The Fart-Secret

What creates the Shift-Sounds, desperately wanted by many!

Besides the usual 0-60 and 60-120 times, there are little things discussed that much, as the distinct sound created by the more sportive gasoline (not Diesel!) vehicles during shifting. A lot of our support-tickets and social media comments somehow relate to the sounds (aka „shift-farts“) created during shifts. Usually people are complainig about „not having them at all“ or „my buddy’s car being louder“. With this article, we want to do a bit of a deep-dive on this topic, so our customers can get a better understanding what’s actually happening, what’s causing that sounds and why they are there (or not there) in the first place. First thing to know is, that those sounds aren’t created on purpose, they are just a by-product of a specific way to reduce torque during a shift, without slowing down the turbos. In automatic transmissions shifts are always carried out as overlap shifts, which means the offgoing clutch is opened at the same time, the ongoing clutch is closed. During the ratio change phase of the shift, the on-going clutch has to overcome the torque of the engine to pull down the RPM’s to the new gear. The more torque during this phase, the more pressure and friction is needed inside the transmission to shift to the next gear. As a consequence, the engine’s torque needs to be lowered during the ratio shift phase, to help the transmission complete its shift. The typical time of the ratio shift phase varies between 100 and 500ms, depending on how „sporty“ the shift is. A faster shift has a shorter ratio-shift phase and vice-versa. It’s this time-frame, where the transmission takes control over engine-torque and lowers it to an appropriate level for the actual shift carried out. Keep in mind, torque reduction is not a bad thing and on really fast shifts that phase is just 100ms (so 0.1 sec) long. That’s is a really short time-frame. Basically there are 3 ways to lower torque in a gasoline engine: 1. Close the throttle (or lower valve-lift in BMW valvetronic engines) 2. Retard ignition 3. Cut Ignition