Editing Variable displacement (section)

===Aftermarket systems===
A number of companies have developed aftermarket cylinder deactivation systems, with varying degrees of success. The 1979 EPA evaluation of the Automotive Cylinder Deactivation System (ACDS), which allowed eight-cylinder engines to be run on four cylinders, found that carbon monoxide and nitrogen oxide emissions were increased beyond the legal limits of the emission standards then in force.<ref name="acds">{{cite web|url=http://www.epa.gov/oms/consumer/devices/511817.pdf|title=Vehicles and Engines|first=US|last=((EPA, OAR, OTAQ))|website=www.epa.gov}}</ref> While fuel economy was increased, acceleration was seriously compromised, and the loss of engine vacuum led to a dangerous loss of braking assist when the system was in four-cylinder mode.<ref name="acds"/> In addition to these issues, while the company proposed a hydraulically controlled system that could be switched from within the car, the version they implemented had to be manually changed in the engine compartment using hand tools.<ref name="acds"/>