Don't be fooled-size does...
Don't be fooled-size does matter!
Before getting to the effect of turbo sizing and A/R ratio, a brief refresher course on turbocharging is in order. Generally speaking, the turbocharger is comprised of a compressor section and a turbine section. The compressor section is what provides airflow in the form of boost to the motor. The compressor is attached to a shaft that is in turn connected to a turbine wheel. The turbine section is placed in the exhaust stream via a turbine housing that directs the heat energy to the turbine wheel. The exhaust energy is used to spin the turbine section, which in turn spins the compressor section, thus wasted exhaust energy is converted into horsepower.
When it comes to street turbos for 5.0L or modular Fords, there are basically two common sizes: the T3 and the T4. Sure, there are a few crazy individuals out there who opt for the large frame monsters, but the T3 and T4 families can serve the needs of just about any V-8 from mild to wild.
For the most part, choosing between the two sizes is a simple matter of whether you wish to run a single or twin-turbo setup. On most 5.0L and 4.6L motors, a twin-turbo kit will consist of a pair of some form of T3 turbos. It should be pointed out that we're referring to the turbine side of the turbos when we stipulate between the T3 and T4 sizing, as even the T3 turbine sections are usually combined with a larger T4 compressor housing to optimize power in a twin-turbo setup.
One of the more common questions regarding turbo motors (well, any motor, for that matter) is, how much power will it make? This is an honest-enough question, and one that deserves a closer look. Perhaps the best way to attack this question is with an example. Suppose we have a stock modular Ford motor that produces 300 hp, and we want to know how much power it will make once we install our aftermarket turbo kit. The same calculations can be applied to determine the power potential at different boost levels (and help turbo selection) of any motor, so pay attention as the formula can be very useful. Suppose we add a turbo kit to our 300hp 4.6L motor and want to know how much power it should make. Before we can calculate the power potential of our newly turbocharged combination, we need to understand that our 4.6L is already operating under boost, or, more accurately, atmospheric pressure. Our normally aspirated mod motor operates with 14.7 psi of (atmospheric) pressure forcing the air into the cylinders when the valves open. This atmospheric pressure is a constant at sea level and a given temperature. Changes in altitude and temperature can naturally affect ultimate pressure.
If we all agree that the normally aspirated 300hp 4.6L is operating at an atmospheric pressure of 14.7 psi, then all we have to do to calculate the power potential of any turbocharged application is to multiply it by a percentage of the additional boost pressure. If we configured our turbo kit to supply 14.7 psi (1 BAR) of boost on top of the atmospheric pressure, theoretically our 300hp mill should produce 600 hp (2 x 300). This same formula can also be used to calculate different boost levels. If we run only 10 psi, then our mod motor would produce (10/14.7 or 0.68 +1 x 300) 504 hp. To calculate the power gains offered at a given boost level, simply divide the boost number by 14.7, add 1, and multiply that number by the original power output of the normally aspirated motor. If we upped the boost pressure on our 300hp 4.6L to 20 psi, our new (theoretical) power output would be an impressive 708 hp. This same formula can be applied to any motor and boost level as long as you know the original power output of the normally aspirated motor and the desired boost level.
 What better way to improve...  What better way to improve the power output of your 5.0L than to add a turbo kit? This single turbo system from HP Performance is capable of pushing even a stock 5.0L Mustang into the 10s. On a modified motor, the sky is the limit in terms of power production. |  For some applications, like...  For some applications, like this 4.6L modular motor, a pair of smaller turbos are utilized. Twin turbos are usually chosen for packaging reasons over a single system. |  When it comes to turbocharging,...  When it comes to turbocharging, size matters for your motor as well. Building a stroker version (4.6L shown) will improve both spool up and ultimate power production, since the boost response is a function of the normally aspirated power output. In this case, bigger really is better. |