Chevrolet Introduces 6-Speed Auto Transmission to Small Car Class in Sonic Sedan

  • 6-speed auto transmission a first for Chevrolet in small car segment
  • Innovative packaging provides compact form for transmission
  • Adaptive shift control allows for engine braking
  • Excellent spread of ratios allows for brisk acceleration and excellent cruising economy
  • Precision manufacturing processes reduce noise levels

Chevrolet Sonic Sedan

The introduction of the Chevrolet Sonic Sedan to South African brings with it a first for Chevrolet in the small car segment with the availability of a 6-speed automatic transmission as an option on the 1.6 LS model.

Chevrolet Innovative design provides compact form

The Chevrolet Hydra-Matic 6T30 is an advanced electronically controlled automatic transmission designed specifically for small car applications.

The key concept behind this all-new 6-speed auto transmission is the compact design and packaging of the unit. By placing the gearsets on the same axis as the engine crankshaft centreline Chevrolet transmission engineers have been able to realise a far more compact form for the transmission, particularly in the fore-to-aft dimensions.

Rather than “fold” the transmission around the end of a transversely mounted engine, a normally bulky installation, the 6T30 transmission has its gear train arranged in line with the crankshaft centreline.

Direct advantages of this compact form are the ability for the chassis engineers to enhance the crush zones at the front of the vehicle, increase interior space, and lower the bonnet line.

To save even more space the traditional “freewheel” gear change mechanism is replaced by a multiple clutch system with gearshifts accomplished by engaging and disengaging clutches simultaneously during gear shifts. Sophisticated electronics enable the precision timing required to synchronise the clutches.

An exception is the shift between first and second gears where there is a large difference between the ratios. In this instance a regular “freewheel” mechanism is used to provide a smooth transition.

Chevrolet Sonic Sedan 2012

Chevrolet Quick acceleration combined with efficient cruising

The selection of a 6-speed transmission has allowed a wide range of ratios to be used to optimise performance and economy. The first gear has a high ratio to facilitate good acceleration from rest. Conversely sixth gear is an overdrive ratio that allows for low engine revs at highway cruising speeds.

The electronic control of the transmission provides for adaptive shift control including automatic gradient braking. This instructs the transmission to remain in a lower gear rather than shift up if it senses that the vehicle is decelerating or coasting on a downgrade. This reduces the need for the driver to apply the brakes during a hill descent. The transmission control module receives inputs from sensors that monitor brake usage, vehicle acceleration rate, and throttle position.

The torque converter used in the Chevrolet 6T30 transmission uses a single plate lock-up clutch which provides a mechanical coupling to the drive with no slippage and minimal power loss. The oval, or hyper-elliptical, form of the torque converter reduces the overall thickness of the component for an additional space and packaging advantage. GM’s electronic controlled capacity clutch (ECCC) technology assists in dampening vibrations.

The helical gears used in the transmission are precision ground and honed for the finest of tolerances. With closer tolerances the gears are less prone to producing transmission noise in the form of a whine or humming sound often associated with gear trains.

Chevrolet Proprietary transmission fluid lasts lifetime of transmission

The Chevrolet 6T30 automatic transmission fitted to the Sonic Sedan is filled with GM specification DEXRON-VI fluid that in normal usage will not need to be changed over the life of the transmission. This fluid was developed to have a very consistent viscosity profile for more consistent shift performance in extreme conditions. It also is highly resistant to degradation over time.