DCT Bible, Chapter 2- Transmission Hardware
Updated: Jan 6
So now that we went over how capable the DCT is, let's go over how you're going to get it in your car!
The BMW 7-Speed DCT, aka the GS7D36SG, comes in four versions. They are essentially the same except for gear ratio and bell-housing combinations. They all feature the same input spline, filters, oil pan and weigh about 175 pounds. For reference, the bell-housing is the part of the transmission where it bolts to the engine.
The first version of the GS7D36SG came in the 2009-2013 135i, 2008-2013 E9X, and 2008-2016 Z4 E89. The bell-housing is the standard for BMW straight 6 engines, and has the starter off to the side to fit in the area created by the BMW slant-6 engines leaning to one side. At this time this is the most common and inexpensive DCT option. They can usually be found for between $750-1100 on eBay and even less from from certain scrap yards if you haggle a bit. This version features a 1:1 top gear ratio, similar to the popular ZF 5-speed used in E36 and E46 models for years, so it makes for a great swap. It also features a 105mm bolt circle output flange tripod, while all the rest feature a 110mm bolt circle tripod. The full gear ratio chart can be seen below.
The next most common DCT is also the slant-6 style with the starter off to the side. It came in the 2012-2018 F80 M3, 2013-2019 F82/F83 M4, and the 2014-2019 F87 M2. These look the same as the earlier 135i/335i model and have the same bell-housing bolt pattern. However, they feature a longer gear ratio set. The top gear ratio is .671:1. These units are usually more expensive, ranging from $1300 to $2000 on eBay. As mentioned before, it's usually best to haggle a scrap yard for the best price and not go through eBay.
The next two options feature a different bell housing as they came standard on BMW V8 engines instead of the slant-6 options. So, instead of having the starter off to the side, they put it right on the top in the center. The starter sits right in the engines "V" between the cylinder heads on factory engines.
The first iteration comes from the 2006-2013 E9X M3 models and has a 1:1 top gear, like the 135i/335i DCT transmission. These usually range from $1100-1500. Its cousin comes from the 2010-2016 F10 M5 and 2011-2018 F06/F12/F13 6-Series. This DCT version features a .671:1 top gear ratio like the F80 M3 transmission and are usually priced between $1600-2000.
Full Gear Ratio Chart
Attaching It To Your Engine
At this time, people are making all sorts of adapter plates and flywheels to match their chosen engine and transmission combination. Most of these manufacturers are smaller shops, build in small quantities and make the adapter plate and flywheel together to match and have correct input spline depth. It is important to choose your adapter setup before you decide on a transmission type as there is a wide selection of adapters for almost all popular engines.
If you have a BMW engine and it's a straight 6, you're in luck because chances are with a little grinding and welding, the DCT box can bolt on and all you need is a flywheel. The grinding required is to slightly open up the two top mounting holes. The welding is required to add a bolt hole for the original starter to bolt on. You can either build up welded material and then drill a hole, or cut off a section and then weld on a section from an older transmission.
Another option for older BMW 6 cylinder engines is to run a N5X 8 bolt flywheel and a starter spacer with the N55 style starter. The reason you need a spacer is because the N55 block spaced the starter back, and you use it to re-align the starter gear with the teeth on the flywheel like factory. Note: The flywheel teeth style is different between the older S5X/M5X engines and newer N5X style engines. They are not compatible and you cannot mix-and-match their starters and flywheels.
For non-BMW engines it is common practice to bolt a spline adapter to a standard flex plate from an automatic transmission of the engine you're using. This works great, is usually low cost and light-weight. However, you must make sure you stay compatible with the adapter plate if one is used, and be wary if you use a inline-4. The 4 cylinder engine harmonics are very hard on the transmission clutch baskets and can lead to cracks and eventual failure. It is best in these cases to use a flywheel with some mass to it so you don't have any issues. This is also why the original DCT flywheels from BMW are pretty dang heavy and not the same as an automatic flex plate. If you have an Inline-6 or V8 it's much easier to get away with this though, as these engines run much smoother.
If you have any questions about adapting a DCT to your engine, BMW or not, please shoot us a message and we would be happy to support you with the best options currently available.
So now that your DCT is attached to your engine, you have to get the power to the differential. The DCT comes with an odd tripod output flange. This originally went to a rubber guibo and then to a BMW 2-piece driveshaft. We found the best way to create strong and affordable driveshaft setup is to use one of our DCT flange adapters which bolt to the tripod and then lets you attach a standard 1350 style U-Joint yolk. Then, we can easily make you a single piece driveshaft with a slip-shaft (since there is no slip yoke) to suit whatever rear end you are using like a BMW M3 or Ford 8.8 differential, for example. We have the adapters and yolks for many differentials in stock. Please contact us with any questions about making the perfect driveshaft setup.
Parking Brake Options
The BMW DCT features a small parking brake lever that locks the transmission when the engine is off. When the engine is idling/running the transmission has oil pressure and this lever automatically lifts itself up, taking itself out of "Park" and into Neutral. This is great in a perfect world, however, if you ever break down, get towed or need to push your car on a trailer you are out of luck. The good news is that the aftermarket controllers may be able to control the stock parking brake module soon, but in the meantime, to get around this you have a few options.
There are a few "locks" available. These bolt to the transmission and the parking brake lever, keeping it in neutral all the time. With these you don't have the option to put the car in Park. However, it is is simple, lightweight, and works in some race/track car situations.
The factory parking brake motor can also be mounted up. It drives a small cable to control the parking brake. Controlling this module can be as easy as a 12V switch. The downsides are that it's kind of bulky and you may need a longer cable depending on where you can fit it.
We believe our solution is kind the best of both worlds. We are hoping it will be available very soon. We tried to keep it as simple as possible so it can be easily implemented in almost any build. It's a cable attached to a handle with a mount that has two positions to place the handle. If you keep the handle in the top position, the vehicle stays in neutral if the engine is off. If you place it in the lower position the transmission enters park when the engine is turned off, and drives like normal when the engine is on. This is nice because you get to retain the park feature, don't have to wire up any electronics, and can place it anywhere in the vehicle as it's adjustable and comes with a long cable. Prototypes shown below
Oil Cooler Systems
The DCT is cooled by the oil that also lubricates it. It is important to keep the DCT in the proper temperature range for the best reliability and performance. The DCT has two ports on the side of the transmission, these are where the oil comes in and out. The lower port is the "Oil Out" and the upper port is "Oil In". There are a ton of adapter options to attach a standard AN hose directly the transmission and either face froward or back for any kind of oil cooler install. A budget option is to weld a AN bung directly to the transmission case.
The DCT has a few fluid pumps, and there is no need for a scavenge pump setup to run your own cooler. The DCT will pump the fluid through the cooler without any problems. We have a large cooler in the trunk that draws air through the back windows, via a used NASCAR duct, and out the bottom of the car via a puller fan. This cooler is available in the universal section of our store. Regardless of use, it is important to at least have a small cooler for your DCT's fluid.
The optimal operating temperature for the DCT is around 175F +/-15. If you cannot reach this temperature, you should install an oil thermostat in-line between the cooler and transmission ports to help it achieve the proper temp. If you have a track dedicated car, this is usually not necessary. To monitor temps you can either tap into the aftermarket controllers fluid temp sensor and take a reading for you digital dash/gauge, or install a sensor into the side of an aluminium oil pan.
There are two oil filters in the DCT. The one in the pan is the primary filter while the round filter that is inserted in the side of the transmission case is the pressurized filter. This filter removes any contaminates that the pumps may create before the fluid gets to the clutches. We recommend changing the filters whenever you install a used transmission. A stainless mesh pressure filter is available from SSP. This is sort of like a K&N filter; it's good for a lifetime as it can be cleaned. However, it's pretty expensive and stock replacement style filters are cheap.
The factory oil pan is made of brittle plastic, has a plastic plug, and uses an expensive seal. Case in point- It's a typical BMW cover that warps and leaks all over the place like a BMW valve cover. We at SLG designed some great replacement pans. They are made from billet 6061 aluminum, include stainless hardware and a re-usable high-temp viton seal. They also feature a magnetic flush mount and pipe threaded drain plug that you don't have to worry about breaking every time you touch it with a tool.
We have two versions of oil pans. The first is the slim pan that is best used on vehicles that don't have much ground clearance. This pan is the same height as the OEM pan. It has a bullnose finish on the inside and out to increase surface area and promote oil cooling without decreasing ground clearance. Our larger pan is much deeper and has large internal baffles that reduce oil aeration and act as a heat sink. On the bottom it has a bunch of cooling fins to further promote heat as much dissipation as possible. The OEM pan holds about 1150ml, slim 1500ml, and the "Big Momma" 3000ml.
I hope chapter 2 has helped you better understand the working parts of the BMW 7 speed transmission. In Chapter 3 we will go over how we can control it!