
We can see the crossover points graphically in the plots, particularly the third graph. Once you know that speed, you can find out the engine RPM for a given gear. (Column A)

Fun stuff. Thanks for posting.
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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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Quote:
Originally Posted by CanAutM3 View Post
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.
Rut roh Shaggy. Propeller head wars.

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      07-15-2014, 06:00 PM   #15
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Originally Posted by GBPackerfan1963 View Post
Rut roh Shaggy. Propeller head wars.

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      07-15-2014, 06:52 PM   #16
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Quote:
Originally Posted by GBPackerfan1963 View Post
Damn, there are a lot of propeller heads on this board.
Agreed, and it is one of the reasons this board is so interresting

Quote:
Originally Posted by GBPackerfan1963 View Post
Just go out and drive the damn thing and enjoy it.
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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Quote:
Originally Posted by CanAutM3 View Post
Agreed, and it is one of the reasons this board is so interesting



But I do, and at the rate of 18~20 track days a year, it is probably more than the majority here
Fixed and one of the little facets.

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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Quote:
Originally Posted by CanAutM3 View Post
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.
I tend to agree. If the dyno alters the shape of the torque curve, it can effect the shift point prediction. As I don't have access to the raw data or corrections, I've run the study with a second data point. Enter Sport Auto. I've grabbed the data and calculated the torque at the wheels. I prefer to use this data over OEM supplied curves as they have incorporated drivetrain losses. EDIT: The data below includes the drivetrain losses of the car as well as the drag of the Maha dyno. It appears that it will be exceedingly difficult to separate torque transfer losses and dyno drag, so I'll include both the dyno-influenced wheel torque and a another group of plots to show the flywheel torque. The three dyno-influenced plots below will predict LOW shift points due to the shape of the torque curve incorporating dyno drag. When I post the group of plots based on the flywheel torque curve, that data will predict HIGH shift points since no losses are incorporated.

In reality, the optimal shift points will fall somewhere in the middle.

Same three graphs as before
  • All gears
  • Gears 2-4
  • Optimal shift points







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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      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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Quote:
Originally Posted by CanAutM3 View Post
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

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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Originally Posted by Sparta1 View Post
I'm going to Montreal at the end of the month. Where do you track days? Tremblant? You ever go to james bay?
Tremblant is my main track. But I also track at Calabogie, Mosport and WGI. There are also the smaller occasional tracks like Sanair, St-Eustache and Shanonville.

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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