How Compound Turbocharger Systems (Turbos) Work

Updated on June 4, 2015

Forced induction is a term car guys throw around: it just refers to a process that compresses air into the intake side of an internal combustion engine. How you go about doing this is actually pretty complicated, but the concept behind it is not. You want cooler, denser air to flow into your engine so that you can produce more power per unit of fuel. This compression of air flowing into your intake is referred to as boost.

There are many ways to go about converting your naturally aspirated engine into a forced-induction engine, or alterng the airflows in an already boosted power plant. I would like to focus on just one of these forced-induction processes right now: compound turbo systems.

Compound Turbocharger

Turbocharger A "feeds" turbocharger B
Turbocharger A "feeds" turbocharger B

Twin Turbo Vs. Compound Turbo: There's a Difference

Not every turbocharger system that utilizes two compressors is a twin turbocharger system. In a twin turbo system, the two turbochargers that compress the air are the same size, and they are set up to split the job of feeding air into the intake between them; the turbos are set up "in parallel."

In compound turbocharged systems, you again have two compressors, but unlike in the twin system, these turbos are different sizes, and they are arranged in series as opposed to in parallel. Instead of splitting the job of forcing air into your intake manifold (or intercooler), they work together, one after the other, to get the job done.

How Compound Turbo Works

Within a compound turbo system, you have a low-pressure turbocharger (the larger one) and a high-pressure turbocharger (the smaller one). Air from the atmosphere flows through the low-pressure turbo, from there into the high-pressure turbo, and from there into your intake manifold or intercooler. This "compounds" the boost effect, which is exactly what you want.

If you have trouble visualizing this effect, think about it like this. Air is a fluid, just like water is a fluid. When water freely flows from a large pipe to one that's significantly smaller, both the pressure and velocity of the water within that pipe greatly increase. The same concept is being applied here. Instead of water flowing through pipes, you have air flowing through turbochargers. Remember, more boost means extracting more power per unit of fuel and greatly improving the performance and efficiency of your platform.

Compound Turbochargers Reduce Turbo Lag

In a previous article on how anti-lag systems (ALS) work, I described the phenomenon known as turbo lag. While there is no way to fully overcome turbo lag in a turbocharger system, there are ways of making throttle response nearly instantaneous.

As it happens, reducing turbo lag happens to be one of the best attributes of a compound turbocharger system. Adding a second, smaller, high-pressure turbo provides boost in the lower end of the RPM range without causing a delay (that you can perceive) between the time you depress the accelerator and the time you accelerate. The high-pressure turbo will continue to provide most of the boost into your intake until enough exhaust is produced to spool the low-pressure turbo. Once the low-pressure turbo is spooling, the amount of boost being fed through the intake is dramatically increased.

Again, to better understand what's going on, it helps to think about this in terms of water in pipes. Before, I mentioned that when water is freely flowing from a large pipe to one that's significantly smaller, both the pressure and velocity within the smaller pipe are significantly higher. Well, now picture the water as RUSHING through the large pipe. You can imagine that if the water in the smaller pipe was already flowing pretty well, that now it's just ridiculously pressurized and flowing VERY well! That's exactly what's going on within a compound turbocharger system. The level of boost it can produce is sinister!

In short, a well-designed compound turbocharger system provides just about everything you want from a turbo setup without any of the normal drawbacks. Excellent throttle response, a dramatic increase in platform performance levels, and a great time in the driver's seat. The only real drawback is a complicated design, and a lot of head-scratching when it comes time to tune the fuel and ignition maps.


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      Fnbend09 4 months ago

      Would the high pressure( or smaller diameter turbo) not begin to restrict flow once the low pressure turbo( or larger diameter turbo) spools up? I guess I was thinking that there might be a bypass valve of some type allowing air to circumvent the HPT once the LPT had reached it's peak efficiency. I think I called those by the proper names. Additionally, is it fair to assume that where fiesible, 3 or more turbos could be used in series?

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      Newbie 5 months ago

      So the smaller one spools up first and draws air through the bigger one spooling it up. So you get boost even at low rpm but can reach higher pressures when the larger one spools up. Did i get it right?

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      9 months ago

      Here is an easy way to think about it. Large turbo is for volume. It works really well at high engine RPMs when the engine needs a lot of volume, horrible for low engine RPM. The small turbo is for pressure. Quick spooling= good for low engine RPM but cant supply volume needed at higher engine RPM. So with combine the two you get both worlds, quick spool up at low RPMs and high pressure/high volume at higher engine RPM. You get a flatter power delivery that rises quick. Many things can effect turbo performance characteristics, turbine and compressor wheel geometry and diameter sizes for example, but a single turbo only works good at a certain part of the spectrum. To widen that spectrum you compound turbo.

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      Jason 11 months ago

      I always thought that it reversed where the smaller turbo helps the bigger turbo build boost. would it be better to say have a rb26 or 2jz motor in a r32 would it be better to do compound turbos over twin turbo

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      anonymous 11 months ago

      now i always thought the smaller turbo would help build boost to spool the bigger one do i have everything correct or not

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      Abi 15 months ago

      Steven, you're wrong about several things with your statement. First, air is not a compressible gas below a certain velocity, so you treat it as incompressible flow. Perhaps some research into incompressible and compressible fluid flow will benefit you're general understanding. Second, the air is only more dense per fixed unit of volume if you can reduce temperature. There is no heat sink between the two "compressors", so the air is not any more dense than the ambient air of the environment you're in. In fact, you can argue that it's less dense due to the heat generated in the engine bay. Third, the function of the compressor wheel in a turbocharger is to increase intake air velocity. Flowing more air allows for you to also feed more fuel, so you can increase specific output of your system. Fourth, PSI is not a unit of weight, but of pressure. In this particular case, it would likely be measured in your intake manifold. Fifth, your comparison about oil and water versus forced induced air is completely irrelevant, and it's also incorrect. While oil may float on top of water due to a differential in densities, the true reason they do not mix is due to polarity. Seeing on how ambient and pressurized air maintain the same polarity, and are also being mixed almost continuously into homogeneous mixture...the oil and water comparison simply makes no sense.

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      steven 15 months ago

      this is incorrect. changing velocity of the air is not a compounding effect. you are using the large turbo to trick the high pressure turbo into thinking the densified compressed air is atmospheric pressure. it then takes this air and further increases the density and sends it into the engine, hence a compound effect. a turbo is a unit that compresses air increasing its density, and by increasing density it increases in weight (psi) and shows as pressurised air because the air around it is at a lower density. the same reason oil floats on top of water.

    • ChrisCampbell05 profile image

      Chris 2 years ago from Tampa, FL

      I love the article maybe you could expand it by writing on compound setups using turbos and root superchargers. Insane low end torque but pulls like a train up in the RPMS. Check out the Hellraiser twin turbo setup for the supercharged 03-04 mustang cobras.

      Also Lloyd, here is a post on how manual boost controllers work

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      Auto Faction 5 years ago

      Yes, there will definitely be a post about both in the near future. Thanks for the comment!

    • Lloyd Barnhill profile image

      Lloyd Barnhill 5 years ago from Hartford, Connecticut

      Will there be a follow up article about sequential turbo systems or boost controllers?