Choosing the proper turbo...
Choosing the proper turbo for your application is a matter of selecting the proper compressor section from the available compressor maps. A single-turbo (T4 shown for 5.0L) application will naturally utilize a larger turbo than a comparable twin-turbo setup.
The ideal situation is to have the intersection appear in the island of optimum efficiency. Looking at the map for the T72, we see that the intersection of a pressure ratio of 2.0 and 60 pounds of flow per minute puts us in the 76 percent efficiency island-just about perfect. Of course, that's for a single turbo, and you'd need to cut the airflow numbers (but not the boost) in half to produce the desired compressor maps on a twin system. Looking at the intersection of a pressure ratio of 2.0 and a flow rate of 300 lbs/min, we see that a TO4E-60 is a pretty good match for our 600hp twin system. Making turbo selection even easier is the fact that turbo companies have already done the choosing for you, especially if you provide your current combination and desired power output.
Another variable that helps determine not only the eventual power output, but the overall response rate of the turbo, is the A/R ratio of the turbine section. (Don't worry, we won't get too technical here.) The A/R ratio is the area of the circular flow passage in the turbine housing, divided by the distance between the center of the turbine impeller to the center of any point on that flow passage.
Twin-turbo kits rely on a...
Twin-turbo kits rely on a pair of smaller turbos (usually T3/T4 hybrids). Using the smaller T3 turbine sections provide improved boost response while the (relatively) larger T4 compressor housings offer plenty of boost potential.
OK, so that's not an easy one to remember, especially without some sort of drawing. If you want to get a better understanding of turbochargers in general and A/ R ratio in particular, check out Corky Bell's book Maximum Boost. The reality is that physics and math are less important than understanding one important fact about A/R ratio. Both the T3 and T4 turbine sections used on most single (T4) and twin (T3) turbo kits are available with a variety of different A/R ratios. For the T3 turbos, the most commonly used A/R ratios are the 0.48, the 0.63 and the 0.82. The important point to remember here is that the larger the decimal number, the greater the total flow potential of the housing. Thus, a 0.63 housing will outflow a smaller 0.48 housing.
While the immediate response might be to go with the "bigger is better" theme, you better hold onto your hat, as the overall flow comes with a penalty. Improving the peak power output of your turbo motor might be as easy as installing a 0.63 housing in place of the smaller 0.48 housing, right? The 0.63 does indeed offer more absolute flow potential, but as always, there's more to the equation than simple flow rates. The 0.63 turbine housing will flow more than the 0.48, but the larger turbine housing will also spool up slower, thus you'll likely lose power in the low and middle power ranges. You might also run into a situation where the compressor flow is the limiting factor and not the turbine, where the addition of a 0.63 housing may not gain you a thing. The additional power offered by an A/R ratio upgrade is (if anything) application specific. This same scenario holds true on single turbo applications, as the large turbo must first be spooled to provide the required boost response. While it seems that smaller twin turbos will always spool much quicker, remember that they are provided only half the exhaust energy of the single turbo. Thus choosing the A/R ratio for a single and twin system will likely be different, with the larger A/R being used on the single application.