It’s likely more work than you think it is…
The TCU (Transmission Control Unit) defines the way how a transmission operates and reacts to driver inputs. From a driver perspective the operation looks very simple. You stomp on the throttle and the transmission starts to up-shift the gears and downshifts them once you slow down the car. The complex task behind this, is that the transmission must pre-calculate constantly what the driver expects it to do next, as there is no formula to calculate technically correct Shift-Points.
Such a thing as a correct shift-point does only exist, if you focus on a single variable like optimal acceleration under full-throttle. In reality this is a multi-variable situation, where many things have to be considered like uphill/downhill driving, current brake-pressure, torque reserve of the engine in the current state, current steering wheel angle (cornering), current wheel-slip (e.g. ice or snow driving), special operating modes of engine/transmission (e.g. Particulate Filter regeneration, cold-start, overheat protection mode etc.), data from the Navigation System (e.g. tight corner or incline ahead), current gradient of the throttle operation by the driver and many, many more.
In a perfect world, the TCU could gather all those inputs, sum them up, apply a formula and spit out the perfect shift-strategy. Actually, that’s what OEM’s are doing to a high degree in their efforts to reduce cost. Many maps get “pre-calibrated” by the computer nowadays. Mapping starts exactly where the computers capacity ends, and that’s the fine line between “technically correct” and “perceived correct”. All those “hard” data points now get layered by the engineer doing the mapping with a certain target group in mind. The permanent question is, if the customer will deem a certain behavior, in a certain situation to be correct or not. What customer A sees as correct behavior, can be judged as unbearable action by customer B.
A simple example: Let’s assume, that for moderate acceleration on a flat surface, the transmission is programmed to up-shift all gears at around 1500 (Engine-)RPM. Technically this would be OK for most vehicles and would lead to an overall relaxed, high economy, low wear driving style. Nevertheless, that behavior will be perceived totally different in a 1-Series 4-Cylinder, compared to a 7-Series with 8-Cylinder engine. And even within the same vehicle, there will be certain customer groups judging this as wrong behavior and they will do so for varying reasons. In the aftermath of this article, there will be for sure vital discussion on Facebook about that. Manufacturers try to solve this, by implementing multiple driving programs into the TCU, e.g. BMW with the DEC-Controller and it’s Eco-Pro, Comfort, Sport, and Sport+ settings, which can be combined with the 3 positions of the Gear Shift Lever (D-Mode, S-Mode, M-Mode).
Each combination of those inputs can be configured independently (more or less) and activates different maps, that lead to different shift-strategies, which are of course again very different between vehicles. S/Sport+ in a M135i will behave different, as it does in a 550i and will again be different in a X5 M50d. It’s all about the specific vehicle and the target group driving with it. To give another easy example: Some cars are allowed to direct-shift from 8th to 2nd Gear when the driver mashes the throttle, while others aren’t. The transmission is always able to do it, but for some models the OEM decided to not allow such a huge jump in gears, as it could be seen as unfavorable by the planned target group.
Everything above only refers to the most obvious part of transmission control: The shift-points, so the moment when a transmission switches from one gear to another. Other control areas are the clutch pressures inside and outside of shifts, the clutch-timing on shifts (which greatly influence the shift-comfort) the slip of the Torque Converter Clutch, clutch-stress calculations, traction management during Launch Control etc. Overall a modern transmission controller sports around 15.000 maps. Many of which are only there to alter the behavior in specific situations, e.g. Towing. For instance, a customer living in a totally flat area and never driving in the mountains, won’t touch thousands of those maps. They just want come into play, until there’s an incline.
With the above said, it should become more clear what “mapping a transmission” means and why a single calibration can’t be used for all cars. In terms of xHP this means a Stage 3 file is mapped specifically for a certain car. There is no relation whatsoever between Stage 3 on a 330d and Stage 3 on a 330i. In some cases not even a M135i and a 335i can share the same setup. (for instance, if different base-programs are used by the OEM) This is foremost the reason, we sometimes can’t release support for some vehicles, although they look very similar to already supported ones, from a customer’s perspective.
Our customers (in most cases) are searching for a more sportive behavior of their Transmission and that’s what we have in mind, when doing the Maps. We remove compromises the OEM had to make for various reasons (for instance to keep distance to the next higher model), we re-think the logic of calibrations from scratch sometimes and we do our development on the street. This is a job that can’t be done on a Dyno. While the OEM has to keep in mind a broad range of target groups for each vehicle, we are able to sharpen up things as xHP customers can not only switch between 3 Stages, but can also alter things like the Shift-Points to their personal liking with the built-in Editor. For a Stage 3 map the main goal is of course always to deliver more raw performance, but this is in turn only a very small part of mapping. Way more time is necessary to re-define the overall behavior during street-driving, so that it leads to more fun with your car on a daily basis! Not only on the Racetrack, but also on the way there.