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DCT Bible, Chapter 2- Transmission Hardware

Updated: Jan 4, 2023

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!



Transmission Versions

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 $1500-2000 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 $3000 to $4000 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.


The first iteration comes from the 2006-2013 E9X M3 models and has a 1:1 top gear, like the 135i/335i DCT transmission. There stand out by the starter portion which is positioned right at the top. These usually range from $1500-2000.


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 $2000-2500. The bell housing is the most large on this one, so they are not desirable for swaps that have tight transmission tunnels and engine bays.


Full Gear Ratio Chart

Attaching It To Your Engine

At this time, people are making all sorts crazy combinations of adapter plates and flywheels to match their chosen engine and transmission combination. We carry a huge selection of adapter plates on our online store. If we don't have it posted please reach out, we do custom adapter combinations all the time. The most important part of the adapter kit is the sprung hub flywheel we offer. This sprung hub dampens the engine pulses that shock the clutch basket over time until it fails. You should choose the torque level closest to your goal. Transmission NVH becomes louder with higher rated sprung hubs. The top rated sprung hub can handle over 1000ft/lbs. of torque.


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 does not work well and can harm the clutch basket.





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.



DCT Driveshafts

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 HTG GCU can control the OEM parking brake module if you like, but it is not required if you use our park lock system.



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 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.






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. Ours is the lowest profile 360 degree option available that allows full flow without the restriction of banjo fittings. A budget option is to weld a AN bung directly to the transmission case.



The DCT has a fluid pump, 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. We pair this with a thermostat and a E54 water-to-oil cooler in the front. This is the ultimate track package. We recommend water-to-oil coolers when possible as they regulate temperature well and are very efficient. These coolers are 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 get that info from the HTG GCU, or install a sensor into the side of an aluminum oil pan.



Oil Filters

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.



Oil Pans

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.



Conclusion

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!










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