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      03-18-2008, 08:28 AM   #89
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Originally Posted by godot View Post
There's another big factor that I didn't see anyone mention: The Corvette's manual tranny has a magnetic gate that forces you to skip from 1st to 4th gear, except when accelerating hard. So in casual driving, you physically cannot enter 2nd or 3rd. (Of course, a lot of people find this massively obnoxious and disable it with an aftermarket wiring harness.) Presumably, the software for the automatic tranny does something similar.

As to highway mileage, coeff of drag is <0.29 in the Vette vs. 0.32 in the M3. Part of the tradeoff of having more passenger and cargo room, I suppose.
Are you sure about this? I have driven both the the ZO6 and MY2008 C6 and did not find this to be the case even once. These were new, un-sold cars at the dealership.
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      03-18-2008, 11:31 AM   #90
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Originally Posted by swamp2 View Post
Good about gearing.

Still about shift times? Are you positive?

What about some evidence on the driving style to promote the best mpg? I am not trying to challenge you on this I am very genuinely interested and again I have heard the same exact argument. Doesn't it make a ton of sense that if your contention about how to best obtain mph in a meat machine/MT vehicle is valid then a very likely conclusion is that shift times must be a big part of the gain?

As far as skipping gears that really should not be a problem. Gear changes to the same shaft are likely to be slightly problematic (by that I simply mean about as fast as SMG) but any changes, up or down like 1<->4, 2<->5, 3<->6 and 4<->7 should be easily possible. It just mean a different synco is moved but all clutching and other operations are identical. For instance we do know already than in D1 the car acutally starts in 2nd gear, combine that with a skip right over to 5th and that seems like a good start to a lot better mpg. As well we know on the performance side that there is a multi-shift "kick down" shift pattern that will drop as many gears as required to obtain maximum acceleration. Whether or not BMW will offer gear skipping patterns specifically to obtain improvements to mpg is entirely another question. If all it took was another D mode option accessed by the Drivelogic mode lever/button I don't see why that would not be available. Hell call it the green or hippie tranmission setting! As long as guys like us get S5/S6 I can still hope for a D=hippie setting as well.
As far as the best way to drive for maximum fuel economy..... The theory is that you consume less fuel by accelerating briskly (but not WOT) to your cruising speed rather than accelerating slowly to get to your crusing speed.

I did a search last night on some of the crazy people who are mpg freaks and found this:

http://www.crxmpg.com/accelmpg.html

Accelerating briskly will yield a ~3% improvement in fuel economy versus accelerating slowly.


As for the shift times, they matter for pure acceleration but for fuel economy, I don't think they matter at all (or at least it is noise).

If MDCT puts in a "Hairy Hippie Legs" mode and it shifts 1-3-5 then sure, this will be more efficient than 6MT. Short of a special alogrithm for fuel economy for MDCT, there should be no difference between MDCT and 6MT (save the gear ratios).
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      03-18-2008, 01:52 PM   #91
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Quote:
Originally Posted by T Bone View Post
As far as the best way to drive for maximum fuel economy..... The theory is that you consume less fuel by accelerating briskly (but not WOT) to your cruising speed rather than accelerating slowly to get to your crusing speed.

I did a search last night on some of the crazy people who are mpg freaks and found this:

http://www.crxmpg.com/accelmpg.html

Accelerating briskly will yield a ~3% improvement in fuel economy versus accelerating slowly.


As for the shift times, they matter for pure acceleration but for fuel economy, I don't think they matter at all (or at least it is noise).

If MDCT puts in a "Hairy Hippie Legs" mode and it shifts 1-3-5 then sure, this will be more efficient than 6MT. Short of a special alogrithm for fuel economy for MDCT, there should be no difference between MDCT and 6MT (save the gear ratios).
It is only a hypothesis but I still believe the shift is important. It is not easy to figure out if the brisk acceleration/gear skip thing saves from more operating time at higher efficiency or from the losses during the deceleration and shifting as I described previously. At this point we each have our theories but mine is stonger because I acknowledges both effects, you insist that the shift time can not be significant without any evidence other than opinion/speculation. Much DCT/DSG marketing information does support the gains purely from shift time effects as well.
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      03-18-2008, 02:02 PM   #92
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Quote:
Originally Posted by swamp2 View Post
Well I think we are disagreeing. My point is that the entire shift system, most notably the shift time will noticeably contribute to the efficiency gains had by M-DCT. You clearly stated nearly the exact opposite to that. Do note again my list abouve WAS NOT in order of importance. I am not prepared to speculate on the proper order.

I have also heard about the high rpm in 1st then shift to a very high gear for the best fuel economy. Sounds possible but also a bit counter intuitive. If that is true it would also seem to strongly support my theory that the shift time is a critical piece of the mpg gains of M-DCT. Do you have some references/actual test data on this topic, I would be keenly interested? Also, don't you think it would make sense for one of the automatic modes of M-DCT to be an efficiency mode, and if a driving method like you mention really does work, this mode could implement something along those lines? Sure it probably would not be best to implement it absolutely literally if it requires always shifting at 6k rpm from 1st then over to 5th or 6th, but the algorithm could apply lessons/results from the method to improve mpg and to let the silicon computer do the work. Surely your contention was not that you (i.e. the meat computer) can best silicon in a pure contest of mpg? That is a bit contradictory...
Somewhere around 25 years ago or so, BMW reported on their own method for best economy, and if memory serves, they recommended using large throttle openings and minimal rpm. Something like three quarter throttle and 2,000 rpm shift points. The rationale was to reduce pumping losses (therefore large throttle openings), and also reduce overall friction levels (therefore shifting at very low rpm).

The downside, of course, is that you get this lunge-pause-lunge thing going, which is not very comfortable for either the driver or passengers.

It seems to me that DCT may allow that driving style while still providing smoothness.

Imagine! You get to haul some ass while explaining to your S.O. in the right seat that you're saving money, reducing foreign oil imports and attacking the global warming phenomenon, all at the same time.

Cool.

Bruce
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      03-18-2008, 02:38 PM   #93
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Quote:
Originally Posted by swamp2 View Post
It is only a hypothesis but I still believe the shift is important. It is not easy to figure out if the brisk acceleration/gear skip thing saves from more operating time at higher efficiency or from the losses during the deceleration and shifting as I described previously. At this point we each have our theories but mine is stonger because I acknowledges both effects, you insist that the shift time can not be significant without any evidence other than opinion/speculation. Much DCT/DSG marketing information does support the gains purely from shift time effects as well.

To discredit your theory in favor of mine around shift times.....

The steptronic has quicker shift times than a human and you don't see better fuel economy. Sure there are other factors like gearing, weight, drivetrain losses etc... To really think 0.5 seconds for 1st to top gears shift times will result in any tangible fuel savings....the logic doesn't flow....

The quicker you can get your engine to stoichiometric air:fuel ratio of 14.7:1, the quicker you can put the engine in optimal fuel conserve state for a low-load cruise. This is the key to fuel "economy" for the M3. Which means stepping on it until you hit cruise speed, which means 1 gear change for any legal speed limit in most countries..... 0.3 seconds is not going to affect fuel economy either way specially since you are off the throttle for shifting.
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      03-18-2008, 03:21 PM   #94
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Quote:
Originally Posted by T Bone View Post
To discredit your theory in favor of mine around shift times.....

The steptronic has quicker shift times than a human and you don't see better fuel economy. Sure there are other factors like gearing, weight, drivetrain losses etc... To really think 0.5 seconds for 1st to top gears shift times will result in any tangible fuel savings....the logic doesn't flow....

The quicker you can get your engine to stoichiometric air:fuel ratio of 14.7:1, the quicker you can put the engine in optimal fuel conserve state for a low-load cruise. This is the key to fuel "economy" for the M3. Which means stepping on it until you hit cruise speed, which means 1 gear change for any legal speed limit in most countries..... 0.3 seconds is not going to affect fuel economy either way specially since you are off the throttle for shifting.
That is reasonable. All except that the shift time gains from Stept. are almost for sure countered and eclipsed by the higher losses it has. As well if the gear ratios are not identical (did not check but very unlikely) then you have a confounding effect that must massively weaken (or totally remove) such evidence.

If you are not doing any shifting in your drive schedule for comparison then of course shifting can not logically play a role. However, don't forget particularly in city driving how much shifting actually occurs, .5 s x big number = reasonable amount of time. Further don't forget the losses during shifting are multi faceted as I posted. You have to lift, and reapply the throttle, there are losses right there. The car definitely decelerates during shifting which is energy totally lost and has to be regained by accelerating, which in turn burns much more fuel than cruising by a large margin. There are some losses from the clutch as well. This debate is testable, just by a DCT owner, but would require great care to get unquestionable results.
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      03-18-2008, 04:51 PM   #95
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Help!!! I think I inadvertently drifted onto the Prius forum. J/k

Actually I find it all intersting.

If the E92 has two MPG displays like my E36 I will not reset one fo the first 1200 miles or so and report back with my actual mileage on my DCT.
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      03-18-2008, 07:42 PM   #96
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Quote:
Originally Posted by devo View Post
Are you sure about this? I have driven both the the ZO6 and MY2008 C6 and did not find this to be the case even once. These were new, un-sold cars at the dealership.
The Corvettes do have a CAGS system that does sometimes force a 1st to 4th shift, but as you experienced, it does not happen all that frequently. Plus, you can buy a simple after market part for about $25.00 that disables the "feature" if it becomes too much of a problem.
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      03-23-2008, 10:33 PM   #97
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Originally Posted by T Bone View Post
The S65 / S85 BMW engines achieves maximum volumetric efficiency very high in the rpm band, which means it doesn't make power until higher in the rpm band. This is great for acceleration when coupled with a close ratio gearbox.

Fuel economy was determined by BMW's preference for high revs ala F1 style. Nature of the beast.

My fuel bill for the M6 and 335xi last year was ~$6000.
  • My M6 had 13.4 mpg (17.6 l / 100 km) over the past 15,000 kms. Sucks gas like crazy.
  • My 335xi has an average of 11.0 l /100 km (21.4 mpg) over the past 5000 km.
  • My E46 M3 had an average of 12.2 l / 100 km (19.3 mpg) for the 50,000km that I had it.
  • My E36 M3 had an average of 10.8 l / 100 km (21.8 mpg) for the 60,000 km that I had it.
I predict the E92 M3 will give most people 16 mpg. The best bang for the gas buck car that I have had is the 335xi......

See this thread....

http://www.m3post.com/forums/showthread.php?t=128525

16 mpg as I estimated As more data points roll in, it may improve to almost 17. M-DCT will not greatly improve this.
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      03-23-2008, 11:40 PM   #98
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Still better than the M6. I could not manage more than 11.5. M5 is like a hybrid coming in at 15.
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