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. A larger expansion chamber allows for more power at lower RPMs because of the extra time needed for the impulse to "bounce" back for the supercharger effect. It was used on their CR motocrossers, GP bikes and MTX, MVX, NS and NSR road bikes. Honda Power Port valve HPP valve. A centrifugal governor opens and closes a two-blade exhaust valve (using over 50 parts) Honda V-TACS The "V-TACS" - Variable Torque Amplification Chamber System - works differently from the "ATAC system" and it will only work when it is used in conjunction with a tuned muffler. Tuned mufflers/expansion chambers increase power but only at the RPM they are designed for and can actually cause a power loss outside their tuned RPM. "V-TACS system" takes advantage of using an expansion chamber without losing power outside the expansion chamber's tuned RPM. Within the head and cylinder of the engine, there is a chamber that is sealed by a valve. This sealed chamber is vented onto the exhaust port when the valve is open. At low RPM this valve is open, this has the effect of increasing the exhaust manifold volume and negating the power loss that would normally be apparent at low RPM with an expansion chamber. At mid RPM the valve is closed, this enables the expansion chamber to work. It is identified by the head and cylinder, being much larger than normal for its displacement, the cylinder wall is also cast with the wording VTACS on it. V-TACS was a foot-operated power valve system made by Honda on some of its small two-stroke bikes and scooters, like the Honda FC50. Honda RC-Valve The Honda Revolution Control valve is designed and works in principle like the "AETC system." A small computer monitors engine RPM and adjusts a two-blade exhaust valve with an electric servo. Honda equipped many two-stroke motorcycles such as the NSR125 and NSR250 models with RC - Valve power plants. Kawasaki KIPS Kawasaki uses a power-valve system called KIPS (Kawasaki Integrated Power Valve System) on their two-stroke bikes. The KIPS utilizes both alterations in port height, closing of the secondary port ducting, and a resonant chamber. KIPS is operated by a mechanical governor on single cylinder machines.