Thread: DCT tranny
View Single Post
      11-23-2012, 05:53 AM   #12
Major General

Drives: 340i M-sport AT
Join Date: Mar 2006
Location: Chicago

iTrader: (1)

Originally Posted by SamS View Post
This is simply untrue. BMW DCT can downshift without going through each gear. You give it judicious acceleration, and hold the downshift lever. It can go directly from 7th to 4th, no problem.
Yes it can. I shouldn't have said it can't.
I should have the point I was trying to make better, correctly.
I corrected that post to not leave incorrect info.
Also, Getrag makes the DCT not ZF. DOH!
ZF makes the dual clutch for Porsche.

A dual clutch can skip gears on downshift, but it can't skip like the ZF does, directly to the gear requested, depending on the gear it's in and the gear called for.
I've posted before on this forum about how the ZF AT works after researching it quite a bit.
That info comes from ZF info and other tech sources.

The reason why a dual clutch can't downshift as directly is because of how a dual clutch is made, using 2 shafts, one for even number gears and one for odd.
Lets say we're in 6th gear and you call up 4th, the first gear change is going to be to 5th on the opposite odd number shaft. It can't go directly to 4th because 4th is on the same shaft as 6th. Once in 5th, then the even number shaft can select 4th and engage. That's 2 shifts, and it's very fast.
If you're in 6th and call up 2nd, the dual clutch may be able to go to 5th first and then select 2nd on the even number shaft.
Although I'm not positive that it will do that.
If it does, then it's still just 2 shifts. If not, then it's more.

With the ZF AT in 6th and you call up 4th, it can go directly to 4th, one shift.

Upshifts are 200 milliseconds for a single shift, but downshifts can be as fast as 100 milliseconds.
If you have the AT give it a try, it's very cool. But, you have to be fast on the lever or the paddles.

ZF 8spd info:
Company product development director Dr Klaus Draeger explained that the eight-speed ZF is now able to shift as fast as a good DCT and, he told, offered other advantages too.

"If you want to shift quickly from 6th to 4th, or from 7th to 5th, you can do this in a single step with the eight-speed. On the DCT it takes longer as the shifts are sequential."

Sportier versions of the eight-speed will also be introduced, said Draeger, but he declined to say whether they would use a wet clutch launch device, as on the AMG Speedshift, which allows sharper shifts and higher engine revolutions.

"The torque converter is an excellent launch device," said Draeger. "It has many advantages: we close it out at just 1000 rpm, when the car has traveled only 2 to 3 meters from rest, so it's almost never involved. And besides, its performance stays exactly the same throughout the whole lifetime of the vehicle - we don't ever get warranty claims on torque converters."

This is from ZF and Alpina:
The SWITCH-TRONIC buttons, for use in the Manual (M) Shift Mode, are ergonomically and unobtrusively positioned on the
back of the steering wheel, and allow the driver to take complete control of the transmission. The system allows for multisimultaneous down-shifts in milliseconds, hereby the transmission will skip several gears depending on engine loads and
rpm. In this manner the 8-speed Sport Automatic Transmission with ALPINA SWITCH-TRONIC reveals itself as extremely
multi-faceted, providing the right shift characteristic for any and all situations.

This also from ZF:
Those who appreciate
agility will be happy to know that the most modern
adaptive shift strategies are used which enable
direct skip-shifts over multiple gears, even allowing
for extreme downshifts such as 8th gear to 2nd gear.
In spite of the dry facts, maximum driving pleasure
is guaranted.

Last edited by RPM90; 11-23-2012 at 05:58 AM.