Search
  • SeemsLegit Garage

So you want a DCT... Chapter 1

The world is a crazy enough place, and now you want to swap a 7-speed transmission that shifts in 50 milleseconds into the car in your garage you call a "project car" as an excuse for it never being done? Well...we agree, that sounds like a great idea!



So what is a DCT and why is it so awesome? Well for one, purists will never like it because it doesn't involve holding a knob and shifting in a H-pattern. However, what it does do is make you go faster by letting a computer control the shifting instead of your un-coordinated arms and legs. DCT stands for Dual Clutch Transmission, and since it has two clutches it can enable a perfectly timed clutch “hand-off” where the clutch of the current gear opens as the clutch of the next gear to be engaged closes. This also makes it much faster than a conventional automatic. A dual-clutch transmission is a transmission with at least two clutches, one for the even-numbered gears and one for the odd-numbered gears. Much like with a sequential transmission, to shift through the gears you must go in order and cannot "short shift" (Example: change from 2 to 4 directly). Some consider the DCT a type of automatic transmission since the driver does not apply the clutch during a normal shift, while some consider it a manual as it doesn't use a torque converter and mechanically operates like a standard manual transmission. We just call it awesome.


With precise control of the releasing clutch and the engaging clutch for the next gear implemented via a computer, smooth shift quality approaching that of a conventional automatic transmission can be achieved. More importantly though, shift times like a true racing sequential can be achieved using this much more robust style transmission without any of the insane maintenance associated with true a sequential (i.e, worn dog rings). DCT's also have parasitic losses similar to a manual transmission, which are significantly lower than a conventional automatic transmission because of the benefit of on-demand pumps, use of splash lubrication, and minimized clutch drag losses since only two clutches are used.


The average driver of a conventional manual transmission shift times are about 500 ms - 1 s (vertical gear changes e.g. 1st-2nd, 3rd-4th, 5th-6th): 1s - 2s horizontal gear changes (2nd - 3rd, 4th - 5th, 6th - 7th) when not in anger. Shift times can change depending on gear throws (distance between gears), ergonomics of lever, age of gearbox (technology and synchro condition) and how abusive the driver is feeling. No matter how you look at it though, a professional racing driver still takes around 250-300ms to shift on his best day when driving in a straight line and focusing just on shifting in a proper race car with a lightweight flywheel. A BMW DCT can do this anywhere, such as the middle of a corner, without upsetting the suspension, in around 50 ms. This is even faster than the 150ms required for a Ferrari Enzo to shift! Plus you get an extra gear!


So if these are so great, why don't people use them more often? Well, they have required OEM electronic controller architecture which is very hard and expensive to implement in a vehicle it wasn't designed for. From the factory the DCT relies heavily on CAN messages to learn about whats going on from other vehicle modules such as the Engine Controller, Brake Pedal Shift Paddles,and more. It then takes into account all of the external factors and makes shift and clutch pressure strategy decisions based on a computer inside the transmission. If missing any CAN signals, the transmission can easily not act as it should and can even go into limp mode. All is well in this world though, the aftermarket has come up with a solution to bypass this overbearing and overcomplicated OEM system. We can take the onboard controller out of the equation and essentially hotwire directly to a few sensors and the shift solenoids inside the transmission. From there, the aftermarket transmission control module (TCM), which is mounted in the car like any normal ECU, can control the shifting based on just a few easily inputted signals such as accelerator pedal position (0-5V), brake pedal status, and RPM. Fun fact, we can even implement a 0-5V electronic clutch pedal or steering lever (Like on a F1 steering wheel) to override the DCT clutches and perform conventional standing starts, clutch kicks and burnouts!


The aftermarket ECU's on the market have more upside than just ease of control. They can even make the transmission hold more power. The BMW DCT can usually hold around 450ft/lbs of torque when controlled by the factory TCU. Not bad, but this is limited because the factory TCU limits how much hydraulic pressure the transmission can exert on the clutch packs. If you need more holding power you need to upgrade clutch packs, which can get pretty expensive. No worries if you make more torque than 450 ft/lbs though, the aftermarket controllers have an ace up their sleeve. These don't have to listen to some desk jockey's "limits" and can safely increase the clutch pressure, effectively increasing the holding power of the clutches like a clutch upgrade would do in a standard transmission. The aftermarket TCU's increased hydraulic pressure can make holding up to 740 ft/lbs of torque possible on factory clutches. With upgraded clutch packs over 1000ft/lbs of torque has been tested without slippage. To say these transmissions are strong is somehow still an understatement.


All in all the DCT gives you lightning fast shifts, some insane holding power and the ability to slap some paddles in your rust bucket. Come back for the next chapter of the DCT journey to see how easy it is to swap into your car!







1,190 views