VNT Turbo Guide – Variable Nozzle Technology Explained for Diesel Engine Builds

VNT turbo guide – episode four of the five part boosting series. Today we cover variable nozzle technology, its advantages, its fundamental limitations and why it suits some engines far better than others.

What VNT Actually Does

Variable nozzle technology – also called VTG, VGT or variable turbine geometry depending on manufacturer – adjusts the effective size of the turbine housing using moveable vanes inside the turbine inlet. Closing the vanes creates a smaller effective turbine housing for faster spool. Opening the vanes creates a larger effective turbine housing for better high RPM flow.

This addresses the core single turbo trade-off: a small turbine housing spools quickly but restricts flow at high RPM, a large turbine housing flows freely at high RPM but spools late. VNT attempts to have both simultaneously.

The Compressor Side Problem

VNT only adjusts the turbine side – the hot side of the turbo. The compressor side remains fixed.

A large compressor wheel has significant rotational mass creating inertia. A small compressor wheel spools more easily but flows less at high RPM. No VNT technology exists for the compressor side because changing the aerodynamic performance of a compressor requires physically changing the wheel diameter and shape – you cannot adjust this with vanes alone.

This means the trade-off is only partially solved. You can have an adjustable turbine but you are still committed to a fixed compressor size. If you want the benefits of a large compressor for peak power you must live with its inertia at low RPM.

The Flow Restriction Problem

This is the most important limitation of VNT for high performance diesel builds. Even when the vanes open fully the vane mechanism remains inside the turbine housing. It is never completely out of the exhaust gas path. The nozzle ring structure always creates some restriction to flow.

A conventional turbine housing with no vane mechanism flows more freely when fully open than a VNT turbine housing of equivalent outer dimensions at the same position. The vanes, regardless of how far they open, add restriction.

For high power diesel builds that need the engine to breathe freely past peak power RPM this restriction matters. You cannot simply open the VNT fully and match the flow of a standard turbine housing of the same size.

Why Petrol VNT Engines Need Wastegates

The Porsche 997.2 and later Turbo models use VNT turbos and also have wastegates. This seems contradictory but there is a specific reason.

On a petrol engine with a throttle body the engine load is controlled by air volume. When you lift the throttle air flow drops rapidly. Without a wastegate on a VNT petrol engine boost would continue building uncontrollably once the turbo is spinning at speed.

On a diesel engine there is no throttle body. Boost is controlled by fuel. Reduce fuel and boost drops automatically. This is why diesel engines do not need wastegates regardless of turbo technology – VNT or otherwise.

Porsche needs the wastegate on their VNT turbos because the petrol engine cannot control boost through fuel reduction alone. The diesel does not have this problem.

The High RPM Flow Problem for OM606 Builds

The OM606 with stage one or stage two camshafts makes maximum power at 6500-7000 RPM. The engine must breathe freely through its entire power band to deliver that power.

A VNT turbo that spools well at low RPM but restricts flow above 5500 RPM will choke the engine exactly where it is supposed to make maximum power. The turbo needs to be fully open and flowing freely through the entire upper RPM range.

Most VNT turbos available at reasonable cost are sized for production diesel engines that make maximum power at 3500-4500 RPM. These turbos are fully choked well before the OM606 reaches its power band. Installing one on an OM606 build produces strong low RPM performance followed by a wall at higher RPM where the turbo cannot flow enough.

There are large VNT turbos from BorgWarner and Garrett that flow adequately at high RPM for high power diesel performance builds. These exist and work correctly. They are less common, harder to source and more expensive.

When VNT Is the Right Choice

Common rail diesel engines – OM648, BMW M57, Audi TDI and similar – are designed around VNT technology. They have camshaft profiles matched to make maximum power in the RPM range where their VNT turbos flow adequately. The entire engine system is calibrated together.

For these engines VNT is excellent. The technology suits the application. Fast spool, strong mid-range, controlled boost, high efficiency within the operating range.

For high revving performance diesel builds on OM606 engines with upgraded camshafts VNT requires careful selection of a large flowing unit or it becomes a restriction rather than an advantage.

Progress in VNT Technology

Modern VNT turbos from production vehicles represent excellent engineering. High quality aluminium turbine wheels, Billet compressor wheels, ball bearings, extended tip compressor technology – VNT turbos are at the leading edge of turbocharger development because they are fitted to millions of production vehicles driving continuous investment in the technology.

A modern VNT turbo from a current Audi, BMW or Mercedes CDI application is an extremely sophisticated component. The challenge for performance builds is finding one sized for the power target and RPM range required.

Summary

• VNT turbo guide: adjustable turbine vanes create variable effective turbine housing size
• Adjusts turbine side only – compressor side remains fixed
• Even fully open VNT vanes restrict turbine flow compared to a clean standard housing
• Petrol VNT engines need wastegates because throttle cannot control boost – diesel engines do not
• OM606 with upgraded camshafts needs turbines that flow freely to 7000 RPM – most VNT turbos restrict below this
• Large VNT units from BorgWarner and Garrett can work for high performance diesel builds but are expensive and harder to source
• VNT is excellent on common rail diesel engines designed around it – camshaft profiles match VNT operating range
• Continuous technology development in VNT because of production volume – modern units are highly sophisticated

Episode five coming next – the last in the boosting series. Love you all. See you soon!