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Why? FAQ and information thread.

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Answers to commonly asked questions for reading, learning and general perusing. Like a mini-Renault-Wiki thing.

Please post any questions you would like answered and stored here and when the collective has answered I will edit it all into a nice tidy pile of useful stuff :)
 
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Why are my driveshafts hollow?
The reason hollow driveshafts are used is because the radial component of the polar moment of inertia is to the 4th power.... the material closest to the torsional axis provides much less resistance to torque than material further outward. By getting rid of inner material and increasing the outer radius a bit you get the same torsional stiffness with less weight.
Also, making a hollow drive shaft uses less material than a solid one. Manufacturers can make hollow drive shafts with the same (or better) structural integrity as their solid counterparts. The saved material lowers the cost considerably, and then because of the reduced rotational inertia, the engine can get the same push with less work. This translates into fuel efficiency. With less material to push, it takes less fuel to get the same acceleration.
 
Whats the difference between normal, and direct injection?
The difference is how a direct injection engine delivers fuel and how the fuel mixes with incoming air.
The injectors in a standard engine spray fuel into the intake manifold, where fuel and air mix together into a fine mist. When the inlet valves open the piston in that cylinder sucks the fuel-air in from the manifold above. With a direct injection engine, however, the fuel gets to skip the manifold part and is injected directly into the combustion chamber.

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What is a VNT (Variable Nozzle Turbo)? >>Also knows as VGT (Variable Geometry Turbo), or Variable Vane Turbo.
VNT's are a a type of turbo usually designed to allow the effective AR (Aspect Ratio) of the turbo (exhaust side) to be altered as conditions change.
If the AR is too big, the turbo will lag very badly, but if the AR is too small the turbo will restrict power at high load. By altering the geometry of the turbine housing as the engine accelerates, the turbo's AR can be closer to optimum for a wider range. Because of this, VNT's have a small amount of lag, have a low boost threshold, and are more efficient at higher engine speeds.
At slow engine speeds, the vanes are in the “closed” position narrowing the gap between them which effectively accelerates the exhaust gas onto the turbine wheel. At high engine speed the vanes open up slowing the exhaust gases.

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