"A street application is different because you are dealing with 91-octane pump gas and lower engine rpm," says Exile Turbo Systems' Rick Head. "Most street applications will see between 10 and 15 pounds of boost."
If turbos are capable of producing high, double-digit boost numbers, how is boost regulated to stay within the realm of streetability? "The boost level of a turbo is controlled by a wastegate," Squires says.
Here is a wastegate manufactured...
Here is a wastegate manufactured by Turbonetics. A wastegate is a must-have in any turbocharged application, whether it be a race or street car. The wastegate regulates boost by bleeding off excessive pressure in the system when a predetermined boost level is reached.
Turbos produce boost in what is called a rising boost curve. As engine speed increases, so does exhaust-gas flow and its corresponding velocity. With the ever-increasing flow of exhaust gas, the turbine wheel spins faster and faster, thus the compressor wheel spins faster along with it. This produces more compressed air, and the boost level rises accordingly. In basic terms, the faster the engine spins and the more rpm you have, the more boost the turbo creates-of course, until the impellers become inefficient. The wastegate acts as a pressure-relief valve to bleed off the excessive pressure, thus keeping the boost figure in check.
"The wastegate is basically a valve encased in a body," Head says. "Surrounding the valve is a spring with a set pressure value. When pressure rises enough to actuate the spring, the valve opens and bleeds off, or dumps, pressure. By controlling pressure and the boost level, the wastegate not only bleeds off excess boost, but it also basically keeps the level at a constant figure." Normally, the wastegate is set by either a manual or electronic boost controller. The boost controller can be located either inside the car or under the hood, and the pressure at which the valve opens can be chosen by either a crew chief or the driver. The boost controller is the easiest way for you to "turn up the wick."
Just as choosing the right-sized turbo is key to making reliable and safe power, choosing the right wastegate is vital in keeping the boost curve steady and the motor alive. Having too small of a wastegate can result in a boost curve that does not stop rising. This will lead to boost creep, or, simply put, a runaway boost curve. "Having a big gate is not an issue, but having too small of one is," Horne says. "Sure, using a smaller wastegate initially will save you money, but in the long run, it will end up costing you more and giving you bigger headaches. At 9-10 pounds of boost, a 38mm wastegate will suffice, but more boost than that will require a larger wastegate. A wastegate is measured by the size of its exhaust housing, and if it has too small of an exhaust housing, it won't be able to bleed enough pressure past the turbine."
There has always been a debate...
There has always been a debate over whether or not a single turbo system is better than a twin-turbo system, and vice versa.
The small wastegate will allow the turbo to spin faster, creating more boost. With no way to effectively bleed off the rising boost pressure, the boost curve will continue to rise, going from 9-10 pounds to 12, 13, and so on. The pressure will keep on rising until the turbine and compressor wheels slow down, which will only happen when the throttle blades are closed and/or the engine is shut off. Anyway you put it, though, having a wastegate that is too small can possibly damage an engine in ways such as a blown head gasket or internal engine damage.
Single vs. Twins
If one is good, then two are better, right? When it comes to a single-turbo setup versus a twin-turbo, the divide between whether it's needed on a street-driven car is as wide as the San Andreas fault.
"Mathematically, a twin-turbo setup is more efficient," Horne says. "[The system] will spool up quicker if both turbos are perfectly matched because of the smaller size. With twins, however, you have double the material and double the cost."
Each has its advantages and...
Each has its advantages and disadvantages, but the choice of which will work best depends on the power you want to make and the application in which you will use your ride.
On the outside, if you have two 76mm turbos, which will equate to what would mathematically be one 152mm turbo, not only will the twins flow more air, but when compared to a single 105mm turbo, with both turbos being smaller, the lag time is dramatically decreased. While twin-turbos will, in theory, spool up quicker than a single, if the combination is perfectly matched, a single can be just as effective as a pair of hairdryers.
"In theory, twin-turbos will spool up quicker than one larger, single turbo when comparing apples to apples," Squires says. "This will not be the case if you are not comparing the right twin-turbos to the right single turbo. I've seen twin-turbos that spool up quicker than singles, as well as singles that spool up quicker than twins. So much of this has to do with picking the right turbo or turbos for the application in the first place." Twins can also lead to a more difficult installation since that system will take up more room under the hood due to the extra turbo and the piping.