<P> In some engines the intake runners are straight for minimal resistance . In most engines, however, the runners have curves...and some very convoluted to achieve desired runner length . These turns allow for a more compact manifold, with denser packaging of the whole engine, as a result . Also, these "snaked" runners are needed for some variable length / split runner designs, and allow the size of the plenum to be reduced . In an engine with at least six cylinders the averaged intake flow is nearly constant and the plenum volume can be smaller . To avoid standing waves within the plenum it is made as compact as possible . The intake runners each use a smaller part of the plenum surface than the inlet, which supplies air to the plenum, for aerodynamic reasons . Each runner is placed to have nearly the same distance to the main inlet . Runners whose cylinders fire close after each other, are not placed as neighbors . </P> <P> "180 - degree intake manifolds"... Originally designed for carburetor V8 engines, the two plane, split plenum intake manifold separates the intake pulses which the manifold experiences by 180 degrees in the firing order . This minimizes interference of one cylinder's pressure waves with those of another, giving better torque from smooth mid-range flow . Such manifolds may have been originally designed for either two - 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 . </P> <P> "Heat Riser"... now obsolete, earlier 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 . </P> <P> 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 . </P>

Where is the manifold located on a car