VIS-Variable Resonance Inlet System
You might think that a large wide-open pipe leading to the inlet of a petrol engine would be the best way to get air to the cylinders. After all, the volume of petrol injected into the engine varies only very slightly from tickover to wide open throttle – it’s the air which is drawn into the cylinder which creates the bigger bang and develops the power.
But designing the inlet for an engine is much trickier than one would first
suppose. In fact, an engine designed to draw in languid air would hardly run at
all, because engines rely on turbulence
in order to mix the petrol and air thoroughly together during both the induction
and the compression stroke, and this produces more torque, especially at lower
engine speeds where piston movement is slower and the manifold pressure is low.
To do this careful attention is paid to the design of the internal inlet and
manifold, using advanced gas-flow research to swirl and resonate the air with
mini shockwaves, so that, already broken up into droplets by the design of the
injectors, the squirt of petrol arrives into a seething maelstrom of air which
further breaks up the droplets into the fuel-air explosive which is needed to
greet the spark from the plug
However, at higher engine speeds the inlet system now needs to deliver as
much air as possible for the greedily sucking pistons, and what is needed now is
a greater internal area in order to provide the maximum volume of air.
The resonance chamber is now restricting the volume of air and is a
disadvantage to engine power.
At engine speeds below 3,200 rpm the engine draws its air through a primary resonance chamber which is carefully designed to produce maximum turbulence in the slower moving low-revs intake, while a secondary inlet duct, designed for larger volumes of air, is closed off by butterfly valves. As the engine speed rises above 3,200rpm, the VIS regulator receives a signal from the EEC V and closes, which causes the butterfly valves shutting off the secondary inlet duct to open. The engine is now breathing through both primary and secondary ducts, greatly increasing the volume of air that would otherwise be available. Significant gains in maximum power are made together with enhanced low-end torque – the best of both worlds!
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