Turbocharging 102

Installing – or upgrading – a turbocharger is a great way to increase your vehicle’s total power output. However, before you go on eBay and buy the biggest turbo you can find, you need to know about sizing.

Sizing A Turbocharger For Your Vehicle

This article will introduce you to the various aspects of sizing a turbocharger, the goal being to help enthusiasts learn the basics of turbocharger selection. Our best advice here is to read this article, make yourself familiar with the basics, and then talk to either a tuning shop that installs turbos or a turbocharger manufacturer for vehicle-specific recommendations.

Choosing A Compressor Housing

NOTE: If you’re not a math whiz, skip down to the TL:DR summary at the end of this section.

You start your turbocharger selection by determining what size compressor housing you need to fit your power demands. A racer will determine how much horsepower he needs to remain competitive, and a street car using normal fuel, rather than high-octane, needs to determine how much boost their engine can accept. It usually falls around 10 psi in a vehicle without an intercooler and 15 psi for those with an intercooler. This is based on an engine that is very well tuned and has an 8:1 or 9:1 compression ratio (a rough figure).

Next, you need to determine how much airflow (in lbs per minute) your compressor needs to flow. This is because horsepower is created when you burn fuel, and you need more air to burn more fuel. Thus, more airflow = more horsepower.

Also, you’re trying to pair the compressor’s efficiency with your engine’s most powerful RPM range…you want the compressor to be doing its very best work in the RPM range you find yourself driving in. That way, your compressor isn’t a chokepoint.

To determine the airflow rate in pounds per minute, you need to:

  1. Determine how much horsepower you want to add, with the understanding of what’s possible, what’s practical, and what’s extreme. This is where discussions with tuning shops and turbocharger companies are so helpful.
  2. Once you know your target peak horsepower output, you need to determine engine airflow to reach this horsepower output. You generally do this by assuming assume a specific air fuel ratio and brake specific fuel consumption (BSFC) ratio, only there are other methods.
  3. You use the following equation:

Turbo airflow equation

If we assume a horsepower goal of 300hp, an air/fuel ratio of 12, and a BSFC of 0.55, we end up with a target airflow rate of 33lb/minute. However, this is not the flow rate for the compressor…this is the flow rate of the engine as a whole. The compressor’s flow rate is dependent upon the engine’s size (displacement), as well as ambient air pressure and temperature.
To calculate the compressor flow rate, we actually need to calculate the amount of boost required to meet the flow rate for a specific engine.So, using our airflow rate from the equation above:

Boost equation

You’ll need to know intake manifold temperature (in Fahrenheit), which you can assume to be 130 degrees on most gas powered engines. You can also assume a volumetric efficiency (Volum. Eff., or VE) of 1.0 if you don’t know the VE of your engine at peak horsepower. The RPM should be your peak horsepower RPM. Finally, you’ll need to calculate the engine displacement in cubic inches.

If we assume that we’re trying to get 300hp out of a 2.0L 4-cylinder engine, and we assume a 1.0 VE, a 130 degree intake temp, and a peak HP output of 5,500 RPM, then using the math above we end up with a boost pressure of 37.1 PSI. However, this is not the peak boost of the turbo…this is the total combined pressure of the turbo and atmospheric pressure. While atmospheric pressure changes with altitude, it’s usually safe to assume an atmospheric pressure of 14psi. So, the amount of boost we need to get 300hp out of a 2.0L engine is about 23lbs (37.1-14 = 23 lbs).

Finally, we need to take the ratio of boost to ambient pressure (eg, divide 23 by 14psi) to come up with a pressure ratio, which in this specific example is about 1.65. Technically, we should correct for some pressure loss on both sides (we need to add a couple of pounds of boost to the peak figure to account for some ‘plumbing issues’ and subtract a pound from ambient pressure for the same reason), but since we’re approximating a lot of figures, we’re going to skip this step.

Suffice to say, this math is designed to get you to a good starting point, not to precisely determine the perfect compressor for your needs. The only way to perfect your compressor is to test a few options…but the math will get you close.Once you have a) an airflow rate in lbs/min and b) a compressor ratio, you’re ready to read a compressor map to find the right compressor for your specific engine.

Comp map

This map – along with a better explanation of the math used to calculate both compressor ratios and airflow rates – can be found in Garrett’s turbo tech article section, which is highly recommended.


TL:DR: Choosing a Compressor is Best Done With Expert Help

In order to find the right compressor for your specific vehicle and horsepower goals, you need to calculate two figures:

  1. Airflow rate in lbs/min
  2. Compressor ratio (which is a function of your engine size and peak hp RPM)

While you can calculate both figures yourself, the best thing to do is review the basic match so you understand the principles, then contact either a tuner or ask a turbocharger retailer for a recommendation. Most turbocharger retailers and manufacturers have created handy tables and charts to help you select the right compressor for your specific engine.

Choosing A Turbine Housing

There are two aspects to choosing a turbine housing:

  1. The physical dimensions of the turbine housing (which need to reflect your exhaust system sizing and available space in your engine bay)
  2. Choosing a turbine that has the right A/R ratio based on your turbocharging goals

On the first point, there are a lot of options on the market. You can go with an OEM-size unit that merely has different attributes, or you can integrate a completely different size and make a bunch of changes to your vehicle.

But whatever you choose, the key is to understand the importance of A/R ratio and the impact it has both on peak HP and turbo responsiveness. A/R ratio is the result of dividing a turbo’s cross sectional area by the turbo’s radius, but in practical terms, it’s better to think of A/R ratio as a proxy for “leverage.”

Basically, the higher the A/R ratio, the more horsepower that can potentially be extracted from the exhaust stream. However:

  • If the ratio is too high, the engine will feel like it’s being “held back” until you approach redline, as the turbine is basically a constraint on the engine because of all the leverage it’s trying to extract from the exhaust
  • If the ratio is too low, the turbo will reach peak output too quickly, making the vehicle impossible to drive

Most OEM turbochargers have an A/R of around 0.7. They start to make power almost immediately after they’re engaged, but they’re still very drive-able at all RPMs. Racing turbochargers, on the other hand, are designed to provide power when the engine RPMs are in a specific higher power range. They can have A/R ratios near 1.5 (or even higher), which makes them sluggish to engage at low RPMs but very powerful at or near the redline (which is just fine in a lot of racing situations).

There is no quick solution to determining what your A/R should be, but turbine manufacturers provide some A/R ratio charts that help you determine what you should install based on the compressor you select. Again, the best plan is to converse with turbo manufacturers, retailers, and/or engine tuners. They will be able to recommend turbine and compressor pairings that make sense for your specific vehicle and goals.

Don’t Re-Use Unless You Do The Math

In order to get the most from a turbocharger, the selection of the proper equipment is critical. As you can see from the detailed math above, there are a LOT of items to consider when choosing the right compressor and turbine. Suffice to say, salvaged turbos must be treated with skepticism. While they can be a great pairing for a specific vehicle, they can also be a terrible (perhaps even dangerous) fit for your specific vehicle.

Most of the time, people buying used turbos are looking for replacements. However, it is possible to scrounge up a somewhat modular turbo compressor or turbine on eBay for a good price, and then rebuild/refit the housing to meet the specs you have.

Finally, remember that there’s gold at the end of the “sizing a turbocharger” rainbow. It’s not an easy process (necessarily), but it’s worth it: