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VNT Turbo Explained
VNT Turbo have been a very common addition to todays 4WD but many do not understand how they work. Or the fact that they are incredibly sensitive.
Today, we wanted to show you how these work and common issues that you may come across.
HOW TURBOS WORK
Before we start, we have to explain how (in a simplistic way) how turbochargers work.
Firstly, exhaust gasses drive the ‘turbine’ which spins the shaft and accompanying compressor wheel which makes the boost.
Previously, we’d mainly adjust the boost response by changing the Tubrine housing A/R ratio. (https://www.turbobygarrett.com/turbobygarrett/turbine_housing_AR_and_housing_sizing)
In a very simple sense, what this did was change the degree to which we ‘funnelled’ the exhaust gasses. If we didn’t really restrict it through a large funnel, then the gasses would have little restriction to flow. This means it wil not generate a lot of velocity (= slow turbine speed = low boost). Alternatively, if we funnelled it all through a small funnel, it would restrict the flow but create a lot of velocity and therefore build lots of boost (especially at lower RPM).
The problem was always that you wanted the smaller AR (tighter funnel) for boost in low RPM. With the limited back pressure and high RPM performance of the larger AR ratio.
ENTER VNT TURBO – the Solution (mostly!)
These two videos we found on YouTube that excellently show how the VNT works. In essence, it’s a ‘variable funnel’ that allows for the best of both worlds (mostly).
Its an excellent solution that allows us to build boost at a MUCH lower rpm than we could have previously, especially in performance application where we can use bigger turbos and still get them kicking in much lower than traditionally.
You say ‘mostly’ – what’s the go?
The issue is that with VNT turbo, you never really get the PURE top end efficiency of a traditional turbine housing, and therefore a Larger AR ratio housing will produce more top end HP than an equivalent VNT set up (however it will have NONE of the bottom end).
Also, its possible to use Lower AR ratios on the turbine housing to get the bottom end response and then bleed off the excess with a wastegate. Again however, this is not the most efficient way as the exhaust flow is compromised resulting in higher parasitic losses of the engine.
SO, What are the common Problems
there are 2 types of VNT actuators. Vacuum (but the vacuum is supplied by electronic valve) or electronic stepper motor arrangement. The WORST possible scenario is that the VNT assembly is in the ‘closed’ position and over speeds the turbo.
Common issues for Vacuum set ups are a loss of boost due to vacuum lost or solenoid failure. These turbos are designed to go full open VNT (aka no boost or turbine speed) should this failure occur.
Electronic stepper motors have 2 sensors within their arrangement, the actual duty cycle and ‘signal’ from the ecu to the motor – but they also have a failsafe in the form of a ‘position sensor’. This way, if the position sensor records something unusual, the ECU will go into a limp home mode and grossly limit the fuel.
In all cases, the ECU will be targeting a set boost value (if not always, definitely from 50% load and up), and via a closed loop, change the VNT assembly as necessary in order to get that desired boost.
If the boost is too high, and its VNT adjustments don’t appear to be working, then the ECU will limit boost by limiting fuel (and therefore exhaust gasses driving the turbo).