# ZF 8AT vs Mercedes 9AT Anatomy

Open up a ZF 8\-speed and a Mercedes\-Benz 9\-speed automatic gearbox side by side and meet the converter, planetary gearsets, clutches and valve body hidden inside

By sunny\-moon\-1ce2a64b

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/eWJaIVRvLnVe)

## Transcript

### 1. Two gearboxes, side by side

In front is a ZF 8\-speed automatic gearbox; behind it, the Mercedes\-Benz 9G\-TRONIC with nine speeds\. Both sit between a car's engine and its rear wheels and change gear all by themselves\. Drag to turn them around\.

The ZF weighs about 87 kg and the Mercedes about 95 kg, including oil and converter\.

### 2. Lift the lids

Their aluminium cases, called housings, hide everything inside\. The top halves lift away so you can see parts that almost nobody ever gets to look at\.

Both gearboxes are built in a line: the engine joins at one end and the drive shaft to the wheels leaves at the other\.

### 3. The torque converter

At the front sits the torque converter, a sealed doughnut full of oil\. A fan inside, turned by the engine, pushes oil onto a second fan that drives the gearbox, so the car can stop without the engine stalling\. Once moving, a small clutch locks the two fans together so no energy slips away\.

This oil coupling does the job a clutch pedal does in a manual car\.

### 4. Planetary gearsets

Each gearbox has four planetary gearsets\. A sun gear sits in the middle, small planet gears circle it, and a ring gear with teeth on its inside wraps around them\. Hold one part still and the others turn at a new speed\.

There are not eight or nine separate gear pairs: the same four sets are combined in different ways\.

### 5. Clutches and brakes

Stacks of thin plates squeeze together to choose a gear\. A clutch, shown in purple, joins two spinning parts; a brake, in pink, holds a part still against the case\. The ZF uses five of these plate packs, the Mercedes six\.

Steel plates alternate with plates lined with a grippy friction material\.

### 6. Shift through eight gears

Press Shift up to step the ZF through its gears\. In every gear exactly three of its five plate packs grip, shown in colour, and the studs on the stand count the gear\. First gear turns the output about 4\.7 times slower than the engine; eighth turns it one and a half times faster\.

One pack lets go while another grips, so the power never stops flowing during a shift\.

### 7. Nine gears in the same space

From behind, the Mercedes shows its secret: nine gears from four gearsets and six plate packs\. Its first gear is lower, about 5\.5 to 1, and its top gear taller, about 0\.6 to 1\. It pulls strongly from a stop and cruises with the engine turning slowly\.

It fits in the same space as the 7\-speed gearbox it replaced\.

### 8. Oil, valves and a small computer

Under each gearbox, inside the oil pan, sits the valve body: a maze of oil channels and valves run by an electronic control unit\. It sends oil pressure to exactly the right plate packs, while a pump at the front keeps the oil flowing\.

The pans have been lowered a little so you can see the valve bodies\.

### 9. Two answers to one puzzle

Both gearboxes solve the same puzzle: keep the engine turning at its most efficient speed\. ZF builds eight gears from five plate packs; Mercedes builds nine from six\. More gears mean smaller steps between them, for smooth and thrifty driving\.

The ZF's lowest to highest gear spans about 7 times; the Mercedes spans about 9 times\.