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Turbo Technology - All Spooled Up
Turbos Explained And Secrets Revealed
When it comes to making a sick amount of horsepower, a turbocharger is a quiet and efficient way to get the job done. For many years, turbo technology lagged behind the other form of forced induction, but today there are more kits and improved tuning. It all combines to make a turbo setup just the ticket for your fast Ford.
Before we delve into the detailed portion of this episode, an understanding of the basics is needed. As most of you probably know, a turbocharger works in conjunction with exhaust gasses and the corresponding heat to drive a compressor that creates boost.
To understand how a turbo works, however, we need to take a look and see what exactly is contained inside. "A turbo is made up of five major parts," explains Nathan Page of HP Performance. "First, you have the compressor housing, which contains and directs compressed air into the throttle body [or carburetor]. Second, you have the compressor wheel, which draws in air and compresses it. Next, you have the centersection, also known as the bearing housing. This contains the bearings and seals that control oil for the turbine shaft. Obviously, you have the turbine wheel and shaft, which transfers the exhaust energy through the bearing housing to the compressor wheel. Finally, you have the exhaust, or turbine, housing, which collects and directs exhaust energy through the turbine wheel."
"The compressor wheel, which spins, draws in fresh air, and compresses that fresh air into boost, which is then pushed into the engine," says Squires Turbo Systems' Rick Squires. "This extra air increases the horsepower the engine can produce because it is taking in more air with the turbos' help than it could on its own."
By using the exhaust gasses of the engine, the turbo can efficiently provide forced induction, especially compared to a blower, which is driven by the crankshaft of the engine. "A typical internal-combustion engine wastes about 50 percent of its energy through exhaust gasses going out of the tailpipe," says Jim Napier of Turbo Horsepower. "Turbos recover some of that wasted heat and pressure, using it to push a large volume of air back into the engine."
Size 'Em Up
By far, the most crucial part of turbocharging any engine is choosing the right-size turbo or turbos. There are many different sizes from which to choose, and knowing which is best for you can be instrumental in keeping the power and fun meter up. "Turbo sizing is an art and a science," Napier says. "The science is very complex and can be driven by any number of tables that have been used over the years by professionals.
"Every turbo has a compressor map, which is a graphical representation of how that turbo performs. Turbo sizing is typically accomplished by experience and using any number of formulas that take into consideration engine displacement, camshaft, transmission, gears, type of intended use, weight of the vehicle, fuel injectors, exhaust system, lag time, and so on."
"The trick to properly sizing a turbo is like any performance project," Squires says. "The biggest problem we see is someone getting a 'deal' on a turbo and then starting the project based around the wrong turbo. You figure out exactly what you are trying to accomplish before starting to purchase components. We see too many people wanting turbos capable of high horsepower, when, in reality, they will never be making that much power. This is the case where you end up with turbo lag."
Turbo lag is the time it takes for the turbo to spool up and begin creating boost. "Lag occurs because of poor turbo sizing," Squires says. Simply put, less lag equals quicker boost production and better throttle response. Most professionals will trade a few peak horsepower for better midrange power, especially on a street-driven vehicle.
"Matching the compressor to the power level and boost requirements is where you start," Squires says. "Then, mating that compressor to the proper turbine wheel that will drive the compressor with the least amount of backpressure is next." It is also important to have an impeller designed to make boost in the proper rpm range.
An industry term thrown around among those dealing with turbos is the "A/R," or Area over Radius, ratio. "This [the A/R] is a relationship of the inner size of the conical shape in reference to the distance this inner duct is from the center of the shaft," Squires says. "Smaller A/R numbers will typically spool up the turbo quicker, while larger A/R numbers will offer more boost and horsepower up top."
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."
"A street application is different because you are dealing with 91-octane pump gas and lower engine rpm," says Exile Turbo Systems' Rick Head. "Most street applications will see between 10 and 15 pounds of boost."
If turbos are capable of producing high, double-digit boost numbers, how is boost regulated to stay within the realm of streetability? "The boost level of a turbo is controlled by a wastegate," Squires says.
Turbos produce boost in what is called a rising boost curve. As engine speed increases, so does exhaust-gas flow and its corresponding velocity. With the ever-increasing flow of exhaust gas, the turbine wheel spins faster and faster, thus the compressor wheel spins faster along with it. This produces more compressed air, and the boost level rises accordingly. In basic terms, the faster the engine spins and the more rpm you have, the more boost the turbo creates-of course, until the impellers become inefficient. The wastegate acts as a pressure-relief valve to bleed off the excessive pressure, thus keeping the boost figure in check.
"The wastegate is basically a valve encased in a body," Head says. "Surrounding the valve is a spring with a set pressure value. When pressure rises enough to actuate the spring, the valve opens and bleeds off, or dumps, pressure. By controlling pressure and the boost level, the wastegate not only bleeds off excess boost, but it also basically keeps the level at a constant figure." Normally, the wastegate is set by either a manual or electronic boost controller. The boost controller can be located either inside the car or under the hood, and the pressure at which the valve opens can be chosen by either a crew chief or the driver. The boost controller is the easiest way for you to "turn up the wick."
Just as choosing the right-sized turbo is key to making reliable and safe power, choosing the right wastegate is vital in keeping the boost curve steady and the motor alive. Having too small of a wastegate can result in a boost curve that does not stop rising. This will lead to boost creep, or, simply put, a runaway boost curve. "Having a big gate is not an issue, but having too small of one is," Horne says. "Sure, using a smaller wastegate initially will save you money, but in the long run, it will end up costing you more and giving you bigger headaches. At 9-10 pounds of boost, a 38mm wastegate will suffice, but more boost than that will require a larger wastegate. A wastegate is measured by the size of its exhaust housing, and if it has too small of an exhaust housing, it won't be able to bleed enough pressure past the turbine."
The small wastegate will allow the turbo to spin faster, creating more boost. With no way to effectively bleed off the rising boost pressure, the boost curve will continue to rise, going from 9-10 pounds to 12, 13, and so on. The pressure will keep on rising until the turbine and compressor wheels slow down, which will only happen when the throttle blades are closed and/or the engine is shut off. Anyway you put it, though, having a wastegate that is too small can possibly damage an engine in ways such as a blown head gasket or internal engine damage.
Single vs. Twins
If one is good, then two are better, right? When it comes to a single-turbo setup versus a twin-turbo, the divide between whether it's needed on a street-driven car is as wide as the San Andreas fault.
"Mathematically, a twin-turbo setup is more efficient," Horne says. "[The system] will spool up quicker if both turbos are perfectly matched because of the smaller size. With twins, however, you have double the material and double the cost."
On the outside, if you have two 76mm turbos, which will equate to what would mathematically be one 152mm turbo, not only will the twins flow more air, but when compared to a single 105mm turbo, with both turbos being smaller, the lag time is dramatically decreased. While twin-turbos will, in theory, spool up quicker than a single, if the combination is perfectly matched, a single can be just as effective as a pair of hairdryers.
"In theory, twin-turbos will spool up quicker than one larger, single turbo when comparing apples to apples," Squires says. "This will not be the case if you are not comparing the right twin-turbos to the right single turbo. I've seen twin-turbos that spool up quicker than singles, as well as singles that spool up quicker than twins. So much of this has to do with picking the right turbo or turbos for the application in the first place." Twins can also lead to a more difficult installation since that system will take up more room under the hood due to the extra turbo and the piping.
So, if twin-turbos will spool up and produce boost quicker than a single turbo, other than cost, why don't more street enthusiasts utilize a twin-turbo setup? It all comes down to application and driving style.
"A twin-turbo setup is more complicated in terms of fitment and fabrication," Head says. "For a street car, it's probably not necessary. A single turbo would probably be the best way to go." As Head explains, when installing a twin-turbo system, the fitment of the turbos, wastegates, and plumbing makes it tougher for some cars as opposed to others.
Is a twin-turbo system too extreme for a car that does have enough underhood space to install one? "If you are looking for big horsepower, then twins are the way to go," Horne says. "With twins, you get more boost at lower rpm as opposed to what you would get with a single, large turbo."
If you look at as many Mustangs as we do, you will see both single and twin-turbo setups on street-driven cars, showing that more and more, twin-turbos aren't just for race cars anymore. "It was not long ago that only race engines used twin systems," Napier says. "Now, many street systems utilize twins for benefits."
Fuel, Spark, and Internals
As you have read, there is much more to turbocharging than just bolting on a turbo and letting 'er rip. Now it's time to cover the other major aspects of running your engine success-fully with a turbo
It's common knowledge that an engine needs air, fuel, and a spark to run. With the turbo forcibly shoving a larger-than-normal volume of air through the intake and into the cylinders, the corresponding amount of fuel is needed to mix with the extra atmosphere to maintain the desired air/fuel ratio. By upgrading the induction system, an upgraded fuel system is a must as well. This includes fuel injectors, fuel pumps, fuel lines, and more.
"Having a fuel system that regulates fuel pressure based on manifold pressure and/or vacuum is ideal," Squires says. "Additionally, air/fuel ratios are richer on a forced induction application versus a naturally aspirated tune. We typically run our systems in the low-to-mid 11:1 range to stay on the safe side of tuning.
"According to Horne, there is a general guide-line for increasing fuel pressure when related to an increase in boost. "We set the fuel pressure at 43 pounds normally," he says, "and then raise the fuel pressure 1 pound for every 1 pound increase in boost."
The more power you make with the turbo, the more fuel you need to supply, starting with the fuel pump and a rising-rate regulator all the way to the injectors. According to Head, however, there is one item in the fuel system that is sometimes overlooked.
"Both the feed line and return line sizes are very important," he says. "They need to be large enough to support the fuel level but not be too large and cut down on flow velocity.
"Power is made when the ignition system lights the plugs and fires off the air/fuel mixture. In a forced-induction application, an upgraded ignition system and reduced spark-plug gap is often needed to properly induce combustion. "Above 10 pounds of boost, if you do not have a strong enough ignition, the spark plugs won't be able to jump the gap," Horne says. This can lead to a misfire or poor running condition.
"Ignition systems typically need to be upgraded to handle the additional cylinder pressure," Page says. "In addition to an ignition amplifier, the spark plugs need to have a smaller gap to allow the spark to jump the distance between the electrode and the ground strap in the high pressure atmosphere without said pressure blowing it out."
Most of those we interviewed for this article stated that a CD-ignition system like an MSD is the way to go. "A good ignition box, like an MSD 6AL or 7AL, would work," Head says. Along with the upgraded ignition components, which can include the MSD box and a high-output coil, the computer must be tuned as well. "Most modern ignition systems are computer controlled," Napier says, "so an aftermarket chip or flash tuner will also be a likely requirement to help the factory engine-management system adapt to the changes turbocharging requires."
The ignition system and an aftermarket ECM tune allows tuners to be able to set the ignition timing for increased power and to avoid detona-tion. "Ignition tuning is critical for boosted applications in order to prevent detonation," Squires says. "On newer stock engines with compression ratios in the 10:1 range, stock timing can be in the 26- to 30-degree range at wide-open throttle. When we tune this type of engine for the turbocharger, we typically have to drop the timing under boost peak by 10-15 degrees to prevent detonation on pump gas."
By retarding the timing, the engine is far less likely to run into detonation, which could lead to serious engine damage in a matter of moments. The overall timing figure itself is not only based on compression ratio and the boost level, but also off camshaft and cylinder head choices as well. "The boost amount, cam, and heads all affect the overall timing number," Head says.
The internals of the engine must also be able to handle the increased cylinder pressure accompanying turbocharging. Included in this area is piston choice, compression ratio, and the type of cylinder-head retention hardware. "Internally, you want to run forged pistons with a moderate compression ratio, though you do want to run as much compression as you safely can," Head says. "Higher compression with less turbo lag means a snappier system, though with pump gas in a street application, you are limited in the amount of compression you can run."
While a stock component-equipped 4.6 or 5.0 can hold up to about 500 hp at the wheels (many combinations have made more than that), for ultimate reliability, an aftermarket block and rotating assembly are a must. Keep in mind that a 9:1 static compression ratio will, under boost, rise to a level that could possibly induce detona-tion. While on paper a lower static compression ratio, like 8:1 or 8.5:1, might seem weak, when under boost, that compression ratio, known as the dynamic compression ratio, will rise right to where it's needed to be at to make the best power.
Running a lower compression ratio helps to keep cylinder pressure down, which assists in avoiding blown head gaskets and detonation. Also, the forged pistons, rods, and crank are more suitable to withstand the increased pressure. "In the engine, you would need forged pistons, rods, and maybe a crank," Napier says. Page agrees with him on this subject. "As with any high horsepower combination, the internal parts of the motor must be up to the task," he says. "Turbo motors generally will need a slightly lower compression ratio, thicker ring lands, and larger ring end gaps." The thicker ring lands on the pistons will allow the piston rings to stay in place under the increased heat and pressure, and will prevent the ring lands themselves from cracking and/or failing.
"Keeping the pressure in the cylinders is another important thing to consider, and a set of head studs and good gaskets will go a long way there," Squires says. "Heads and cams will increase the amount of boost that gets into the cylinders, which will allow you to run less manifold boost to make the same power."
Camshaft And Valvetrain
With camshaft profiles made to take full advantage of the juice or increased airflow offered by a supercharger, we wondered if there was a dedicated cam profile specifically for turbos. The answer we got was quite surprising.
Consensus was that a stock or mild cam would be best suited in a turbocharged applica-tion. "Blowers like more air out, while turbos like more air in, so you want to have a good amount of intake lift and duration, but not as much as a blower or nitrous cam," Campbell says."
Horne agrees with running a small cam, though he differs a bit on the subject from Campbell. "You want a cam with very small duration but with high lift," he says. "Also, you want very little overlap. You want to look for something around 225 degrees of duration. We've seen a lot of people do well with an F-303 cam."
While a custom-ground cam is the way to go to suit the profile to the power level you want to make with your engine, there seems to be a specific set of guidelines as to what works and what doesn't.
"The key is to keep duration and overlap to a reasonable level," Squires says. "Lobe centers on turbo cams are typically going to be in the 114-116 range, and duration at 0.050 inch is typically not going to exceed 230 degrees. Too much valve overlap will leave your cylinders contaminated with exhaust and rob you of significant amounts of power."
Going along with the camshaft choice is the accompanying valvetrain, specifically the valvesprings. With high lift, without question better valvesprings are needed. If you were running the stock cam, however, you would think the stock springs would work, right? Think again.
"We typically see stock valvesprings start to float around 7-9 pounds of boost," Squires says. "The added intake and exhaust pressure will tend to hold the valves open and rob you of top-end power. Changing the springs is relatively easy and inexpensive, and will help when using the stock cam or cams."
Other Aspects of Turbocharging
Sorry to inform you, but we aren't done yet. We haven't talked about blow-off-or bypass-valves, turbo maintenance, and intercooling. Grab something to drink because we might be here all night.
The wastegate bleeds off boost pressure to keep the boost level and turbo speed in check, protecting the engine. But what protects the turbo when the throttle is slammed shut? A simple device called the blow-off valve. Also known as the bypass valve, this component bleeds off pressure in the intake tubing while allowing the turbo to freewheel. This prevents the boost from causing reversion that can slam into the compressor wheel, causing crazy harmonics when the throttle is shut under boost.
"This component (the blow-off valve, or BOV), dumps the boost out of the intake tubing during a specific condition, which is when there is boost in the intake tubing and vacuum present in the manifold," Squires says. "This is a condition that occurs mostly in a manual-transmission car during a shift. If the BOV is used, the turbo will freewheel during shifts and maintain most of its turbine rpm. Without the BOV, the turbine can dramatically reduce its rpm, which would require more spool-up time when the throttle is applied in the next gear. At high boost level-15 pounds or more-this component becomes critical in turbocharger longevity and prevents a condition called compressor surge, which can damage the turbo."
"Once the throttle is closed after positive pressure is achieved, there is a pressure buildup that occurs between the turbo and the closed throttle blades," Page says about compressor surge. "To prevent that pressure buildup from abruptly stopping the turbo, it must be relieved.
Another important part of utilizing a turbo is maintenance. A turbo is lubricated by oil plumbed from the crankcase, so the oiling system must be properly set up and oil changes must be made on time. "Providing adequate oil pressure and flow through the turbo prevents it from damaging itself in the harsh environment in which it lives, which is one of extreme tempera-tures and extreme rpm," Squires says. According to the experts, that 3,000-mile oil change you are supposed to make should be shortened to 2,500 miles and not missed.
Idling down the turbo isn't needed if you operate the car under normal driving conditions. If you are planning on using the car mostly at the strip, however, idle it after use or employ a turbo timer. If you don't, oil will heat up dramatically in the super-hot turbo and can burn, dramatically shortening its life.
In an effort to make more power and keep the engine out of detonation, use of an intercooler is a no-brainer. An intercooler is a component that lowers the temperature of the intake charge after it comes out of the turbo and before it enters the engine. There are two common types of intercoolers available: air-to-air and air-to-water. You can also employ the usage of a methanol injection system to cool the intake charge.
"Since compressing air increases the temperature of the air, an intercooler is typically used to reduce the temperature of that air," Squires says. "We typically see about a 1/2hp gain for each degree that you can drop the intake's temperature, so an intercooler is a good investment. Lowering the intake temperature also lowers cylinder and exhaust temperatures, which reduces the risk of detonation and cylinder damage."
Let's do the math. If we go by Squires' observation that you pick up 1/2 hp per degree you drop the intake temperature, then by dropping the IAT (inlet air temperature) from 250 degrees to 150, you will pick up 50 hp. Talk about huge! That's because the cooler charge allows for a more aggressive ignition timing curve, and we know that equals more power.
"Turbo engines are more subject to detonation due to the elevated temperature of the pressurized air, so anything that lowers air temperature will help make more power and keep the engine safe by reducing the chance of detonation," Napier says.
So how do you choose an intercooler, and which type is more effective? "[In aftermarket applications] air-to-air intercoolers are generally used on street cars, while air-to-water inter-coolers are generally used on race cars," Napier says. According to Campbell, however, an air-to-water intercooler is more effective that an air-to-air. "With an air-to-air intercooler, on a 100-degree day, you can see an IAT around 130-150 degrees," he says. "The problem is, with an air-to-air, you are dependent on the outside temperature. With an air-to-water inter-cooler, which uses ice water to cool the IAT, the IAT will cool quicker and with more predictability and consistency."
"With an intercooler, fin count is also impor-tant," Horne says. "The more fins an intercooler has, the more efficient it is. Ideally, look for an intercooler that has 12-15 fins per inch."
Methanol injection is another form of intercooling, such as the systems from Snow Performance. A methanol injection system squirts a mix of methanol and water, usually in a 50/50 mix, into the intake tract to cool the incoming air charge. This is also an efficient way to lower IATs.
As you can see, turbocharging your Ford can be a sure-fire way to make some serious horsepower, but to make that power, you need to do things the right way and have all of your ducks in a row. "Obviously, there are many components on the vehicle that need consideration based on the new horsepower and torque available when running a turbo," Squires says. "Turbochargers are the friendliest of the power adders for keeping the stock components and drivetrain in one piece because of their incredibly smooth application of big power." Unlike nitrous, which needs to be refilled after a certain amount of use, a turbo's power is always available. All you have to do is flex your right foot and get all spooled up.