Although we all like to include upgrades to our trucks, selecting the right option can sometimes be a little daunting. Take for instance a turbo upgrade. There are a plethora of options available, including bigger, compound, and even Variable Geometry Turbos (VGT) or non-VGT swap kits. That last option seems to create a lot of questions, as many enthusiasts are contemplating whether to swap out their VGT for a non-VGT. While doing so might offer a few benefits for some truck owners, doing such a swap does have some implications that you should be aware of — legality being the biggest. So, I thought we should get a discussion going about how to make an informed decision. I want to make sure you guys get good information so I reached out to people that specialize in turbos — KC Turbos.

“The VGT vs. non-VGT argument has been something we have been contending with for a long time,” said Cameron Lewis of KC Turbos. “In our experience, a properly functioning VGT is going to spool faster and make more power through the powerband when compared to a non-VGT in just about any setup under 650 horsepower on a 6.0 and 6.7-liter Power Stroke. The ‘larger’ you go, the bigger the advantage you will see from a non-VGT. ‘Smaller’ setups are much better off with a VGT. However, a lot of this decision comes down to personal preference — for the sound, feel, driving style, etc. It is hard for me to believe people when they say that a non-VGT will spool faster than a VGT. We simply have never seen that through all of our years of testing.”

VGT Or Non-VGT — Let’s Talk VGT

For the last decade, diesel trucks coming out of the factory have benefited from Variable Geometry Turbochargers. Before that, older turbo-equipped trucks utilized a fixed vane turbo. The difference? Let me explain.

A variable geometry turbo means the turbocharger can adjust the path of the exhaust gasses to change their effect on the turbine wheel. The most common method is to direct the exhaust gases onto different regions of the turbine by angling the “variable” teardrop-shaped “vanes” placed around the turbine impeller. These vanes are moved simultaneously by a rotating component called a unison ring.

These teardrop-shaped vanes control exhaust flow through the turbine wheel as they open and close to match the engine’s exact boost requirements. At low engine RPM, the vanes remain mostly closed, restricting exhaust flow towards the impeller. This restriction increases air velocity, helping the exhaust gases get the turbine up to speed faster. This brings down the boost threshold and reduces turbo lag.

However, when the engine reaches higher RPM levels, this restriction would be a problem since the engine needs to pump out more exhaust to make power. It is at this time that the vanes open to allow as much exhaust gas to flow through the turbo as possible, avoiding a restriction that would increase backpressure and reduce power. This ability to change as needs dictate makes a VGT a great option for most truck owners.

Radius Revelation?

The movable vanes allow the effective aspect ratio A/R (area over radius) of the turbocharger to be altered as conditions change. A turbocharger’s A/R number is typically found cast into the exhaust-turbine housing, as it is a function of the various sizes in the turbine housing. The smaller the A/R number, the faster the turbo will spool, at the cost of maximum output. The larger the A/R number, the longer the turbo will take to spool but will have more potential output — all within a given turbine size.

The velocity at which the exhaust gas is pushed through the housing plays a major role in a turbo’s spool-up time. At low RPM, there is a low volume of exhaust gasses being produced. In order to maintain that gas’ velocity, you want a housing with a smaller volume. When RPM is increased, so is the volume of exhaust gas. Ideally, you’d want a larger housing to handle the larger volume of gasses. Instead of having to pick one or the other, the variable geometry allows the housing to act as both a small and large housing as the needs change.

If a turbo’s A/R is too large, it will fail to create the proper boost the engine requires when at low RPM. Conversely, if the A/R is too small, the turbo will choke the engine at high speeds. This can lead to high exhaust manifold pressures, high pumping losses, and ultimately decreased power output. By using a VGT with the ability to alter the geometry of the turbine housing as the engine accelerates, the turbo’s effective turbine housing size can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, a low boost threshold (not enough exhaust gas pressure at the lowest RPM levels to spin the turbocharger and create the necessary boost), and high efficiency at higher engine speeds.

A Sticky Situation

The VGT offers the benefit of better power and torque supply across the RPM range because the turbo is always operating at the highest possible efficiency. The downside is that the internal, movable “gates” can get stuck because of carbon buildup. This happens over time and can, depending on idle time, driving habits, and exhaust brake use.

If the gates become harder to move, this places extra load on the electronic actuator, eventually causing it to burn out. This is the main reason truck owners think about installing a non-VGT to replace a stuck actuator of a VGT. You will know this is happening when you experience sluggish acceleration, low power, and a non-functioning exhaust brake. Unfortunately, when this does happen, there is no real way of cleaning the internals without removing the turbo and disassembling it to clean all internal components.

VGT Or Non-VGT — Let’s Talk Non-VGT

The benefit, however, of a working VGT is higher power and torque across the RPM range because the turbo is always operating at the highest possible efficiency. The downside is, there’s a lot that can go wrong. Unison rings wear and can stick, and precise control depends on multiple sensors and electronics. In contrast, a non-VGT tends to last longer and be more reliable, therefore, it is often considered more desirable.

A non-variable geometry turbo is an older design and is less complex as nothing inside of the housing is variable (fewer moving parts and no actuator). Without gates to fluctuate air flow within the housing, the volume of the turbine housing is fixed, and exhaust gas velocity (and subsequently, turbine speed) relies solely on exhaust gas volume. Many truck owners feel that replacing a VGT with a non-VGT unit is an upgrade as a non-VGT tends to last longer and be more reliable because of those traits.

Since there are no moving vanes within a non-variable geometry turbo, the exhaust gas passes through the turbine without being regulated by the vanes. Unfortunately, if the A/R is too large, at low engine RPM, the engine does not generate enough exhaust flow to spin the turbine fast enough to generate any meaningful boost levels. This is when the turbo is said to be below the boost threshold. A small A/R turbo will perform well at low RPM, but be choked at higher RPM. It’s a tradeoff in efficiency zones you make when you select a specific A/R housing.

Once the engine reaches an RPM that is suitable to generate boost, it still takes some time to get the turbine spinning fast enough to deliver an appropriate amount of boost. This is known as turbo lag. Turbo lag and boost threshold are both higher when dealing with larger turbines that take more energy to get spinning. However, these higher-flowing turbines are capable of generating more power at a higher RPM. It’s a trade-off.

Bigger Is Not Always Better

In a non-VGT, the critical parameter that needs to be addressed when choosing is the A/R ratio. When buying a non-VGT, you select the A/R you need for the driving characteristics you want. Like most things, bigger is not always better.

Unfortunately, there is no Facebook forum or one-size-fits-all solution to getting the correct unit for your truck. How you intend to use your rig could require a different turbo than your buddy who has approximately the same horsepower. Getting the best turbo for your particular needs should begin with a call to the turbo manufacturer of your choice.

So, getting back to whether the swap to a non-VGT turbo is a good idea…

“Once you start trying to exceed the 650-horsepower mark, you start to lose what makes the VGT shine — low-end torque,” Cameron states. “Since a VGT has to be massive to flow enough air on the top end to create 700 horsepower, this is where going to either a single non-VGT or switching to compounds is a better fit. For example, we have a built 6.4-liter truck, that came with compound turbos from the factory. We equipped the truck with our Stage 2 low-pressure and Stage 1 high-pressure turbos and all the supporting mods. We were able to make 1,000 rear-wheel horsepower on the dyno. Even at that power level, this is still the truck that Charlie, KC Turbos’ owner always chooses to tow with because it still has all of its low-end power. If we tried to get that same power level on a big single Non-VGT turbo, the truck would be next to impossible to tow with.”

Cameron went on to affirm that any person with a 6.0L, 6.7L, or any other VGT-equipped diesel who wants to tow with their truck should stay with a VGT. If you want to tow and have big power, then you need to get a compound setup. “We have always found that VGT will outperform a non-VGT every time on the low end,” said Cameron. “If you want a comfortable, all-around daily driver, going to a non-VGT isn’t the answer. If someone is considering going to a non-VGT setup, I would rather push them towards a compound setup so they have the best of both worlds — The first turbo being a VGT with quicker spool up to keep low-end power and a massive non-VGT on top of that to get all the big top-end power without the associated turbo lag that non-VGTs can’t escape.

Cameron concluded with, “It’s all about managing expectations for your truck. What is it that you are looking to get out of your diesel? If you are okay with the drawbacks of a non-VGT to chase big power on a purpose-built sled-pulling or drag-racing truck, then it makes sense. If you are doing the non-VGT swap to be different on the street because you like how they drive, that’s okay too. What we don’t want to happen is for the guys who like how a non-VGT turbo drives and performs to blindly recommend them to new to the diesel scene truck owners who don’t have experience with either because there is a good chance they may not like what their truck becomes. The majority of people we know who drive diesel pickup trucks, whether that be Cummins, Duramax, or Powerstroke, enjoy having the low-end power a VGT can provide so that they can use their truck for what it was made for.”

Keeping It Legal

If you still feel that a non-VGT is the right option for a turbo swap in your truck, you should also know that you could be subjected to legal trouble. I say this because the EPA feels that since the turbo is part of the exhaust, changing to a non-emissions-compliant part is against the law. That said, I could not find anything that explicitly talks about this, but that “grey area” could leave room for interpretation and we all know in whose favor that will go.

So, have I added to the confusion about whether a VGT or non-VGT is right for you? I thought so. That is why I will reiterate that making a call to the turbo manufacturer of your choice needs to be your first call. Explain — and be honest — your current and future plans for your truck, including usage and upgrades.