Twin Turbochargers: Benefits and Drawbacks

Twin Turbochargers: Benefits and Drawbacks
In the mid to late nineties, twin turbochargers were the feature to have on many flagship performance vehicles from Japan. These vehicles owned large amounts of power, superb acceleration, and extensive potential for tuning. Though, as time has passed, both the benefits and drawbacks of the twin turbocharger layout have become quite apparent.
Parallel twin turbochargers
Parallel mounted twin turbochargers are used on a variety of upscale performance and luxury cars alike. This design, most commonly found on V type engines, uses a turbo for each bank of cylinders, with both running full time. The purpose of this layout is to reduce the amount of piping required for the plumbing of the intake and exhaust systems, which can increase response and reduce turbo lag. To this end, parallel twin turbochargers also tend to be smaller than what you would typically find on a single turbo engine of similar displacement. Because there is less exhaust gas available to each turbo, a smaller turbo must be used to maintain quick response, and the cost for this response, is limited high end power delivery and boost potential. For those seeking power, this can be the parallel twin turbocharger's greatest weakness next to cost.
Twin sequential turbocharger
A sequential twin turbocharged layout generally uses a paired combination of a small turbocharger for low end response, and a larger turbocharger for high rpm power. To avoid lag problems, the smaller turbocharger maintains quick response at lower engine speeds and also helps to spool the larger turbocharger via the increased exhaust flow. At higher rpm or at a factory determined switch over point, the workload is shifted to the larger, secondary turbo, allowing for high boost without the presence of the lag that would be apparent with a single large turbocharger. While sequential turbochargers help bridge the gap between top end power and low end agility, they tend to be overly complex, and in the case of the Mazda RX-7 Twin turbo, can even contribute to reliability concerns. In addition, for the custom tuner, the profiles of the smaller and larger turbocharger can be quite difficult and expensive to properly match. Because of this, many enthusiasts will simply upgrade to the simplicity of a larger, single turbo.
Twin turbo charging should not be confused with twin charging, which consists of the use of both a supercharger and turbocharger plumbed inline of one another. Twin turbo charging deals only with the use of two turbochargers, either in a sequential or parallel layout. Both of these layouts tend to suffer from large maintenance and modification costs, but besides this point, each has its own unique strengths and weaknesses. With parallel twin turbochargers, the biggest downside is likely the limited high rpm power, while sequential twin turbo setups suffer mainly from the complexity inherent of their designs. For both of these designs, single turbocharger kits are generally sold as upgrades due to their reduced complexity, higher potential power and improved reliability. For those who are willing to deal with the trouble and expenses however, a properly matched sequential turbocharger may be hard to beat.