Naturally, there are a number of variables that can affect the eventual power output, which are not accounted for in the simplistic formula. The power gains offered by the boost pressure will likely be less than the ideal due to things like turbo sizing, intake design, cam timing, and even exhaust flow. Will your fuel system (pump, lines, and injectors) support the additional power? How about the management system? Don't expect magic from a stock computer that was originally designed to operate a normally aspirated motor. Adding a simple inline fuel pump and rising-rate FMU can only take you so far (remember those days?). How about ignition timing and spark energy? Adding 15 psi of boost to any motor can seriously tax the ignition system. In all likelihood, it will be necessary to retard the timing, something that usually reduces the power output to prevent detonation.

Will the throttle body, intake or cylinder head restrict the power output? How about the exhaust system? You can't expect a stock exhaust designed to support the needs of 300 hp worth of air to work equally well at double (or more) the power output. Of course, it's also possible to produce more power than the result predicted by the equation. This happens on highly efficient motors (such as Four-Valve mod motors), especially those with lowered compression, though we often better the formula on 5.0L applications as well. The low compression reduces the normally aspirated power, so the gains are more pronounced once boosted.

The final note on computing power outputs is that any change made to the normally aspirated motor will be multiplied by the boost pressure percentage. Suppose we have our 300hp 4.6L and want to make 650 hp. Using our handy-dandy formula, we see that it would take roughly 17 psi of boost, if everything went according to the calculations. The other route is to increase the efficiency of the normally aspirated motor before applying the boost. Using this method, we can produce the same power output with less boost pressure (and attending problems). If we increase the power output of our normally aspirated motor to 350 hp, we can reduce the required boost pressure to just 12.6 psi. If we increase the power output to a solid 400 hp (using ported heads, cams, and exhaust system), we can reduce the boost to just over 9 psi and achieve 650 hp.

The increase in normally aspirated power can come in the form of an increase in displacement, too. Adding a stroker crank to a stock engine is a surefire way to improve the output of any motor; adding the turbo only compounds these power gains. The added displacement improves spool up, while the increase in normally aspirated power from the hike in displacement is multiplied by the boost pressure.

Though we've covered this power/boost formula time and time again, it's important for our needs here as it can also be used to help size and select a turbo for your application, or at least a compressor section.

Using our power formula, take the normally aspirated power output and calculate the potential boosted power output based on a desired boost pressure. Let's use our 300hp 4.6L again as an example. Suppose we want to produce 600 hp from our normally aspirated mod motor. We know it will take 14.7 psi to double the power output of the 300hp 4.6L. Looking at the compressor maps supplied by the various turbo companies (ours came from the Turbonetics catalog), simply convert your boost pressure of 14.7 psi to a pressure ratio. In this case, our pressure ratio of 14.7 psi would be 2.0 (a pressure ratio of 1.0 is atmospheric pressure). If we're looking for the 4.6L to produce 600 hp, it will require 60 pounds of air per minute (hp/10 = lbs per minute). Armed with the fact that our turbocharged Ford will require 60 pounds of air per minute at a pressure ratio of 2.0, we can now select a compressor from the various compressor maps. All that's required is to take the intersection of the two points to determine where your combination fits on a particular compressor map.