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How Compound Turbocharger Systems (Turbos) Work

Auto Faction enjoys maintaining, improving, and modding cars.

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 altering 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 a 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.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


UncleZ on August 24, 2020:

Twin turbo setup is what you have bc your not compounding anything. Compound turbo setup actually compounds boost from one to the next. It doesn't bypass anything. You boost the boost like 20psi just an example to 120psi. That is what compound boost is. my comment isn't explained in detail but its not completely wrong. Info like this article and video should be taken down bc it is completely false information.

Jonathan on August 20, 2019:

Aex is correct. As I was reading the part where you used the water analogy, my brain kept screaming Bernoulli's theorem!

Alex on November 23, 2018:

There is an IMMENSE amount of misinformation here. Let me try to rectify some:

1. one does not want "cooler, denser air" in the engine, but just "denser" air. The "denser" already includes the "cool" part.

2. There is NOT more power per unit fuel (I'll call this efficiency). "power per unit mass of fuel" is a non-sensical term anyway; it must be "work per unit mass of fuel". A turbo-charged engine is less efficient at first, since the turbine increases work required to push out the exhaust gases. Only through various decidedly INDIRECT ways, the efficiency of a turbocharged engine is also increased (thus, more work per unit mass of fuel). The one and only goal is to have denser air and therefore more "power density", thus more power from the same engine size/displacement/etc. That is all. Only "afterwards", efficiency is increased indirectly.

3. "compression" or "Boost" don't "flow" anywhere, they aren't what is called stream quantities, but quantities of potential. Through their potential, a stream forms. The stream (energy, mass..) flows somewhere, but the potential does not go anywhere.

4. the analogy of water and air is 100% better not be made here. Water and air are both fluids, that is correct. Water as a liquid is however nigh incompressible (negligible influence of pressure on its density -- this does not encompass the temperature influence by the way). Air as a gas is *ALWAYS* compressible. Abi, in an earlier comment, is wrong about this as well. I reckon he refers to gas being treated as incompressible up to Mach numbers of about 0.3 (there is no consensus about this anyway); this is only to say that only beyond certain speeds or Mach numbers does the density play a significant role. However, even at *any* speed below, the density still changes -- compressible flow is yielded.

In fact, the compressor really only increases the gas's pressure and therefore density. At the same time, it SLOWS the gas. This is a key concept: going through the compressor, the gas is slowed! The opposite is true for the turbine.

Water as a liquid and air as a gas just cannot be compared right here.

Also, the water's "pressure" does not rise while going into the smaller diameter pipe.

5. the really nice throttle response is only really true for an actively controlled serial turbocharger. You did not mention this, but if we are referring to two passive compressors just combined in series, the effect is not as strong by far.


First: just wrong, see above

Second: even more wrong. Density is mass per unit volume, that is it. Your mention of "dense per unit volume" would mean "mass per volume squared" or "kg/m^6". That does not make sense. Should a compressor increase a gas's pressure (and at the same time not increase its temperature so much that the effect is counteracted), the resulting gas is more dense, period. There is no alternative way of seeing it. It is not "less dense".

Third: Wrong, see above. The air actually is slowed down if you will. The purpose is to increase pressure. We don't care for velocity. In fact, we care so little for velocity the compressor's diffusor is actually attempting to dissipate it all: that's its job! The highest boost is yielded if the air behind the compressor didn't move!

Fourth: yeah

Fifth: yeah steven is wrong or at least not making sense. But bringing polarity into this is uncalled for, and a poor argument, too. In a room at ambient pressure, warmer air will have less density, resulting in a pressure differential, resulting in an acceleration, resulting in a velocity. So, the same fluid (same polarity throughout) does also flow just from a density differential.

Other commenters have mentioned how the smaller spools the larger one. Nope, the compressors don't really know of each other. They are exclusively spooled by their respective turbine and nothing else.

As far as one compressor goes, they just take whatever is in front and apply their current (depending on their rotational speed) pressure ratio. That is it, granted they are within their operational area! They do not care for what is in front; the intake does not have to be at ambient pressure.

The reason the small one spools up quickly is its tiny polar moment of inertia. Its mass and geometry is simply so small that it can do that.

Fnbend09: exactly on the money! As mentioned earlier, you only truly get the benefits of compounding if it is controlled to some degree. There are multiple bypasses in play, yeah.

ryansmith on August 26, 2018:

where could I find the mount for the second turbo that comes from the first turbo?

Fnbend09 on November 23, 2017:

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?

Newbie on November 05, 2017:

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?

D on July 06, 2017:

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.

Jason on May 24, 2017:

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

anonymous on May 23, 2017:

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

Abi on December 28, 2016:

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.

steven on December 27, 2016:

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.

Chris from Tampa, FL on September 09, 2015:

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


Auto Faction (author) on May 18, 2012:

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

Lloyd Barnhill from Hartford, Connecticut on May 18, 2012:

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

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