Diesel Tuner's Blog

High Flow Vanes (HFVs) For Variable Geometry Turbos and Why You Need Them

Posted by Ben Lawson on Dec 3, 2022 10:19:00 AM

One of the biggest advancements in turbo design since we first started applying them to diesel engines over 70 years ago has been the invention of Variable Geometry Turbos. These turbos utilize a set of movable vanes on the inside of the exhaust housing that allows the actuator to vary the area in which exhaust gasses can pass through them. This allows the exhaust side of the turbo to increase or restrict the flow of exhaust hitting the turbine wheel. In some designs it also changes the flow direction of gasses hitting the exhaust turbine as you can see illustrated below. 

Vane position diagram 1.1-01
When restricted, the velocity of the exhaust gasses increases which helps spin the turbine faster than a fixed geometry turbo would. This gives the turbo the ability to spool the turbine and compressor wheel up much better than it normally would even with a low volume of exhaust pressure traveling through the system. This helps to spool the turbo at lower RPMs to create boost where it would normally be lacking. This is why variable geometry turbos (VGTs) are such an advancement. They have the ability to significantly reduce turbo lag and provide much better response and turbo efficiency across a range of RPMs. 

6.0L Stealth 67 Unison Ring, vane, exhaust housing (1)Nozzle Ring-01

The vanes used in VGTs come in a few different variations including pivoting vanes which move each individual vane in unison and sliding ring designs which use a set of stationary vanes that are moved in and out of a ring that acts as a wall to trap and channel exhaust gasses. However no matter the design they all create a restriction in the exhaust system even when in the fully opened position. Our goal when we started trying to improve the designs of these turbos was to improve the design of these vanes to create less of a restriction in the exhaust system along with adapting them to work with larger turbine wheels that can further improve turbo efficiency.


Stealth 64G2 Turbine and Stock LB7 Turbine


Upgrading to a bigger compressor wheel is one of the best ways we have found to improve the efficiency of VGT turbos. Yet attempts at solely upgrading the compressor wheel on VGTs have proven fruitless. Doing so only made the truck smoke more and gave us negligible improvements to power over the stock turbo. Through our rigorous testing on our first VGT project (an LML VVT) we realized that upgrading the compressor wheel alone would not be enough and thus a turbine upgrade would also be necessary to produce the power we were asking for. After redesigning our LML VGT turbo to accept a larger compressor wheel and turbine we again started testing. Only to find that there was still far less of an improvement to the turbo's efficiency than we had expected. This, we discovered, was due to the limiting design of the stock vane pack.


Since the vane height is designed to match the size of the turbine wheel inducers to provide the same size gap for exhaust gas to pass through evenly and onto the turbine. Upgrading the turbine wheel alone was ineffective at improving the efficiency of the turbo. We realized that upgrading the vane pack would be the only way to take advantage of our upgraded compressor wheel and turbine wheel.


Most vane packs in variable geometry turbos like in the HE351ve and L5P turbocharger leave only about a 9mm gap for the exhaust gasses to flow through before being directed through to the turbine. This restrictive gap along with the volute of the exhaust housing corresponds with the size of the inducer of the turbine wheel and creates a smooth transition for the exhaust gasses to flow through the vanes and onto the turbine blades. However this nearly 9mm gap is too small to effectively flow more exhaust gases through to the larger turbine. Not only that but the gap only reaches 9mm when the vanes are fully opened and even then there are still the individual vanes themselves filling some of that space.


We designed our HFVs with an approximately 12mm gap in order to match the larger inducer size of the upgraded turbine wheel used in our Stealth (VGT) Turbos. Along with that we also machine the volute of the exhaust housing out to the same size gap as the upgraded HFVs. Machining the volute opening bigger is key in allowing us to take full advantage of the larger vanes and turbine wheel.

L5P Stock VS Upgraded Turbine Wheels-01

A larger gap for exhaust gasses to pass through means the exhaust gas can flow onto the full width of the larger turbine inducers and transfer more exhaust energy to the turbine. This lets the turbo take better advantage of the exhaust energy and turn it into more rotational force. Resulting in higher boost levels and more pounds of air per minute. It also means less kinetic energy is lost to the compression of exhaust gasses being forced through the smaller 9mm restriction.


L5P Vane Nozzle Comparison-01

More kinetic energy making it to the turbine also means less kinetic energy is transferred into heat energy. The result is lower exhaust gas temperatures (EGT's), which means you get to take advantage of those higher power numbers for longer which is great if you’re towing or using your truck in competition. Lower temperatures also keep things like your emissions control systems happy and in functioning order. This means your truck should not only produce more power but also run more reliably with this upgrade. More energy being transferred to the turbine wheel also means that we have more energy spinning the compressor wheel and this results in 30% more air being transferred through the compressor side of the turbo.

HE351 Nozzle Ring Comparison

Not only does our HFV technology help with controlling engine and turbo temperatures and improve energy transfer but it also significantly reduces drive pressure. One way to get more power is by adding more boost however the same result can be achieved by getting rid of drive pressure. When you reduce the drive pressure you allow higher exhaust flow and more energy put into spooling the turbine.


6.0L Stealth 67 Unison Ring, vane, exhaust housing (1)

Overall variable geometry turbos have a distinct advantage over fixed geometry turbos. Yet when it comes to modifying them in order to improve their performance their key difference is ultimately a limiting factor. This is where the necessity of our HFV design comes into place. Modifying the vane packs of our VGTs was the only way to truly reap the benefits of an upgraded compressor wheel and turbine. The result is a variable geometry turbo that can produce up to 700hp in 64 versions and up to 800hp in larger 67 versions.

Here is the list of turbos that utilize our proprietary HFV design:

L5P Steath 67G2
L5P Stealth 64
LML Stealth 67G2
VVT Stealth 67G2
HE351ve Stealth 64
HE300vg Stealth 64

2.8L LWN DT-44


If you'd like to learn more about VG turbos please check out the link below:

How VVT/VGT Turbochargers Work on Your Diesel

Topics: Turbo, Stealth, High EGT's, LML Turbo, Turbo Upgrades, Stealth VVT, Stealth 67G2, L5P, VGT, L5D, Nozzle Ring, High Flow Vanes, Turbo Vanes, HFV