Honda CR250 Cylinder

The CR250 went water cooled in 1981 and early models have a steel bore surface and no exhaust valve system. The 1984 & 85 models have the ATAC exhaust system, and 1986 models onwards have different versions of an HHP exhaust powervalve system and a Nikasil plated bore.

ATAC System: The Honda Automatic Torque Amplification Chamber system works by effectively increasing or decreasing the volume of the exhaust system with a small butterfly valve located just before the exhaust connection. At low RPM a centrifugal crankshaft driven gear opens the valve into a small chamber and increases the volume of the exhaust by allowing the exhaust gases to flow through the chamber. At high RPM the ATAC valve is closed and the exhaust simply exits into the expansion chamber. For a reason that is probably very clear to two stroke designers, a larger exhaust volume benefits power at low RPM and visa versa. The 1984 model had a remote chmaber on the exhaust header (as shown) and the 1985 model had the chamber as part of the barrel assembly.

The HPP System: The Honda Power Port system works by using a complex system of linkages to change the height of the exhaust port openings within the barrel. At low RPM the two sliding valves on the top edge of the exhaust port are closed. At higher RPM they begin to open which effectively raises the height of the port, until they are fully open at high RPM. This changes the exhaust valve timing depending on the RPM range which allows the best exhaust timing at any RPM.

Top View
Left Side View
Right Side View