Richard Holdener
November 1, 2008
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.

Since the induction system has such a pronounced effect on the power output of a nor-mally aspirated, it has a similar effect on a turbo application. Forget the short-runner intakes and stick with something that optimizes the power curve in the desired rev range. Runner length works the same in N/A and boosted applications.

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.

Improving the head flow by porting can yield significant power gains since the power gains produced in normally aspirated trim are multiplied by the boost level.

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.

Turbo motors are sensitive to cam timing as well, but know that most aftermarket cams that improve power on an N/A application will also yield extra power on a turbo motor. For street applications, resist the temptation to run long-duration cams with excessive overlap, as this will ultimately hurt boost response. Most of the off-the-shelf aftermarket performance cams work well on turbo motors as well.

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.

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 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.

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 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 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.