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 turbo options are available for a diesel truck focused on speed and performance?

Turbo Options for High-Performance Diesel Trucks

When transforming your diesel truck into a speed and performance machine, turbo upgrades play a critical role. For enthusiasts who prioritize acceleration and power over utility, several turbo options are available, each with its unique advantages.

Variable Geometry Turbo (VGT) Upgrades

For those looking to maintain some versatility without compromising on performance, upgrading to a larger VGT is a popular choice. These turbos, like those in the 67mm family, offer improved airflow and can significantly boost your horsepower, pushing you past the 700hp mark. They are ideal for 5.9 or 6.7 Cummins engines and similar setups. VGTs provide a broad powerband and are relatively simple to install as drop-in replacements.

Fixed Vane Turbochargers

If you’re aiming for peak horsepower and are less concerned with low-end torque, a single fixed vane turbocharger might be the way to go. Although these turbos may lack the wide powerband of a variable geometry system, their free-flowing exhaust housings make them excellent for high-performance builds. Options are available that can handle up to 67mm and beyond, delivering impressive power with reduced exhaust gas temperatures (EGT).

Turbo Kits and Conversions

For serious truck enthusiasts, complete turbo kits that convert your existing setup into a high-performance beast are available. These kits often include different turbo sizes, ranging up to 72mm, catering specifically to brands like Cummins or Powerstroke engines. While these installations might demand more effort, the resultant power gain and performance enhancement make the investment worthwhile.

Performance Considerations

When choosing the right turbo for your truck, consider what aspects of performance are most critical to your driving needs. Whether you opt for a VGT for its ease and versatility or a fixed vane turbo for its robust power output, preparing your vehicle with larger injectors, a more powerful fuel injection system, and supportive engine modifications will help maximize your gains.

In summary, turbo options for those focused on speed and performance range from sizable VGT upgrades for balanced power and ease of use to high-output fixed vane turbos for pure horsepower. Each choice offers distinct advantages depending on your specific performance goals.

What happens if you install a turbo that is too large for your intended usage?

When considering turbo installation, size matters more than you might think. Opting for a turbo that is too large for your needs can introduce a range of challenges.

Slow Spool-Up Time

A large turbo requires significant exhaust energy to get moving. If your engine can’t provide that, you’ll experience a slow spool-up. This delay means sluggish performance, especially noticeable during city driving or cruising on highways.

Increased Throttle Demand

Because the turbo isn’t generating boost efficiently at lower RPMs, you’ll need to press the throttle harder to maintain speed. This leads to higher fuel consumption, as your engine works overtime without the expected boost in performance.

Elevated Exhaust Gas Temperatures (EGT)

The combination of more throttle input and low boost levels can result in increased exhaust gas temperatures. These higher temperatures can stress engine components, potentially leading to damage over time.

Power Surge

On the flip side, if conditions allow the turbo to spool up, you’ll experience a significant surge in power. This rush can be exhilarating and your EGTs will stabilize and even cool at maximum output, indicating the turbo is working efficiently at high power levels.

Poor Street Performance

Despite the benefits at peak performance, the downsides of everyday driving make a too-large turbo impractical for typical street use. It provides a power imbalance—great at high speeds, but inefficient and cumbersome at lower ones.

In summary, installing a too-large turbo can diminish driving comfort and engine efficiency, leaving you with high fuel costs and potential damage unless you’re constantly pushing the limits. Choose wisely to balance power and practicality.

What are the characteristics of a turbo suitable for a dual-purpose truck with some performance modifications?

Characteristics of a Turbo for a Dual-Purpose Truck with Performance Modifications

Driving a dual-purpose truck with performance modifications involves seeking a balance between fun and functionality. Owners of such trucks often have an appetite for speed, which translates into a few key modifications under the hood. These trucks commonly feature enhancements like performance tuning programs, cold air intakes, possibly upgraded intercoolers, larger-than-standard tires, and sometimes even reinforced transmissions due to past failures.

Power & Performance Balance

The turbocharger chosen for these trucks must be capable of delivering consistent power output while ensuring the vehicle remains practical for everyday use. This means it needs to handle increased power without elevating exhaust gas temperatures (EGT) excessively and provide efficient spool-up characteristics to prevent lag during acceleration.

Recommended Turbo Options

1. Drop-In Turbochargers:
   • Ideal for straightforward installation and balanced performance.
   • These turbos typically offer a good mix of power and utility without requiring extensive modifications or affecting the vehicle’s emissions systems.
2. Compound Turbo Kits:
   • Comprise two differently-sized turbochargers working together to cover a wide performance range.
   • While installation is more complex, these kits provide exceptional boost potential and improved engine torque. The setup helps to maintain quick spool-up thanks to the continued use of the stock turbo alongside a larger secondary turbo.
   • Particularly beneficial for those who utilize their trucks for towing, as they enhance power while reducing EGTs, effectively supporting over 600 horsepower on a stock fuel system.

Conclusion

Opting for either a drop-in turbo or a compound turbo kit depends on your priorities—whether you’re looking for easy installation or maximum towing capability. In both cases, the key is ensuring the turbo can handle increased power demands while staying true to the truck’s dual-purpose nature, balancing both performance and practicality.

What are the differences between variable geometry turbos (VGT) and fixed vane turbos?

Differences Between Variable Geometry Turbos (VGT) and Fixed Vane Turbos

When exploring turbocharger options, it’s crucial to understand the distinctions between variable geometry turbos (VGT) and fixed vane turbos. This knowledge can guide your decision-making in selecting the right performance enhancements for your vehicle.

Variable Geometry Turbos (VGT)

One of the standout features of VGTs is their adaptability. Modern engineering allows these turbos to adjust the position of their vanes, which are situated in the exhaust housing. These adjustable vanes direct exhaust gases optimally at varying engine speeds:

• At lower speeds, the vanes close, accelerating the turbo and minimizing turbo lag. This offers a quicker response for enhanced drivability.
• At higher speeds, the vanes open, allowing exhaust gases to flow freely. This maximizes power and reduces exhaust gas temperatures (EGT).

The key advantage of VGTs is their ability to offer the benefits of both smaller and larger turbos by adjusting within a broader operational range. This flexibility is why they’ve been a popular choice in the automotive industry for years.

Fixed Vane Turbos

In contrast, fixed vane turbos have a set geometry. This design does not allow for internal adjustments as engine speeds change. Consequently, these turbos are optimized for a specific performance range:

• Simplicity and reliabilityare often highlighted as strengths due to fewer moving parts.
• Consistent performancewithin their designated range, without the need for adaptive mechanisms.

However, their fixed nature means they may not perform optimally across a wide range of driving conditions like VGTs.

Key Differences

• Operational Range:VGTs have a broader range, whereas fixed vane turbos are confined to a predefined performance band.
• Complexity and Maintenance:VGTs are more complex, potentially requiring more maintenance due to their adjustable parts, while fixed vane turbos offer straightforward reliability.
• Performance Adaptability:VGTs can adjust to provide both quick acceleration and high power, whereas fixed vane turbos are tuned for consistent output in a narrower band.

Deciding between these two types depends on your vehicle’s needs and your performance goals. Whether you prioritize flexibility or simplicity, understanding these differences will lead to more informed decisions.

Why is there no one-size-fits-all solution for turbo sizing in diesel trucks?

Compound turbos can be a bit more difficult to size than single turbos, and can be geared towards response, all-around performance, or peak power. In general, the larger turbo should flow roughly twice what the small turbo will flow.

However, the right turbo setup isn’t a one-size-fits-all solution. It largely depends on how you plan to use your truck. Are you looking for quick response times, or is your priority achieving peak power for towing heavy loads? Perhaps you’re seeking balanced performance for daily driving and occasional off-road adventures. Each scenario demands a different approach to turbo sizing.

To simplify the decision-making process, consider breaking down your usage into categories:

1. Quick Response: Ideal for those who need immediate power delivery.
2. All-Around Performance: Perfect for balanced driving needs.
3. Peak Power: Suited for maximum output and heavy-duty tasks.

By identifying which category best fits your requirements, you can better determine the appropriate turbo size. If you’re still uncertain, consulting with experienced professionals can provide tailored recommendations to ensure your setup meets your specific needs.

What turbo considerations are necessary for a race truck aiming for high horsepower?

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.

For those venturing into the race truck realm, where horsepower exceeds 850, choosing the right turbo setup becomes even more critical. At these elevated levels, a single S400-based 80mm turbo can be a gateway to 1,000 horsepower, while a compound setup with a larger 106mm turbo can push into the 2,000-horsepower range. This choice depends heavily on whether you prioritize peak power or a balanced performance across various conditions.

Compound Turbo Considerations

Compound turbos can be a bit more difficult to size than single turbos, and can be geared towards response, all-around performance, or peak power. In general, the larger turbo should flow roughly twice what the small turbo will flow. This setup is perfect for those seeking a balance between quick spool-up and maximum power output.

However, achieving such high power doesn’t stop at the turbos. You’ll need a fortified engine with stronger pistons and rods, a performance camshaft, and a robust valvetrain. A massive fuel system is essential to support these modifications, and a built transmission is critical to handle the immense stress.

Safety and Practical Considerations

Don’t forget the racing-specific equipment like a roll cage and drag slicks, which are essential for safety and performance on the track. While these setups deliver breathtaking power, their street usability can be limited due to slow spool-up times, so it’s crucial to weigh your priorities.

In summary, understanding airflow and turbo sizing is just the beginning. Comprehensive vehicle modifications and strategic planning are key to unlocking the full potential of your race truck.

How do engine modifications affect turbo 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.

Figuring out exactly how much a turbo will flow requires a bit of help from the manufacturer. Companies like Garrett and BorgWarner provide access to compressor maps, which show the maximum pressure ratio, flow, and compressor speed.

Engine Modifications and Turbo Performance

While turbo sizing is crucial, it’s important to remember that other engine modifications can significantly impact turbo performance. For optimal engine performance, the air and fuel your engine consumes must be balanced. If you decide to increase fueling with larger injectors or a dual injection pump, you must also increase airflow. Failing to do so could lead to issues like high exhaust gas temperatures (EGT), which compromise engine efficiency.

Imagine a turbo that’s perfectly matched with a stock fuel system. With a set of larger injectors and a high-performance tune, that same turbo might be too small, unable to provide the necessary airflow. When upgrading, consider installing a larger turbo first and keeping the stock fuel system. This approach minimizes risks and ensures smoother transitions between modifications.

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.

Compound turbos can be a bit more difficult to size than single turbos, and can be geared towards response, all-around performance, or peak power. In general, the larger turbo should flow roughly twice what the small turbo will flow.

By considering both turbo sizing and the effects of engine modifications, you can fine-tune your setup to achieve the ideal balance of power and reliability.