Thanks to Honeywell, who supplied...
Thanks to Honeywell, who supplied this example of an efficiency map, you can see how you or your hired turbo expert chooses a turbo for your engine. The efficiency map shows how much pressure a turbo creates at a certain airflow in a graphical representation.
If you have the smaller A/R ratio, the turbo will create boost quicker, giving you a good deal of power at a lower rpm. At higher rpm, however, this small A/R will cost you some power. The opposite is said for a turbo with a larger A/R, which will give up some lag time, but flow a large amount of air and boost at higher engine speeds. For the average street enthusiast who may only tach his or her engine up to 5,500-6,000 rpm, the smaller A/R turbo would be the hot ticket.
Another thing to consider is how hard the turbo has to work to reach the horsepower level you want to create. "You want a turbo that uses only 75 percent of its capability to reach the desired horsepower level," says Dalton Campbell from Pro Turbo Systems. "When running a turbo at 75 percent of its capacity, you're not working the turbo as hard, and raising the power level of the engine is as simple as turning up the boost. If you have a turbo that's maxed out, then in order to increase power, even minimally, you have to get a larger turbo."
Efficiency Map vs. HP/CI
When it came to choosing a properly sized turbo, the rule of thumb was to use an efficiency map. There are two schools of thought nowadays, however. While the efficiency map is still around and in use, some of those "in-the-know" tuners are using the efficiency map as a guideline before choosing a turbo based on horsepower and cubic inches.
According to Squires, for him there are several points to look at when choosing the right turbo for your engine. "There are basically three components of the turbo that need to be speci-fically sized," he says. "First is the compressor wheel, which is sized for the proper airflow and boost capabilities. Second is the turbine wheel. Each turbine wheel is designed to flow a specific amount of energy which, when properly matched to the compressor wheel, will drive the compres-sor efficiently. Finally, the turbine housing, which must match the size of the turbine wheel but also be sized right to deliver the proper exhaust velocity and flow. This gives the best possible spool time with the least amount of top-end restriction. An efficiency map, which each compressor has, shows by using graphs and points on those graphs, how much airflow a specific turbo can flow at a certain boost, or pressure, level.
For years, the efficiency map was the way to go. Turbo gurus would look at the map, figure out the boost level and power level they wanted to run at, and pick a turbo sized accordingly. According to Brian Horne of B&G Custom Turbo Fabrications, the efficiency map isn't a true representation of how effective a turbo can be in a given situation, though.
"Every turbo has an efficiency map," Horne says. "Recently, however, turbo size and choice has been based off of horsepower and cubic inches, along with the corresponding amount of air the engine can use at a given or desired horsepower level. We still use a map, but it isn't always accurate. We now use the map more as a guideline, and then figure out what the max airflow would be and use a turbo that will flow that amount of air efficiently.
While the differences between an efficiency map and the horsepower/cubic-inch relationship are numerous, so are those that are split between using one over the other. The fact remains, however, the more information you have at your disposal will aid you in choosing the right turbo.
Controlling And Regulating Boost
The amount of boost you run normally depends on a few important variables, including engine internals, compression ratio, type of fuel, ignition, and application. One of the biggest factors in figuring out what boost level would suffice for your powerplant depends on how and where you drive your Ford.
If it's a pure-bred race car that will only see action at the dragstrip, then chances are you have a dedicated race engine and can run upwards of 20-40 psi or more in some race applications. For those who tool around on the street and dabble a bit at the track, the boost level must be on the conservative side so the engine will live.
"A 5.0 block will take around 450-500 rwhp before it explodes, so for a street application, between 6 and 9 pounds of boost is more than enough," Horne says. Napier puts the boost figure slightly higher. "Typical turbo boost for a street car is between 8 and 15 pounds," he says. "As for boost in a race car, you can easily double or triple that number."