Sizing Turbos for Diesel Engines

Turbo Size: Understanding the Numbers behind compression-ignition

Diesel guys are number guys whether they know it or not. You’ll see the Big Three touting numbers in all of their ad campaigns: best in class towing, most torque, highest horsepower. In a day of little turbo economy cars, diesel numbers are almost always big. For many aftermarket enthusiasts, 500 rear-wheel horsepower and 1,000 lb-ft of torque is sort of the starting performance benchmark. Whether you’re good at math or not, numbers infiltrate virtually every aspect of the diesel market, so let’s have a look at them! We promise to try and keep it basic.

Engine Displacement

Almost all of the diesel engines we work with in the performance industry are large, but if there’s a displacement you can’t place (or you’re talking to an old hot rod guy) you can mix liters and cubic inches by taking the size in cubic centimeters (5.9L for instance = 5900cc) and dividing it by 16.38. This gives us 360.2 cubic inches. That means an 8.3L (8300cc) is 506 cid, and a 1.9L TDI is a tiny 116 cubic inches. It may be off by a decimal place or two, but the cubic inch to cubic centimeter can be helpful in everything from calculating airflow to sizing turbos, since most American hot rod math is non-metric.

Airflow

Another handy notation to have concerns airflow, which is simply cubic inches x rpm / 3456. That means a 360 cid engine (can you tell we like Cummins?) spinning at 3,000 rpm would consume about (360 x 3000 / 3456) 312 cfm of air when operating at 100 percent volumetric efficiency. However, volumetric efficiency is usually pretty far from 100 percent; more around 80 percent. So that means we now get an answer of (312 x .8) 249.6 (let’s just say 250) cfm of air. The great thing is, with these simple formulas you don’t even have to be that good at math, you just plug in a couple numbers and there’s your answer. If you’re wondering what airflow has to do with anything, we’re getting to that.

Turbos and Engines

So let’s say you have your Cummins spinning at 3,000rpm and you want to add some boost. Every atmosphere above outside air adds another bar to the pressure ratio, so if ambient is 14.7 psi (it varies by elevation) then 14.7 pounds of boost would be a 2:1 pressure ratio. That means 29.4 psi would be a 3:1 pressure ratio, and 73.5 psi would be a 6:1 pressure ratio. If we take our earlier theoretical 250 cfm figure, that means to make 14.7 pounds of boost you’d need a turbo that flows 500 cfm, 750 cfm for 3:1, and a whopping 1,500 cfm for 6:1. More than anything, this can be very helpful in sizing turbos for your desired power levels.

Turbos and Airflow

We get the question all the time: how do you size turbochargers? In reality, the answer is simple: turbos should be sized as small as possible for your desired horsepower. Luckily, airflow is one of those rare areas where the starts align, as roughly one pound per minute (lb/min) of air is equal to about 8 rear-wheel horsepower on a good running engine. That means a turbo that can flow 50 lb/min (we’re talking stock-ish here) would be good to about 400rwhp. For those looking to be right at 1,000rwhp, an S480 is a popular choice, which at 120 lb/min (120 x 8 = 960 rwhp) we can see why. Note that it’s also popular to make more power than the “formula,” but if you do it’s usually at the risk of overspeeding the turbo.

Fueling

Engine airflow math is fairly simple (with some exceptions, like intercooling), but fueling is a tough one to crack. Lots of things come into play, including injection pressure, timing, duration, nozzle size, and so on. One thing we can calculate fairly easily though, is the needs of a lift pump. Starting with a factory example, let’s take a truck that has a turbo that flows 50 lb/min of air and calculate the fuel needs to support that power level. Most factory engines run at around a 20:1 air/fuel ratio (or even higher) to keep smoke to a minimum, so we can see that at that airflow level, we’d need (50 / 20) 2.5 lb/min or .36 gal/min (diesel is around 6.93 pounds in weight) or (.36 x 60) 21.6 gph. Not very much. However, just richening the air/fuel ratio to 14:1 increases that need to 30.9 gph. On a big horsepower mechanical truck (let’s say 120 lb/min turbo, and 12:1 air/fuel ratio) we can see the need goes all the way up to 86 gallons per hour. Now, you may be wondering why there’s 100 and 150 gph pumps out there, if there’s only a need for 86 gph, even on a hot engine, and that answer has to do with pressure. As pressure goes up, flow decreases, as the pump has a harder and harder time pushing the fuel to the engine. A pump that free-flows 150 gph might only flow 120 at 20psi, 100 at 40 psi, and 80 at 60psi. Some pumps aren’t even made to run at those types of pressures, and might not get there at all. Also, the instant need for fuel once the pedal is mashed means that most people go with overkill so there’s no pressure drop when the engine suddenly demands fuel.

Nitrous Oxide

Nitrous oxide can be great fun on a diesel, or it can lead to a lot of headaches, depending on how it’s used. The most common misconception about nitrous is that a certain size jet equals a certain size “shot.” While this is true on gas engines (a 0.062 jet for instance, is a 150hp shot) on diesels it works totally different. Since diesels operate over a wide range of air/fuel ratios, more air doesn’t always mean more power. In a lightly fueled truck, an 0.080 jet might equal 50 hp; in a heavily fueled truck, the same 0.080 jet might be worth 200 horsepower. Also, since nitrous flow is based upon the area of a jet (3.14 x radius^2), a .100 jet is not twice the size of a .050 jet, it’s four times larger. Diesels are also capable of huge amounts of nitrous ingestion, even with the turbo. Two or three 0.125 jets on a diesel can be worth 500 hp or more.

Transmissions, Axles, and Tires

It’s hard to fit everything about transmissions, axles, and tires in one category, but it’s important to remember that they all are interrelated to each other. Change the gear ratio? The overall speed versus rpm of the vehicle changes. It’s the same with transmissions, and tire sizes. We’ll start with the easiest calculation, estimated speed versus rpm. Let’s say that our test vehicle is a ‘01 GMC with a five-speed transmission, 3.73 gears, and 32-inch tall tires. In fifth gear, the engine is overdriven via a .71 gear ratio, which means the effective final gear ratio is (3.73 x .71) a 2.65. So to figure out speed at say, 2,000 rpm, our equation is mph = (rpm x tire diameter) / (gear ratio x 336). The number 336 is just a constant, that gives you a correct result. So, our speed is (2,000 x 32) / (2.65 x 336) = 71.8 mph. This formula is probably the most useful in determining tire size changes, as we can see switching to 35’s would give us a big speed increase to (2,000 x 35) / (2.65 x 336) = 78.6 mph. Just like the other formulas, it’s not 100-percent accurate due to slippage, tire deflection, or other unknown variables, but usually it’s darn close.

In closing, breaking out the calculator can help you plan the next move for your truck, calculate power, or just help you day dream. As useful (and accurate) as most of these formulas are, there are still “freak” vehicles that can break the mold, and run quicker or make more power than physics should allow. So take all of these formulas with a grain of salt, and remember that math isn’t scary, it’s helpful!

FREQUENTLY ASKED QUESTIONS

What is the “goldilocks zone” for turbo sizing, and why is it important?

Understanding the “Goldilocks Zone” for Turbo Sizing

When it comes to turbo sizing, finding the perfect balance is crucial for optimal performance. This “Goldilocks zone” refers to a size that’s neither too large nor too small, but just right—offering the best performance for your diesel pickup.

Why Size Matters in Turbochargers

• Performance: A turbo that’s too large can deliver impressive power but may suffer from slower spool-up times. This delay can impact your vehicle’s responsiveness, making it feel sluggish off the line.
• Efficiency: On the flip side, a turbo that’s too small might spool quickly but can limit your engine’s overall power potential. This could lead to higher exhaust gas temperatures (EGT) and reduced efficiency.
• Installation and Versatility: Generally, stock turbos or those around 64mm in size often hit this sweet spot. These options are engineered to provide a balance of quick spool time, effective EGT control, and seamless installation. Their design aims to complement both everyday driving and more strenuous tasks.

The Importance of the Right Fit

Choosing the appropriate turbocharger size ensures that your vehicle can handle a variety of tasks smoothly, whether you’re on the highway or tackling heavy loads. It enhances overall driveability and mechanical efficiency, making it an ideal choice for most diesel pickup owners. In short, identifying the Goldilocks zone in turbo sizing transforms your driving experience, maximizing both performance and fuel economy without sacrifices.

What turbo upgrades are recommended for a diesel truck focused on speed and performance?

Turbo Upgrades for Speed and Performance in Diesel Trucks

If you’re looking to transform your diesel truck into a powerhouse of speed and performance, turbo upgrades are a key focus. At the third stage of modifications, where speed takes precedence over utility, several options stand out.

VGT Turbo Upgrades

Transitioning to a variable geometry turbo (VGT) with a 67mm turbocharger serves as an ideal upgrade. This size provides a significant power boost beyond the common 600hp mark, aiming for over 700hp. VGTs offer versatility by adjusting the vanes to optimize airflow and acceleration, making them suitable for various diesel engines, including Cummins, Powerstroke, and Duramax models.

Single Fixed Vane Turbochargers

For those pursuing maximum performance, a single fixed vane turbocharger is an excellent choice. Although they might lack the broad powerband of a VGT, they excel in high-performance scenarios with their larger exhaust housing. These setups can accommodate turbos up to 72mm, delivering impressive peak horsepower and cooler exhaust gas temperatures. However, be prepared for a slight delay in spool-up time compared to VGT turbos.

Supporting Modifications

Enhancing your turbocharger goes hand-in-hand with other performance upgrades. Consider adding components such as larger injectors, an upgraded injection pump, intercoolers, valve springs, and an intake horn. These additional modifications work in harmony with your turbocharger, ensuring optimal power delivery and engine efficiency.

In summary, for a diesel truck geared towards speed, a combination of VGT or fixed vane turbo upgrades and supporting modifications can rouse remarkable performance gains, setting you apart on the road.

What problems can arise from using a turbo that is too large for a diesel truck?

The Pitfalls of an Oversized Turbo in Diesel Trucks

Choosing the right turbo for your diesel truck is crucial. While a small turbo can lead to damages, opting for an overly large one brings its own set of issues. Here’s why an oversized turbo might not be the best choice.

Slow Spool-Up Time

One of the primary concerns with a turbo that’s too large is its slow spool-up time. The larger turbine and compressor wheels require a significant amount of exhaust energy to function effectively. At low RPMs, such as driving in stop-and-go traffic or cruising on a flat highway, there isn’t enough exhaust flow to spin the turbo efficiently. As a result, you’ll often experience a sluggish response when you need that boost.

Increased Fuel Consumption

With an oversized turbo failing to provide the necessary boost during regular driving conditions, you’ll find yourself applying more throttle to maintain speed. This increased throttle input leads to higher fuel consumption, resulting in lower fuel efficiency. Essentially, you’ll be burning more fuel without gaining the corresponding power benefits.

Elevated Exhaust Gas Temperatures

The combination of low boost and high fuel consumption can elevate exhaust gas temperatures (EGT). High EGTs can be detrimental to your engine, leading to potential overheating and the risk of damaging components like pistons and turbos. When not monitored carefully, this could result in costly repairs.

Poor Driving Experience

While it’s true that when the turbo finally spools up, it can deliver impressive power and acceleration, the delay can make for a less-than-ideal driving experience. The performance boost might be thrilling on a long stretch of open road, but everyday driving will likely feel unresponsive and frustrating.

Conclusion

An oversized turbo might sound appealing due to its high power potential, but it comes with drawbacks that can affect your truck’s efficiency, engine health, and overall driveability. A balanced approach in selecting the right-sized turbo ensures optimal performance without compromising on fuel economy or exposing your engine to unnecessary risks.

What is the difference between a variable geometry turbo (VGT) and a fixed vane turbo?

Understanding VGT and Fixed Vane Turbos

When exploring turbocharger options, it’s essential to know the difference between a variable geometry turbo (VGT) and a fixed vane turbo. These two technologies play crucial roles in how efficiently your engine performs, particularly concerning responsiveness and power delivery.

Variable Geometry Turbo (VGT)

A Variable Geometry Turbo, or VGT, features adjustable vanes within the exhaust housing. These vanes can pivot to alter the flow of exhaust gases onto the turbine wheel. At lower engine speeds, the vanes close to increase the velocity of exhaust gases, helping the turbo spool up faster and reducing turbo lag. In contrast, at higher speeds, the vanes open up, allowing for a more efficient flow that maximizes power output and reduces exhaust gas temperatures (EGT).

Advantages of VGT:

• Enhanced Performance Across RPM Range: A VGT can blend characteristics of both smaller and larger turbos, offering more versatile performance.
• Reduced Turbo Lag: The adjustable vanes provide quick response at low speeds without sacrificing high-speed power.
• Adaptability: The adjustable nature allows for improved efficiency under different driving conditions.

Fixed Vane Turbo

On the other hand, a fixed vane turbo comes with a static set of vanes that do not adjust. This means the turbine wheel receives exhaust gases in a consistent manner, optimized for a specific engine speed range. While they don’t offer the same level of adaptability as VGTS, fixed vane turbos are simpler in design, which can lead to increased reliability and lower maintenance needs.

Advantages of Fixed Vane Turbo:

• Simplicity and Reliability: With fewer moving parts, fixed vane turbos tend to be more robust and easier to maintain.
• Cost-Effective: Generally less expensive to produce and repair due to the straightforward design.

Key Distinctions

• Flexibility: VGTS provide variability in performance, while fixed vane turbos are limited to a specific operational range.
• Complexity: The variable geometry design is more complex, possibly requiring more maintenance, whereas fixed vane turbos are simpler and can be more durable.
• Performance: VGTS offer a wider operating range with better response at varying speeds; fixed vane turbos excel at specific, consistent engine conditions.

Choosing between a VGT and a fixed vane turbo depends largely on your performance needs, budget, and maintenance preferences. Each has its unique benefits and drawbacks, making them suitable for different applications and user requirements.

What are the potential issues with installing a turbocharger that is too small for a diesel engine?

Turbos and Airflow

We get the question all the time: how do you size turbochargers? In reality, the answer is simple: turbos should be sized as small as possible for your desired horsepower. Luckily, airflow is one of those rare areas where the stars align, as roughly one pound per minute (lb/min) of air is equal to about 8 rear-wheel horsepower on a good running engine. That means a turbo that can flow 50 lb/min (we’re talking stock-ish here) would be good to about 400rwhp. For those looking to be right at 1,000rwhp, an S480 is a popular choice, which at 120 lb/min (120 x 8 = 960 rwhp) we can see why. Note that it’s also popular to make more power than the “formula,” but if you do it’s usually at the risk of overspeeding the turbo.

However, there are important considerations when it comes to turbo sizing, especially if the turbo is too small for your engine. A too-small turbo can deliver fantastic off-idle torque and quick throttle response, practically eliminating any boost delay. This might sound ideal at first, but it comes with significant drawbacks.

• Limited Peak Power: While the turbo spools up almost instantly, its small size means it has a restricted peak power potential. The turbine and compressor sections can only flow a limited amount of air, measured in cubic feet per minute (CFM), and once this limit is exceeded, the turbo acts as a bottleneck for your engine’s performance.
• Excessive Heat Production: Operating a small turbo beyond its peak efficiency leads to increased production of heat. The compressor side stops generating more airflow and instead, churns out excessive heat, potentially raising exhaust gas temperatures (EGT) and engine coolant temperatures to unsafe levels.
• Risk of Engine Damage: If you push your engine hard over a prolonged period with a too-small turbo—such as when towing heavy loads—the high temperatures can cause severe damage to internal components like pistons and valves, as well as the turbocharger itself.

Balancing turbo size is crucial not only for achieving desired horsepower but also for maintaining engine health and longevity. Understanding these dynamics ensures you make informed decisions about turbo sizing to optimize performance without compromising on safety and reliability.

What turbo options are available for a diesel truck used for both daily driving and occasional performance?

We get the question all the time: how do you size turbochargers? In reality, the answer is simple: turbos should be sized as small as possible for your desired horsepower. Luckily, airflow is one of those rare areas where the stars align, as roughly one pound per minute (lb/min) of air is equal to about 8 rear-wheel horsepower on a good running engine. That means a turbo that can flow 50 lb/min (we’re talking stock-ish here) would be good to about 400rwhp. For those looking to be right at 1,000rwhp, an S480 is a popular choice, which at 120 lb/min (120 x 8 = 960 rwhp) we can see why. Note that it’s also popular to make more power than the “formula,” but if you do, it’s usually at the risk of overspeeding the turbo.

Turbo Options for Dual-Purpose Diesel Trucks

When considering turbo options for a diesel truck that you use for both daily driving and the occasional performance burst, it’s important to find a balance that maintains utility without sacrificing power. Here are some options to consider:

1. Drop-In Turbos: For an easy installation and a steady boost in performance, a 64mm drop-in turbo might be your go-to. It offers improved power without the need for extensive modifications, perfect for those who want enhanced performance with minimal hassle.
2. Compound Turbo Kits: If you’re aiming for a broader performance range, a compound turbo kit could be the solution. This setup uses two turbos of varying sizes to deliver a wider operational range. While it involves a more complex installation process, it retains quick spool-up times and enhances power and torque, making it ideal for towing and performance driving.

Performance Considerations

• Exhaust Gas Temperature (EGT) Management: Managing EGT is crucial for maintaining performance and longevity. The right turbo setup can help reduce EGT, ensuring your truck runs efficiently even under stress.
• Torque and Power Gains: Upgrading to a compound turbo can significantly boost torque and power, providing the extra oomph needed for heavy-duty tasks or spirited driving.

Whether you choose a drop-in turbo for simplicity or a compound turbo kit for maximum versatility, understanding the balance between turbo sizing and performance needs is key to enhancing your diesel truck’s capabilities. By selecting the right setup, you ensure your vehicle remains powerful yet practical for everyday use.

How do engine modifications impact turbocharger performance?

Turbos and Engines

So let’s say you have your Cummins spinning at 3,000rpm and you want to add some boost. Every atmosphere above outside air adds another bar to the pressure ratio, so if ambient is 14.7 psi (it varies by elevation) then 14.7 pounds of boost would be a 2:1 pressure ratio. That means 29.4 psi would be a 3:1 pressure ratio, and 73.5 psi would be a 6:1 pressure ratio. If we take our earlier theoretical 250 cfm figure, that means to make 14.7 pounds of boost you’d need a turbo that flows 500 cfm, 750 cfm for 3:1, and a whopping 1,500 cfm for 6:1. More than anything, this can be very helpful in sizing turbos for your desired power levels.

But before you rush into turbo sizing, consider how other engine modifications can impact your turbo’s performance. A turbo isn’t an isolated component; it interacts closely with the rest of the engine. For instance, the balance between air and fuel is crucial. If you increase the fuel supply with larger injectors or a dual injection pump, you’ll need to boost airflow to match. Failing to do so can lead to high exhaust gas temperatures (EGT), which can harm your engine over time.

Imagine your turbo is perfectly sized for a stock fuel system. Adding more fuel with a hot tune and bigger injectors might make even the perfect turbo seem inadequate. This is why it’s often safer to install a larger turbo first, and then adjust the fuel system. This approach helps maintain balance and prevents issues like overheating.

In summary, while calculating pressure ratios and airflow needs are essential steps in turbo selection, understanding the interplay between engine modifications and turbocharger performance is equally important. With the right combination, you can optimize your engine’s power and efficiency without compromising reliability.

What is the recommended sequence for upgrading a turbo system?

When considering upgrades, it is advisable to install a larger turbo before making changes to the fuel system. This approach helps to maintain balance and prevent performance issues.

What are the consequences of unbalanced air and fuel intake?

If the air and fuel intake are not balanced, it can result in high exhaust gas temperatures (EGT), which can be detrimental to engine performance and longevity.

How do changes in the fuel system affect turbo performance?

Altering the fuel system, such as upgrading to larger injectors or adding a dual injection pump, requires a corresponding increase in airflow. Without this balance, the turbo may not perform optimally, leading to potential issues.