By Featured Writer Mike McGlothlin
How many of us still call a tissue a Kleenex or have that one relative who refers to every type of soda as a “coke?” We’ve all generalized words in our lives, and in most instances it’s harmless. But within the automotive world—and especially in the diesel realm, where turbochargers are everything—one wrong word can cause endless confusion. Case in point, for years any dual-turbo or two-turbo system in this niche has generally been called a twin-turbo system. Even though actual twin-turbo configurations are rarely employed on diesels and the term is used inaccurately the majority of the time, most gearheads and purists chalk it up to twin-turbo being the buzzword (or rather, buzz term) that simply ‘stuck.’ And stuck it has. Aftermarket manufacturers even use twin-turbo language to sell their compound turbo systems…
Now that you understand that any two-turbo system can (correctly or not) be referred to as a twin-turbo configuration, we’ll explain the three primary types of dual-turbo arrangements. First and foremost, a true twin turbo arrangement—comprised of two identically-sized turbochargers, driven off of half of an engine’s cylinders—will be formally defined and clarified. Then compound turbos, the most popular type of dual turbo system in the diesel industry, will be detailed. After the confusion between twins and compounds is straightened out, we’ll break down sequential turbocharging. Whether these turbocharger concepts are new territory for you or you’re an automotive veteran who simply needs a refresher, this guide is intended to clean up the confusing twin-turbo lingo once and for all.
Twin, Compound, And Sequential
There are three primary types of dual-turbo configurations: twin, compound, and sequential. Whether you plan to spec out and install your own turbos or simply want to know what you’re looking at when you attend the local car show, it’s important to understand how each system functions. The technical term for twin turbos is parallel turbos, and this system is often referred to as a “true” twin-turbo arrangement. Compound turbo systems consist of two (and sometimes more) turbochargers of different sizes, which operate in series. Although sequential turbo arrangements are very similar to compounds, in this system one turbo takes over the other one’s duties in certain operating conditions.
Exactly What Are Twin Turbos?
A true twin-turbo system is a parallel system and can best be broken down by focusing on the word “twin.” Think identical twins here, as each turbo is identically sized and a mirror image of the other. Each turbo is exhaust driven off of half of the engine’s cylinders. In a V-8 application, that means one turbo per bank. In an I-6 application, both turbos are positioned on the exhaust manifold (or header) with each unit being driven via three cylinders. Because the turbos operate independently of one another, there is only one stage of compression.
Twin Turbo Sizing
By receiving exhaust flow from just half of the engine’s cylinders, the turbos employed in twin arrangements are kept relatively small. The fact that diesels only have limited engine rpm to work with also factors into their sizing. On high-end performance applications that utilize twin turbos, external wastegates are all but a prerequisite to keep the small-ish turbos from overspeeding. Overspeed failure is one of the most common killers of a turbocharger, and it occurs when excessive drive pressure pushes turbine shaft speed beyond what it was engineered to handle.
Independent Operation
Twin turbos operate independently of each other, with each unit scavenging half of the engine’s exhaust gases. In V-8 applications, each turbo receives its exhaust energy from a dedicated exhaust manifold (or header). On I-6 engines, one turbo is driven off of the front three cylinders, while the second is driven via the rear three cylinders. Also working individually on the compressor side, each turbo utilizes its own dedicated intake piping—although both turbos typically route boost through the same intercooler and ultimately the same intake manifold.
True Twin Turbos On A Cummins?
While the idea of two identical turbos hanging from the exhaust manifold of an inline-six Cummins, effectively splitting the cylinders, may sound like a reasonable performance proposition, it doesn’t work very well in the real world. This is primarily because you can’t get a favorable (i.e. big) pressure ratio out of them—and large pressure ratios are vital in diesel in order to overcome the engine’s airflow restrictions. To be fair, with the right amount of R&D invested in a twin-turbo I-6 Cummins configuration parallels can be made to perform well, but for most diesel owners and aftermarket enthusiasts, it isn’t feasible when a compound arrangement can be added for less cost and headache.
Twins Vs. Compounds 101
The obvious difference between twin-turbo and compound turbo systems is present in each configuration’s physical appearance. Turbo location and packaging, the corresponding intake and exhaust plumbing, and two air filter elements (twin) vs. one (compound) are all dead giveaways, but the sizing difference and number of compression stages that drastically set these two systems apart might not be as front-and-center. The multiplication of boost that takes place in compound turbo systems is what makes them a much more viable performance option on diesel engines. However, in the aftermarket “twin-turbo” is the buzzword that sells parts—and a heck of a lot more parts than the proper terminology (compound turbos) does.
The Mechanical Makeup Of A Compound Turbo System
By combining a smaller, high-pressure turbo (also known as the primary charger) with a larger, low-pressure turbo (also known as the atmospheric or secondary unit), a compound system operates in series. On the boost side, the atmosphere turbo sends its compressed air into the high-pressure charger, where the already-boosted air is compressed once more. Turbo sizing can vary tremendously depending on the engine application (as well as the engine’s intended use), but for a B series 5.9L or 6.7L Cummins, a high-pressure turbo can be as small as a 54mm to 62mm unit (compressor inducer) while the low-pressure turbo can be larger than 88mm.
How Compounds Work
In a compound turbo arrangement, the high-pressure turbo receives exhaust gases first—and because turbochargers are exhaust-driven, this means the smaller, high-pressure turbo kickstarts the boost-building process. At low engine speed, the high-pressure charger is performing most of the work, but that changes as rpm increases. The atmosphere, low-pressure turbo contributes most of its boost production at moderate to higher engine speeds. As the bigger turbo comes online, the relentless torque curve experienced at lower rpm thanks to the high-pressure unit doesn’t go away. Rather, it’s sustained—sometimes even all the way out to 4,000 rpm. This exceptional torque curve is part of what makes a compound arrangement so attractive.
Compounds—The Best Way To Overcome An Engine’s Lack Of Airflow
Because a compound turbo arrangement compresses air twice, it is what’s known as a two-stage system. By comparison, and as was previously mentioned, a parallel (i.e. twin-turbo) configuration is a single-stage system—even though two turbos are part of the equation. When a lack of airflow exists, such as is the case with most OEM diesel engines, compounds make components like restrictive cylinder heads a non-issue. Building inordinate amounts of boost can overcome restrictions in airflow, at least to an extent.
Compounds Offer Superb Exhaust Efficiency
Compared with a twin-turbo arrangement, compounds typically call for less exhaust plumbing, although a more compact exhaust system doesn’t always guarantee easier packaging in a vehicle. In addition to its ability to multiply boost pressure, a compound system lends itself to better efficiency thanks to a shorter path between the high-pressure and low-pressure turbos. The shorter the length of the hot pipe (the piece that connects the turbocharger’s exhaust housings) the less amount of exhaust energy is lost, the result of which is improved transient response and ultimately better drivability.
Making Large Compounds Efficient
To cut down on heat loss (i.e. drive energy) between the high-pressure and low-pressure turbo, fiberglass or silica yarn-based exhaust wrap is often employed on the hot pipe to keep the heat required to drive the low-pressure turbo contained. The same goal is at play when turbo blankets are installed. A high-quality turbo blanket encases the turbo’s exhaust housing, keeps heat where it needs to be, and also keeps heat away from the compressor (intake) side of the turbocharger. In larger compound turbo arrangements, exhaust wraps and turbo blankets can provide a noticeable improvement in streetability.
Why Compounds Are More Popular
Performance, practicality, and cost—in no particular order—all play into why compound turbo arrangements are vastly more popular than twin-turbo setups on diesels. A huge bonus for compression-ignition engines is the fact that pre-ignition problems aren’t a concern—which means they can utilize high boost pressures. But beyond the big boost and horsepower potential, the increased flow from compounds introduces cooler, denser air into the engine, carries enough oxygen to effectively burn all fuel present in the cylinders (i.e. cleaner emissions), reduces exhaust gas temperature, and oftentimes improves fuel efficiency.
The Best Of All Worlds: Superb Low-End, Mid-Range, And Top-End
From a performance, streetability, and work standpoint, it’s extremely hard to beat a well-designed compound turbo system. With the right low-pressure turbo in series with a well-spec’d high-pressure unit, both chargers will complement each other perfectly. From low rpm to redline, compounds offer the potential to produce an extremely broad torque curve and strong mid-range power while also providing enough flow to make considerable horsepower up top. Better yet, compounds can be catered for specific purposes, be it towing, daily driving, racing, or owning the dyno.
Sequential Turbos
Sequential turbo arrangements are similar to compounds in that they consist of a big turbo (low-pressure) paired with a small unit (high-pressure), but the overall functionality of the system isn’t exactly the same. Instead of an open-type system, such as in compounds where both turbochargers are always contributing something regardless of engine rpm, either turbo in a sequential configuration can effectively be taken out of the equation. An advanced version of a sequential turbo system is employed on BMW’s M57 3.0L inline-six diesel engine (found in ’09-’11 U.S.-spec 335d sedans). Using internal bypass valves, the BorgWarner K26 atmosphere turbo sees almost no exhaust flow at low rpm, which allows the K39 high-pressure (primary) charger to spool instantly. As engine speed increases, a bypass valve begins directing flow to the atmosphere turbo but does so in a way that keeps the primary humming along at full song. At high rpm, the low-pressure turbo sees virtually all of the exhaust flow.
Best Supporting Actor: The Intercooler
Most compound, twin, and sequential turbo arrangements route compressed boost through an intercooler before the air enters the cylinder head(s). In some instances, intercooling takes place between stages on a compound and sequential arrangements, but by and large, most intercoolers are located after both turbos. A time-tested heat exchanger, an intercooler allows for lower intake air temperatures, which ultimately results in reduced exhaust gas temperature (EGT). An efficient intercooler (be it of the air-to-air or air-to-water variety) will experience very little boost drop from its inlet to its outlet.
Internal Wastegate
In any dual turbo system, wastegating is a good idea. A wastegate’s purpose is to act as a bypass valve to bleed off excess drive pressure (exhaust gases). This prevents a turbocharger’s turbine shaft from overspeeding. Right behind foreign debris ingestion, overspeeding is one of the most common types of turbo failure in the diesel world. Wastegates break down into two types: internal and external. An internal wastegate is typically found in OEM or OEM replacement applications (shown). It’s located within the turbo’s exhaust housing and, once a preset boost pressure is reached, an actuator forces open a bypass valve that routes exhaust gases around the turbine wheel rather than through it.
External Wastegate
For performance-minded turbo setups, external wastegates are more the norm. An external wastegate isn’t part of the turbocharger, but thanks to not having the constraints of being located within the turbo’s exhaust housing can outflow an internal wastegate by a wide margin. They’re typically positioned in the exhaust manifold (inline-six) or turbo up-pipes (V-8). Various sizes and a multitude of actuator springs allow for fine-tuning of an external wastegate. In a twin, compound, or sequential turbo arrangement—where you’ve likely spent thousands on quality turbos, piping, and fabrication—this is the best way to protect your investment.
Putting Drive Pressure To Work
In compound applications where a moderately small high-pressure turbo is combined with a fairly large atmosphere charger in order to get an extreme off-idle response yet huge flow at high rpm, an external wastegate is the piece of the puzzle that can make it all work. By wastegating the high-pressure turbo in a way that diverts drive pressure directly into the bigger turbo, the atmosphere charger is driven harder—forcing it to produce as much boost as possible. Of course, in this scenario, the atmosphere turbo can be fitted with its own wastegate as well, for ultimate peace of mind.
Non-Wastegated Applications For Compounds
It’s also worth noting that with a well-engineered twin, compound, or sequential turbo arrangement a wastegate isn’t mandatory. Knowing your engine’s flow data, your target boost level and rpm, and how to read a compressor map is key here. You want to select (or build) turbos that won’t operate near the surge line but also not live on the edge of overspeeding. However, if horsepower goals change and more fuel and (in turn) boost is introduced the once-perfectly-spec’d turbos could be pushed out of their map and into the overspeed zone.
Common Compound Turbo Sizes
The most common atmosphere turbo selection in the most commonly used dual turbo system (compounds) and for the most commonly modified engine, the 5.9L or 6.7L Cummins, is the S400 from BorgWarner. In this highly popular application, an S400 with a compressor wheel inducer ranging anywhere from 75mm to 88mm, the proven 96mm (exducer) turbine wheel, and a 1.32 A/R exhaust housing with a T6 divided inlet flange is preferred. A turbo from BorgWarner’s S300 line usually gets the nod for the high-pressure unit, where compressor wheels typically range from 62mm to 69mm, with a 68mm or 73mm turbine wheel, and a T4 or T3 inlet flange.
Add-A-Turbo Compound Kits
Anytime a second turbo can be added to an engine without having to completely rework its induction system, it’s going to sell. For this reason, aftermarket add-a-turbo kits—which mount an atmosphere unit out in front of your stock turbo—are the hottest ticket items in the diesel performance segment. These bolt-on systems offer the most affordable path to running compound turbos. Not only are they budget-friendly, but they come with a proven low-pressure turbo (such as the aforementioned S400 variety), install relatively easily, and allow you to retain quick, factory-like spool-up in addition to increasing high rpm airflow.
VGT’s Can Be Used In Any System
In either type of dual-turbo arrangement, variable geometry turbos can be employed. In compound and sequential systems, a VGT acting as the high-pressure unit can provide unmatched spool up and transient response when compared to a fixed geometry counterpart. VGT technology has also campaigned on twin-turbo engines with success. In twin setups, and because they’re typically sized on the small side, the use of VGTs almost always requires the inclusion of wastegates to avoid overspeeding.
Spool Valves
When a VGT (or VGT’s) isn’t part of a dual-turbo equation but a quicker spool up is needed, what’s known as a spool valve (or turbine diverter valve) can be a lifesaver. By blocking one of a turbocharger’s two exhaust volutes, exhaust flow is restricted, which leads to quicker boost production. However, at a preset pressure, the valve opens, allowing exhaust to rush through both volutes, restoring full exhaust flow. A spool valve is a great addition to high-horsepower compound turbo arrangements where even the primary turbo is oversized in order to move enough air at high rpm to achieve a target power level.
Bonus Round: Triple Turbos
If you want to get really wild with boost, you can look into triple turbos. Here too there is much confusion, as most “triple turbo” configurations are actually two-stage compound arrangements like the one pictured here, where two-atmosphere units effectively act as a single low-pressure unit and work in conjunction with a high-pressure, exhaust manifold charger to produce boost. A true triple turbo setup, which consists of a large atmosphere turbo, an intermediate turbo, and then a high-pressure turbo kick-starting everything (usually in the valley on a V-8) is a rare find. The latter arrangement would be a three-stage system.
