25-09-2012, 04:09 PM
Imagine a hosepipe, if you stick your finger over the end, you get a lot of backpressure, but the speed at which the water exits the nozzle is very high - resulting in it goes miles, release your finger and the backpressure drops but the flow increases... This is often known as the turbos aspect ratio - A/R... Being variable, it gives all the advantages of a large turbo, with none of the drawbacks of a large unit...
That's essentially how a VNT works, the Variable Nozzles sit infront of the exhaust gas flow and accelerate the gasses into the turbine wheel, obviously this increases backpressure - not good, however at low rpms it doesn't matter, as soon as the turbo spins up to speed and the inlet manifold pressure increases, the vanes open up, dropping the backpressure, but allowing a greater flow in the process... Controlling this is a nightmare as on the motorway, the backpressure is too high, a normal boost controller isn't intelligent enough to open them a little bit whilst cruising, but be closed when you stomp on it and it doesn't allow for the fact that at low rpms it doesnt stop the turbo producing too much backpressure and therefore too much boost, causing surge...
That's essentially how a VNT works, the Variable Nozzles sit infront of the exhaust gas flow and accelerate the gasses into the turbine wheel, obviously this increases backpressure - not good, however at low rpms it doesn't matter, as soon as the turbo spins up to speed and the inlet manifold pressure increases, the vanes open up, dropping the backpressure, but allowing a greater flow in the process... Controlling this is a nightmare as on the motorway, the backpressure is too high, a normal boost controller isn't intelligent enough to open them a little bit whilst cruising, but be closed when you stomp on it and it doesn't allow for the fact that at low rpms it doesnt stop the turbo producing too much backpressure and therefore too much boost, causing surge...
