Venture.  A specially shaped restrictor in a tube designed to  speed up the flow of fluid passing through it. According to  Bernoulli’s principal, any time the flow of fluid speeds up  without losing or gaining any energy from the outside, the  pressure of the fluid decreases. Updraft carburetor.  A carburetor that mounts on the bottom  of a reciprocating engine. Air entering the engine flows  upward through the carburetor. Upper-deck pressure.  The absolute pressure of air at the  inlet to the fuel metering system of a turbocharged engine.  Upper-deck pressure is the same as the turbocharger  discharge pressure. V-blocks.  A fixture that allows a shaft to be centered and  rotated to measure any out-of-round condition. Vernier coupling.  A timing coupling used with base- mounted magnetos. The vernier coupling allows the timing  to be adjusted in increments of considerably less than one  degree. V-engine.  A form of reciprocating engine in which the  cylinders are arranged in two banks. The banks are separated  by an angle of between 45º and 90º. Pistons in two cylinders,  one in each bank, are connected to each throw of the  crankshaft. Vertical tape instrument.  A tall rectangular instrument that  displays the quantity of the parameter being measured by a  movable strip of colored tape. The presentation resembles a  vertical bar graph. Vibration loop.  A loop in a rigid fluid line used to prevent  vibration from concentrating stresses that could cause the  line to break. Valence electrons.  Electrons in the outer shell, or ring,  around the nucleus of an atom. It is the valence electrons  that give an atom its electrical characteristics and are the  electrons that may be pulled loose from an atom to cause  electrical current. VIFF.  Vectoring in forward flight. A method of enhancing  the maneuverability of an airplane by vectoring the exhaust  gases and/or fan-discharge air to produce thrust components  not parallel to the longitudinal axis of the aircraft. Valve overlap.  The portion of the operating cycle of a four- stroke-cycle reciprocating engine during which both the intake  and exhaust valves are off of their seats at the same time. Vapor lock.  A condition of fuel starvation that can occur  in a reciprocating engine fuel system. If the fuel in the line  between the tank and carburetor is heated enough for the  fuel to vaporize, a bubble will form in the line. If the vapor  pressure of the bubble is high enough, it will block the fuel  and keep it from flowing to the engine. Viscosimeter.  An instrument used to measure the viscosity  of a liquid. The time required for a given volume of liquid at  a specified temperature to flow through a calibrated orifice  is used to indicate the viscosity of the liquid. Viscosity.  The resistance of a fluid to flow. Viscosity is the  stiffness of the fluid, or its internal friction. Vapor pressure.  The amount of pressure needed above a  liquid to prevent it from evaporating. Viscosity index (VI).  A measure of change in viscosity of an  oil as it changes temperature. The higher the viscosity index,  the less the viscosity changes. Vaporize.  The changing of a liquid into a vapor. Vectored-thrust engine.  A turbojet or turbofan engine with  the fan and/or exhaust nozzles mounted in such a way that  they may be rotated in flight to produce forward, vertically  upward, or rearward thrust. Viscosity index improver.  An additive used to produce a  multi-viscosity lubricating oil. The polymer additive expands  as temperature increases and contracts as temperature  decreases. VI improvers cause viscosity to increase as oil  heats and decrease as it cools. Volatile memory.  Computer memory that is lost when the  power to the computer is turned off. Velocity.  A vector quantity that expresses both the speed  an object is moving and the direction in which it is moving. Velocity turbine.  A turbine driven by forces produced by the  velocity, rather than the pressure, of gases flowing through  the vanes. Volatility.  The characteristic of a liquid that relates to its  ability to vaporize or change into a gas. Volumetric efficiency.  The ratio of the volume of the charge  of the fuel and air inside the cylinder of a reciprocating engine  to the total physical volume of the cylinder.  G-30