The hot (turbine) side must...
The hot (turbine) side must also be sized correctly for the intended application. Smaller A/R ratios offer improved boost response, while larger A/R ratios improve exhaust flow and ultimate power.
Choosing the right turbo for the job is much like choosing the right camshaft or intake manifold. It's important to be honest about the intended application and intended usage. Turbo selection for a daily driven street car on pump gas will most certainly be different than a system for a drag racer looking to run 10s at all costs. Actually, 10s are pretty easy with almost any turbo system (we ran 10s with a stock 5.0L running a single turbo kit from HP Performance), so maybe a better example would be low-9s. When choosing a turbo for your car, know that no turbo will be able to offer the mythical combination of immediate boost response and 1,000-plus horsepower on your otherwise-stock motor. Turbos are simple but efficient airflow devices; unfortunately, they possess no magical powers. With that in mind, don't ask for the 1,000hp turbo if you never plan to run it (or them) over 500 hp. If 500 hp is your goal, then sizing the turbos-both compressor maps and A/R of turbine section-will provide much better overall results. Properly sized turbos will offer much-improved boost response, something that will bring a big smile to your face every time you step on the gas. Certainly more than bragging about your laggy 1,000hp turbo(s) that are running inefficiently at half their ultimate potential.
Twin systems will rely on...
Twin systems will rely on smaller T3 turbine housings and wheels. One way to improve boost response is to eliminate any restrictions in the exhaust system downstream of the turbo.
In fact, a case can be made for undersizing rather than oversizing turbo(s), as the undersized turbo will offer immediate boost response that can be enjoyed every time you are behind the wheel. The downside may be a loss of peak power, but how often do you accelerate through the gears and enjoy all that massive torque? Unless you're running your car at the strip, you'll never miss the possible loss of top-end power.
To illustrate the importance of turbo sizing, we ran a couple of tests on the chassis dyno with our friends at HP Performance. In our series on Project Pro Stock, we ran a 5.0L motor with both a 60 and a 67mm turbo. Our modified 5.0L was equipped with a stock (high-mileage) short-block but augmented with TFS heads, a Holley SysteMAX intake, and a Lunati Voodoo cam. Running the motor at 10 psi with the 60mm turbo resulted in 575 rwhp. Replacing the smaller 60mm turbo with a larger 67mm one resulted in a jump in peak power from 575 hp to 604 hp at the same 10 psi of boost.
A single turbo system will...
A single turbo system will usually consist of a T4 turbine section. While the T4 turbine section is theoretically less responsive than a smaller T3 (used on twin-turbo application), remember that a single turbo has twice the exhaust flow feeding it. For this reason, boost response is actually more a function of actual turbo choice than the number of turbos chosen. As a general rule, single turbos will have larger A/R ratios than the turbine sections used on twin systems.
The reason for the significant jump is that running 10 psi pushed the 60mm turbo near its absolute flow limit. Because the turbo had reached its flow limit, installing a larger turbo allowed the motor to make more power at the same boost level. It should be mentioned that the smaller 60mm turbo offered better boost response in the lower rev ranges and would be the turbo of choice for power levels below 550 hp. If the goal was to produce a mild turbocharged 5.0L that made up to 550 rwhp, the 60mm turbo would be the best choice, especially for a daily driver.
If you had to exceed 600 hp, then the only route would be to install the larger 67mm turbo. Due to the tremendous torque production offered by the turbo motor, care must be taken when attempting to exceed 600 rwhp with a stock 5.0L block, as that may be the limiting factor in terms of turbo selection.
Example number two is a 4.6L Three-Valve mod motor equipped with a single turbo kit (again from HP Performance). In this test, the '05 mod motor was first run with a single 67mm T4 turbo. Running just under 14 psi, the motor produced 570 hp at the wheels.
The 67mm turbo was then swapped out for a larger 76mm unit. Despite the fact that the larger 76mm turbo was capable of supporting over 800 rwhp, the motor produced better power with the smaller 67mm turbo. As expected, the smaller 67mm turbo was considerably more responsive than the larger 76mm turbo, offering full boost 400 rpm sooner. This increase in boost pressure earlier in the rev range equates to some serious torque gains. What wasn't expected (which is why it's so important to actually test) was for the 4.6L to produce more peak power with the smaller (67mm) turbo. Running the same boost, the motor produced an additional 10-15 hp. Both of these tests (and so many others) illustrate the importance of turbo sizing.
Choose wisely and you'll be rewarded with a tenacious turbo motor blessed with an impressive power curve that offers both tremendous power and torque. Choose poorly and you can have a lazy turbo motor that offers neither.