Air Equipment - General Information Section 12 - Air Equipment - Group 12 M11 Page 12-4 E-Type System With Air Dryer Vehicles equipped with air dryers vented to the atmo- sphere during unloaded compressor operation, using the Holset E-Type air compressor, require the installation of an econ valve to prevent excessive oil consumption. E-Type System Without Air Dryer Air systems without air dryers, or with air dryers not vented to the atmosphere during unloaded compressor operation, can use the Holset E-Type unloader valve with- out modifying the air system.