LINE PRESSURE VS. ENGINE TORQUE
... and what this has to do with the durability of your ZF6/ZF8 Auto-Trans!
Auto Transmissions can sometimes play bad with your engine tune, with the outcome of bad shifting, extended slip times or low torque in certain gears. But why is that?
One of the main inputs for the Transmission to do it's job is the engines actual torque. Around 50% of the maps in the TCU use actual engine torque (or some stuff calculated from it) as one of it's inputs. For example Line pressure (thats the base pressure the transmission operates it's clutches with) has a direct relation to engine torque. The higher the torque, the higher the pressure in the trans is and vice versa (of course this is limited within certain boundaries).
So where does it know this "Actual Torque" from? As a car obviously hasn't got a built-in Dyno to measure torque, it must be calculated somehow during operation. This is the job of the ECU. Simply put, it has various maps to factor the actual load situation (based on fuel delivery, boost pressure, ignition angle and many more) and output a calculated torque value from that. This torque value is transmitted to all other units via the Bus-System. One of the most important units (among others) to use that Torque Value is the TCU, you Transmission Controller.
And here's comes the bad news for people tuning their car: All automatic transmissions have "Torque Limits" stored in their TCU. Once the calculated "Actual Torque" breaches the maximum, the TCU will request the ECU to back down boost and stay within this limits. Sneakily, those limits are different per gear. For the ZF6HP the 5th Gear has the lowest value, for the ZF8HP it's 3rd and 6th Gear. (sometimes only 6th Gear) The outcome is power loss, as soon as one of those gears is engaged. (on tuned vehicles)
The common workaround for tunes to operate properly and not hit torque limits, is to "bend" the torque calculation in the ECU, so it well never calculate a value above the transmissions stock limits. Problem solved? Well, not really. Given what's said above, the trans will now operate with wrong clutch pressures. The torque calculation is calibrated to match real world torque very closely. If you alter this calculation, you create an offset between calculated and real world torque, which directly triggers the transmission to operate with too low line pressure.
The effects can be various and are not necessarily recognized instant. Let's suppose your 335i F30 makes around 450 Nm at crank from factory. With a typical Stage 2 tune you add 150 Nm so get 600 Nm at crank. The transmission used is the 8HP45. 45 stands for the suggested max. Torque -> 450 Nm. The electronic limiter sits at 465 Nm, so just a tad above stock torque level. Your tuner now adjusts the torque calculation, so it never triggers this trans limit. The offset between real world and calculation is 150 Nm, or nearly 30%. This means, your transmission will operate with approx. 30% lower pressures, as it would with adjusted torque limits and proper torque calculation.
Short term you may feel that shifting is sometimes a bit off, delayed or not as crisp as it should be. Long term this will lead to extended clutch wear as there is more slip on every shift and even in gear. (Microslip) Adapting the torque limits and using a tune with proper torque calculation will avoid all that AND will increase holding power of your transmission dramatically!
Especially important for people with upgraded Turbos, which are pushing the limits of their platform. The ZF transmissions are incredibly well built and can last forever, even when operated well above stock torque, as long as they are adjusted properly.
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