Launch Control

What it does and not does!

Since introduction of our first Maps for the F-Series x35i (N55) cars in January 2019, there is one question popping up constantly: "Does xHP add Launch Control (LC) to my car?" Short answer: Yes. But there's a more elaborate one as well. Today we'll be trying to tackle the most common questions and misunderstandings around the LC function. First, lets start what LC actually is. BMW added the LC mode to their vehicles around Summer 2013. (Press Release: bit.ly/2uUFCq4)

The Press Release from back then also describes "LC" as a measure to "accelerate with optimal drive slip". Thats a pretty broad description and does not tell what actually has changed from a technical point of view. From the driver's seat, the first difference is the fancy checkered flag in the dash, together with the sentence "Launch Control Active", when carrying out the procedure noted in the Press-Release. In a nutshell: Vehicle must be fully warmed up, steering wheel straight, doors closed, no trailer attached (yes...that actually gets checked), put your gear lever to "S", press the DSC button once or go to Sport+, press the brake pedal VERY hard, quickly go full throttle, release the brake after around 1 - 2 seconds. There you got it, your BMW accelerates. How exciting. For people having LC from factory, you can even read it up in the user manual on the iDrive ( https://bit.ly/2UGyiN4 ) and here's a Video from BMW explaining the usage. ( https://bit.ly/2FSdYPi ) Unfortunately I found it only in German, but I think you get the idea anyway.  (Note: The procedure is different on cars with Double-Clutch-Transmissions)

So now we know how to activate it, but what's going on under the hood? There are a few of the car's computers involved to make it happen, but most important are the DSC (Stability Control) and the Transmission. Both have to work together for an optimal launch. The DSC needs to be set to "Traction"-Mode. Initially, this mode was meant for Winter-Driving and is there on BMW's for ages, but has been of course refined more and more over time. It allows the wheels to slip quite a bit, before actually cutting power. Optimal Traction is generated in a tiny slip area, when the vehicle's tyres spin around 10-20% quicker than the vehicle itself is actually moving. The root sits in the chaining effects of the rubber with the underlying surface, which are optimal with some slip. Of course the optimal amount of slip and the grip in general vary with temperature, tyres, tyre pressure, surface etc. etc. Thats why it's so hard to match manufacturer 0-60 times with a stock car...they take care the outside conditions are perfect when measuring. Hard to re-produce on a random road on a random day. It's hit and miss. The second important unit is the transmission, taking care of optimal shift times and torque transfer on your way to 60 Mph. 

What the ZF 8-Speed Auto actually does in Launch Mode is to engage a different shift mode, which is only active when the aforementioned procedure is carried out. The characteristic of this mode can be described as "i-don't-care-about-my-clutches-mode". Yes, LC mode makes your transmission wear. Thats the reason, there is an undocumented limit of 50 launches built in to the ZF transmissions. After 50 launches the trans will silently never engage that mode again. How do we know that? Because we don't look at documentations, we reverse-engineer the transmission's code to understand whats going on inside. It's also that exact mode, that is added to pre-2013 cars, when they get flashed with xHP. Even if you have a BMW before Summer 2013, the flash with xHP will update the transmission to the most recent software, which has the LC routine built in. There's only one "downside": You won't see the flag in the dash still, as that flag is not triggered through the transmission. Eventually it can be activated on the older cars with some coding in other modules, but technically it does no difference, wether there is a flag or not. 

But there are more things to consider with LC, when running a car that isn't really stock. There's for example the procedure with pressing the brake and flooring the throttle. On a DCT transmission the trans would be in idle position in that situation and there wouldn't be any power-flow to the rear wheels unless you release the brake. On a torque converter Automatic like the ZF8HP, there is always power-flow to the rears, as soon as you are in D/S or M mode. This means when going full throttle the engine will be instantly transmitting torque to your rear wheels and the brake has to overcome that momentum to keep the car standing still. This works without issue on a stock car, but on tuned cars with lifted torque limiters the engine can actually outpferform the rear brakes and they will start to spin, no matter how hard you press the brake. This does not mean LC is broken, this does only mean your rear brakes can't hold the power anymore.

The solution is simple: Don't let the car boost that high, before releasing the brake. Just release the brake quickly, around 1 sec after going full throttle. This is also a major difference between LC mode on a DCT (Double-Clutch) Transmission and a Torque-Converter Auto. With the DCT the procedure holds a certain pre-set RPM (that's what creates that crazy sound on M-Cars) before dumping the clutch once the driver releases the brake. On a torque converter trans the engine just presses with maximum force against the torque converter until you release the brake. So the RPM you reach while standing still is not controlled. It's just the outcome of engine power vs. the torque converter. (means the RPM will vary depending on engine power) So once the car is tuned, it is to some extent the drivers responsibility to release the brake at the right time.

The next issue is the Traction Control during accleration itself. This is a tricky task as there is no perfect world, where each launch is the same. Actually the DSC has to monitor wheel spin hundreds of times a second and reduce/raise engine power all the time to keep the wheels in the optimal slip range. The engineers configure this process in the DSC for a stock car, with stock power level and stock suspension. Changing the tyres to a very different size, fitting an aftermarket coil-over suspension, or running 100 HP more than stock will all offset this calibration and in the end you can be left with a worse 0-60 than stock.

Last but not least, tuned vehicles offer a different power band, which has influence on the optimal Shift-RPM. Again, the RPMs to shift during LC mode are configured for a stock engine and yes, they will change with tuned engines. By how much depends on the engine tune. People with a Stage 1 tune will likely even benefit from lower Shift-RPMs compared to stock, people with bigger aftermarket turbos may need higher Shift-RPMs. This can only be determined on car by testing. (or by pre-calculating based on a proper dyno sheet)

All issues described above can be tackled with the Custom Module inside xHP. The "Torque-Limit per Gear" function takes care of the first two issues. It allows you to set the maximum allowed torque for 1st Gear. As standard, those limits are set to maximum (1000 - 1200 Nm@Crank) in our maps, but we already included presets for different situations, which you can use as a starting point to find your personal best setting. The last issue will be gone with the next release of xHP, where people will be able to set their Shift-RPMs during LC mode. Please note, that all of this works exactly the same on the ZF 6-Speed Autos, but they do not have the special super-hard shifting mode for LC.