Too often considered as up-selling, an optional modification, it is actually much more important than you might imagine.
Let's address, in a general aspect, the importance of reprogramming an automatic gearbox (TCU) in addition to the engine control unit (ECU).
Torque Limitation:
This is often THE most important point when it comes to performance. Most gearboxes are limited in torque, either globally or sometimes only on specific gears. For example, with ZF 8HP gearboxes, try measuring the torque on the 5th or 6th gear, and you'll notice (depending on the engine) the significant difference in torque.
It depends on gearbox technologies; some are well-sized to accept the increase in torque provided by a stage 1 tune, while others are not. Let's take two concrete examples:
Renault gearboxes (1.6/1.8/2.0 TCE) have a specific operation, and the gearbox limiter directly affects a significant reduction in pressure. This means that if, at any moment, your torque demand exceeds the maximum nominal value, there will be a regulation of boost pressure, not only reducing performance but also affecting the vehicle's behavior.
For example, between 3000 and 4500 rpm, the gearbox limit is reached, even for a few tenths of a second, and the boost will be drastically reduced, until the regulation is released again.
On a Porsche Macan 2.9T (Turbo / GTS version), there's also a specificity: it's the only vehicle in the VAG group with this engine equipped with the DL501 gearbox (while Audi RS4/5, Panamera, Cayenne... use the ZF8HP gearbox), and we can see the consequence of this change.
Looking at this vehicle with a stage 1 tune and the original gearbox:
Like the Renault example, we see a "flat" curve undergoing regulation. Now, look at the log and see the significant throttle closure here in blue.
Now, after reprogramming the gearbox, we get a nicely filled curve, with the throttle wide open.
These are just two examples among thousands.
Torque Rev/Match:
Linked to torque limitation, this is a strategic point. Some of you might say, "I never reprogram the gearbox, yet I manage to exceed the max torque values." In many cases, that's true.
Why? Because the gearbox and the engine are coupled in a certain way with a CAN signal, a torque calculation information sent from the engine ECU to the gearbox (to simplify). In many cases (when the customer doesn't want to tune the gearbox or if the gearbox simply isn't reprogrammable), one can cheat, meaning deceive (or even cut) the torque information sent to the gearbox. In this case, it might, for example, think that the distributed torque is only 400Nm when it's actually 500Nm. So, this method can bypass the torque limitation.
But... it's not ideal. The reality is that a gearbox is generally perfectly calibrated with the engine, for both performance and driving pleasure. So, by using this method, one exposes themselves to issues like abrupt gearbox reactions, lack of smoothness, etc.
The ideal scenario is to have a perfect engine/gearbox match in terms of engine speed and torque range. This way, not only will we maintain perfect and pleasant gearbox operation, but we'll also significantly increase performance by creating a perfect shifting with immediate throttle response, achieving the ideal harmony between the engine and the gearbox.
Clutch Pressure:
Here, we'll talk more about DSG gearboxes. There are two main families of gearboxes:
Gearboxes with torque converters: Those are the most common these days. This technology is very reliable, accepts large amounts of torque, and ultimately outperforms most dual-clutch gearboxes on the market in terms of performance. Some manufacturers still leave doubt to their customers (BMW mentioning "DKG8" or Porsche with the "PDK" of the latest 992), but in reality, they are conventional automatic gearboxes.
Dual-clutch gearboxes: These are the die-hard ones, and they are still around, especially at VAG with the famous "DSG" or "S-tronic" gearbox. On these, there's a clutch system (which varies depending on the model), and through engine mapping, it's possible to modify the oil pressure applied to these clutches. This can not only provide more performance/speed but also increase torque tolerance and limit friction. This type of modification is almost essential for any stage 3 tune, for example.
There are models where we strongly recommend DSG gearbox reprogramming, like the Golf 8R / Audi S3 8Y, as we quickly reach the original tolerance limits, even with just a stage 1 tune. Reprogramming it will not only provide better performance but also much healthier operation.
Additional Options:
Depending on the models and versions of gearboxes, it's also possible to modify many little things that can delight your customers. Activating or changing a launch control's rpm settings, increasing gear shift speed, paddle response, etc.
We can also change the general gearbox operation: shift strategy at mid-load, modify downshift in sport mode for a more aggressive driving experience, etc.
Also, for gearboxes not originally equipped with it, remove the automatic gear shifting in manual mode, meaning each gear will reach the rev limiter without shifting to the next gear.
As you've understood, we STRONGLY recommend gearbox reprogramming whenever available. Many tool manufacturers have recognized this importance and have been working hard in recent years to offer reading-writing protocols. In addition to an additional sale to your customer, this will be a mark of professionalism.