Subaru offered two all-wheel drive systems for the automatic transmission, called ACT-4 or VTD. The first system, called ACT-4 (active torque split) was introduced on the 1988.5 Alcyone using an electronic control unit to vary the torque applied to the rear wheels dynamically based on driving conditions. As an active system, it varied the torque split infinitely based on several inputs. This AWD system was used throughout the entire production run in vehicles manufactured for sale in the US, Canada, Germany, France and Switzerland. A more advanced system called VTD (variable torque distribution), was introduced and used on SVX for sale in Japan, the UK, the Benelux region, Sweden, Australia, Spain, Austria and Brazil. The VTD AWD system adds a planetary center differential. The system retains the use of the ACT clutch and active control, though its size and role are much smaller as it is used only to suppress differential action instead of the complete differential function. When no speed difference exists between front and rear, the entire VTD gear rotates as a unit and torque split is at a mechanical 36/64 biased by the planetary gear ratio. The clutch is incorporated to prevent and suppress any differential action that may occur as the planetary gear will send virtually all torque to the axle with the highest speed if not restrained. By using a similar logic to the previous ACT system, the clutch can theoretically direct any percentage of torque to either axle, but in practice the variation remains between 34-50% front and 64-50% rear. This system is not capable of operating in 2WD and therefore could not be used on 2WD dynos as required for emissions testing in some states. This prevented VTD as an option on Subaru vehicles offered for sale in US until passenger car regulations were changed, which occurred long after the end of svx production. It was later introduced to the US with the Outback VDC.