How To Size A Turbo For A Diesel Engine?

Diesel guys, whether they realize it or not, are number guys. In every commercial campaign, the Big Three will claim figures like best in class towing, greatest torque, and most horsepower. Diesel numbers are virtually usually large in today’s world of small turbo economy automobiles. For many aftermarket fans, 500 horsepower and 1,000 pound-feet of torque is the starting point for performance. Numbers penetrate practically every element of the diesel industry, whether you’re skilled at math or not, so let’s have a look at them! We’ll do our best to keep things simple.

Engine Displacement

Almost all of the diesel engines we work with in the performance sector are big, but if you can’t pinpoint a displacement (or you’re talking to an old hot rod guy), you can convert liters to cubic inches by dividing the size in cubic centimeters (5.9L = 5900cc) by 16.38. This results in a volume of 360.2 cubic inches. That translates to 506 cubic inches for an 8.3L (8300cc) and 116 cubic inches for a 1.9L TDI. Due to the fact that most American hot rod arithmetic is non-metric, the cubic inch to cubic centimeter conversion can be useful in everything from calculating airflow to sizing turbos.


Another useful notation is airflow, which is just cubic inches x revolutions per minute / 3456. When operating at 100% volumetric efficiency, a 360 cid engine (can you tell we like Cummins?) spinning at 3,000 rpm would consume around (360 x 3000 / 3456) 312 cfm of air. Volumetric efficiency, on the other hand, is frequently well below 100%, hovering around 80%. As a result, we now have a result of (312 x.8) 249.6 (let’s say 250) cfm of air. The best part is that you don’t even need to be a math whiz to use these basic formulae; all you have to do is enter in a few numbers and you’ve got your answer. We’ll get to that later if you’re wondering what airflow has to do with anything.

Turbos and Engines

Let’s pretend your Cummins is spinning at 3,000rpm and you want to enhance it. Every atmosphere above outside air adds another bar to the pressure ratio, thus if ambient pressure is 14.7 psi (it varies depending on elevation), 14.7 pounds of boost would result in a 2:1 pressure ratio. That means a 3:1 pressure ratio is 29.4 psi, while a 6:1 pressure ratio is 73.5 psi. If we use our earlier 250 cfm calculation, a turbo with a flow of 500 cfm, 750 cfm for 3:1, and 1,500 cfm for 6:1 would be required to provide 14.7 pounds of boost. This is very useful when it comes to sizing turbos for your desired power levels.

Turbos and Airflow

We’re frequently asked, “How do you size turbochargers?” In actuality, the solution is straightforward: turbos should be as little as feasible for the horsepower you want. Fortunately, airflow is one of the few places where the odds are in your favor, as one pound per minute (lb/min) of air equals about 8 rear-wheel horsepower on a well-running engine. That means a turbo capable of flowing 50 lb/min (stock-ish) would be good for around 400 rwhp. An S480 is a popular choice for those trying to get close to 1,000rwhp, and we can understand why at 120 lb/min (120 x 8 = 960 rwhp). It should be noted that making more power than the “formula” is popular, but doing so usually means risking overspeeding the turbo.


Engine airflow math is rather straightforward (with a few exceptions, such as intercooling), but fueling is a difficult problem to solve. Injection pressure, timing, duration, nozzle size, and other variables all play a role. The requirements for a lift pump, on the other hand, are quite simple to determine. Let’s start with a factory example and compute the fuel requirements for a vehicle with a turbo that produces 50 lb/min of air flow. To keep smoke to a minimum, most factory engines run at roughly a 20:1 air/fuel ratio (or even higher), therefore at that airflow level, we’d require (50 / 20) 2.5 lb/min or.36 gal/min (diesel weighs around 6.93 pounds) or (.36 x 60) 21.6 gph. Not a whole lot. However, simply increasing the air/fuel ratio to 14:1 raises the requirement to 30.9 gph. On a mechanical vehicle with a high horsepower (let’s say 120 lb/min turbo and a 12:1 air/fuel ratio), the need can reach 86 gallons per hour. Now, you might be wondering why there are 100 and 150 gph pumps available when only 86 gph is required, and the explanation has to do with pressure. As the pressure rises, the flow diminishes as the pump struggles to send the fuel to the engine. At 20 psi, a 150 gph pump may only flow 120 gph, 100 gph at 40 psi, and 80 gph at 60 psi. Some pumps aren’t designed to operate at those pressures and may not even reach there. Furthermore, because of the immediate requirement for gasoline once the pedal is mashed, most people choose for overkill to avoid a pressure decrease when the engine quickly needs fuel.

Nitrous Oxide

Depending on how it’s used, nitrous oxide may be a lot of fun on a diesel or a lot of headaches. The most prevalent nitrous misunderstanding is that a specific size jet equals a specific size “shot.” While this is true for gas engines (a 0.062 jet, for example, is a 150hp shot), it is not the case for diesel engines. Because diesel engines operate over such a broad range of air/fuel ratios, more air does not always imply more power. An 0.080 jet in a weakly fueled truck might be worth 50 horsepower; in a strongly fueled truck, the same 0.080 jet could be worth 200 horsepower. Also, because nitrous flow is calculated based on the area of a jet (3.14 x radius2), a.100 jet is four times greater than a.050 jet. Even with the turbo, diesels are capable of consuming large volumes of nitrous. A diesel engine with two or three 0.125 jets can produce 500 horsepower or more.

Transmissions, Axles, and Tires

It’s difficult to lump everything about transmissions, axles, and tires into one category, but keep in mind that they’re all interconnected. Do you want to change the gear ratio? The vehicle’s overall speed versus rpm fluctuations. Transmissions and tire sizes are the same way. Let’s start with the most straightforward calculation: estimated speed vs. rpm. Assume our test car is a 2001 GMC Sierra 1500 with a five-speed transmission, 3.73 ratios, and 32-inch tall tires. The engine is overdriven in fifth gear by a.71 gear ratio, resulting in an effective final gear ratio of (3.73 x.71) a 2.65. mph = (rpm x tire diameter) / (rpm x tire diameter) / (rpm x tire diameter) / (rpm x tire diameter) / (rpm x tire diameter) / (rpm x tire diameter) / (rpm (gear ratio x 336). The number 336 is only a constant that yields the desired effect. As a result, our speed is 71.8 mph (2,000 x 32) / (2.65 x 336). Switching to 35’s would give us a huge speed boost of (2,000 x 35) / (2.65 x 336) = 78.6 mph, hence this method is probably the most useful in predicting tire size adjustments. It’s not perfect, like the other formulas, because of slippage, tire deflection, and other unknown variables, but it’s typically quite near.

Finally, getting out the calculator might help you plan your truck’s next move, calculate power, or simply daydream. Even though most of these formulas are useful (and correct), there are still “freak” vehicles that can break the mold and run faster or produce more power than physics should allow. So take all of these formulas with a grain of salt, and keep in mind that math isn’t scary; it’s beneficial!

How do I know what size turbo I need?

It’s not difficult to figure out an engine’s boost pressure ratio, but you’ll need to put your ego aside for a while. The formula is straightforward: You may calculate your pressure ratio by multiplying the absolute output pressure you desire (14.7 + boost pressure) by the absolute inlet pressure the Earth says you can have (14.7). The most difficult thing is limiting oneself to a respectable number. Start with a realistic boost pressure, such as 10 psi for a normally aspirated engine, and work your way up to higher numbers for higher-power, track-only designs. You’ll also need to figure out the right absolute inlet pressure if you’re not near sea level, as it won’t be 14.7 psi.

Upgrade the Air Intake

Improving the airflow to the engine is a surefire technique to boost a diesel vehicle’s performance. More air will reach the engine using an enhanced air flow kit, resulting in increased power.

In addition, the new airflow kit will pull air from outside the engine compartment, bringing colder air in. The amount of power produced by the engine will rise because cooler air is denser and holds more oxygen.

An enhanced air flow system can boost horsepower while also improving fuel economy.

Change or Reprogram the ECM

Engine performance is controlled by the Engine Control Module (ECM), which alters critical engine parameters such as the air-fuel mixture and maximum RPM.

You may easily change these settings by reprogramming or changing the ECM. This will allow the engine to create more horsepower and torque, which will increase performance.

ECM upgrades not only increase power, but they also help to increase diesel efficiency.

Using New Fuel Injectors

The next step is to upgrade the fuel injectors if you’ve improved the air flow to the engine and set up the ECM to produce additional power.

More fuel will reach the engine thanks to new fuel injectors, resulting in increased horsepower. Individual injector nozzles are found on most performance fuel injectors, which provide higher pressure and better atomize the fuel.


Adding extra power to diesel engines using a performance turbocharger is a wonderful way to do it.

The turbo operates by pressurizing the air intake and forcing additional air into the engine. It is possible to generate more power while improving engine efficiency by using a turbo.

In comparison to a non-turbo engine, a stock turbo boosts air flow three to four times. A performance turbo, on the other hand, can enhance airflow by five to ten times over a non-turbo engine, resulting in a bigger horsepower boost.

Performance Exhaust

You’ll need to update your exhaust system if you want to increase the engine horsepower.

Unlike factory exhaust systems, which are designed to reduce noise, a performance exhaust system will have a wider diameter and fewer bends, allowing for more exhaust flow.

A broader, straighter exhaust system will help reduce exhaust gas temperature and boost the engine’s horsepower and torque.

What happens if your turbo is too small?

A horsepower target is the first step in selecting a performance turbocharger. Each turbocharger is engineered to accommodate a specified horsepower and displacement range. There will be a lot of turbo lag if a turbo is too big for your engine, and if a turbo is too tiny for your engine, you may not reach your horsepower target. These sections provide information created by our engineers to assist you in the process.

How much HP does a turbo add to a diesel?

Adding a supercharger or turbocharger to your vehicle will result in immediate horsepower gains. While this is one of the more expensive alternatives on the list, the power that each of these add-ons provides will astound you. Both of these components, often known as forced induction parts, force air into your car’s engine, increasing horsepower and torque.

A turbocharger works in conjunction with the exhaust system to produce improvements of 70-150 horsepower. A supercharger is a device that adds 50-100 horsepower to an engine by connecting directly to the intake manifold.

What upgrades are needed for a turbo?

If you want to make significant horsepower gains with your turbo system, you’ll need to upgrade the engine’s stock fuel system. The stock fuel pump and injectors are frequently incapable of delivering a substantial volume of fuel over the stock level. This restricts the tuning potential of your engine and, as a result, the amount of boost you can get. Upgrades to higher-flow fuel pumps and injectors will allow you to tune to compensate for the turbo’s increased airflow, increasing horsepower and dependability.

What size turbo is on a 5.9 Cummins?

When it comes to turbocharger sizes, the inducer diameter of the compressor wheel, turbine wheel diameter, and turbine-side housing ratio are the most generally used metrics. All of these variables have an impact on the turbo’s performance. Large exhaust housings, large-diameter turbine wheels, and smaller compressor-side inducers are common features of stock turbochargers. For example, a HX35 turbo from a 5.9L Cummins measures 54/69/14, which indicates it has a 54mm compressor wheel, a 69mm turbine wheel, and a 14cm2 exhaust housing. The size of the exhaust housing impacts how quickly the turbocharger spools up; however, a housing that is too tiny can result in high drive pressures (think of it as boost on the exhaust side), which can be difficult on engines, turbochargers, and exhaust hardware. Non-wastegated turbochargers have larger exhaust housings to keep the turbo’s driving pressure within acceptable limits, even at top engine speeds and fueling levels. If not wastegated, a common aftermarket turbo might be in the 64/71/14 range, but a faster-spooling 64/65/14 (note the small turbine diameter) will need to be wastegated, else high drive pressures, high EGTs, and possibly turbo overspeeding will occur.

What does a bigger turbo do?

Installing a large turbo in place of your turbocharged car’s stock equipment is a common option to substantially boost performance. However, understanding the benefits and drawbacks of this type of upgrading is critical to determining whether it’s something you actually want to do. Although it’s tempting to give in to the urge to have something more, don’t “When you step on the gas pedal, you’ll hear “go,” but the truth is that this tweak won’t help everyone as much as they might imagine. Before you spend the money on a larger-than-stock turbocharger, keep the following points in mind.

To be licensed, all 50 states currently mandate that all vehicles meet some form of emissions criteria. Vehicles with factory-installed equipment are particularly configured to ensure that they meet these requirements. If you modify your vehicle beyond its original settings, you may be obliged to follow specific standards to ensure that your vehicle does not emit any dangerous emissions as a result of the changes. Before you conduct any maintenance on the car, you’ll need to study up on your local and state legislation to ensure that you’re following all of the rules. Failure to do so could result in a fine or, worse, a car that is unable to be licensed due to changes that do not meet state requirements.

In most cases, insurance companies need you to declare any engine modification that significantly reduces horsepower. The reason for this is because cars with greater power are typically driven more aggressively than ones without (for obvious reasons). This may appear to be a little detail, but it’s critical that you follow the instructions in your insurance policy to the letter. If you don’t, your insurance provider can deny liability in the event of an accident, leaving you vulnerable.

Generally, any factory warranty on your vehicle will be voided if you make this type of upgrade. This type of drastic change to your factory equipment might put additional stress on your vehicle’s engine, and your car’s manufacturer will no longer cover it. Before making the transition, be sure you understand what you’re getting into with your car’s warranty.

The truth is that, while you may be looking forward to your new, bigger turbo giving out gobs of extra power, it will, in most cases, have a negative impact on drivability. The larger the turbo, the more detrimental it will be to daily driving. This is because turbochargers rely on exhaust energy to power the compressor that feeds more air into the engine. The more exhaust gases it takes to spin the turbine, the bigger the turbo is. This means that large turbos simply do not work effectively at low RPMs (a phenomenon known as turbo lag) “Most people drive in this area (“turbo lag”). A large turbo produces a lot of horsepower and high RPMs, which is fantastic on the drag strip but bad in traffic. Furthermore, increasing your car’s turbo might reduce gas mileage, which can cost you a lot of money in the long run.