or four-barrel carburetors, but now are used with both throttle-body and multi-point fuel injection. An example of the latter is the Honda J engine which converts to a single plane manifold around 3500 rpm for greater peak flow and horsepower. Older heat riser manifolds with 'wet runners' for carbureted engines used exhaust gas diversion through the intake manifold to provide vaporizing heat. The amount of exhaust gas flow diversion was controlled by a heat riser valve in the exhaust manifold, and employed a bi-metallic spring which changed tension according to the heat in the manifold. Today's fuel-injected engines do not require such devices. Variable-length intake manifold A variable-length intake manifold (VLIM) is an internal combustion engine manifold technology. Four common implementations exist. First, two discrete intake runners with different length are employed, and a butterfly valve can close the short path. Second the intake runners can be bent around a common plenum, and a sliding valve separates them from the plenum with a variable length. Straight high-speed runners can receive plugs, which contain small long runner extensions. The plenum of a 6- or 8-cylinder engine can be parted into halves, with the even firing cylinders in one half and the odd firing cylinders in the other part. Both sub-plenums and the air intake are connected to an Y (sort of main plenum). The air oscillates between both sub-plenums, with a large pressure oscillation there, but a constant pressure at the main plenum. Each runner from a sub plenum to the main plenum can be changed in length. For V engines this can be implemented by parting a single large plenum at high engine speed by means of sliding valves into it when speed is reduced. As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency. There are two main effects of variable intake geometry: Venturi effect: At low rpm, the speed of the airflow is increased by directing the air through a path with limited capacity (cross-sectional area). The larger path opens when the load increases so that a greater amount of air can enter the chamber. In dual overhead cam (DOHC) designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by deactivating the intake valve itself. Pressurization: A tuned intake path can have