Mike Kojima
December 29th '04
http://www.turbo-owners.com/forum/ge...bang-blow.html

INTRODUCTION
It is no secret among the staff at SCC that I am a dyed-in-the-wool turbo freak. I love building and tuning street cars of wretched excess. Nothing gives me a bigger grin than doing a rolling third gear burnout in a Viper-smoking turbo-powered compact. To give me my needed power fix, I have found, after working my way through milder, less addicting recreational power adders like high-compression and nitrous oxide, that only the rush of turbo boost can satisfy my jonesing for speed...

Although I have been accused of exhibiting a warped turbo bias, there is a factual explanation for my turbo slant that can be explained quantitatively. The turbocharger is the primary device responsible for the performance parity that sport compacts currently have with most of the big displacement domestic camp. A turbocharger is the great mystical equalizer that replaces sheer cubic inches. If you want to learn about the great equalizer, then read on. The next three installments of "Suck, Squish, Bang, Blow" will teach you more about turbos than you probably need to know!

So far, we have covered the range of techniques for getting more power from the automotive four-stroke engine, ranging from the basic bolt-ons to the deep internals. Now its time to get past that and start exploring the technology needed to make the amazing amounts of horsepower per litre than modern sport compact engines are capable of.
The one device that is largely responsible for the incredible power than stock-block sport compacts are now generating is the turbocharger. The turbo is what enables Lisa Kubo's and Ed Berganholtz's stock block B18C Honda engines to generate more than 650hp on gasoline from only about 1800cc of displacement. (That's only 110cubic inches for the metrically impaired.) To put this into perspective, if a 350 Chevy could generate the same power per cubic inch, it would have more than 2070 hp! And before you bring up Top Fuel engines, remember, they burn nitromethane, not gasoline. How do you pick a good turbocharger for your application? You will learn all this in the segments to come.

WHAT IS A TURBO?
A turbocharger is a device that compresses the intake charge going to your engine. This is the same thing that a supercharger does. Since we dive into the workings of the supercharger elsewhere in this issue, we will only gloss over supercharging, not going too in depth in this summary. Supercharger kits that are on the market right now are not capable of generating the enormous power of turbochargers, but they do, for the most part, have broad usable powerbands and good throttle response, which makes for excellent streetability, What is a better choice for you, a supercharger or a turbo? Well read on and we will give you more than enough information for you to make a sound and logical decision.

Turbochargers and superchargers both use a compressor to increase the density of the intake charge, but the similarities stop there. Compression of the intake charge means that more air molecules are being forced into the cylinder. When a proportionally larger amount of fuel is injected with the denser air charge, the mixture can pack a much larger wallop on the piston during the power stroke. Although the concept of compressing the intake charge by either turbocharging or supercharging increases the power in the same way, the way that the compressor is driven and controlled is very different between the two. Even the types of compressors used are very different between turbos and superchargers.

HOW A TURBO WORKS
A turbocharger is a very simple device. The turbocharger uses a turbine that is driven by the hot, expanding exhaust stream to power a centrifugal compressor. This steam contains a lot of thermal, sonic and kinetic energy. The turbo uses all of this wasted energy to drive the turbine and power the compressor. That is the beauty of a turbocharger. The turbocharger is recovering wasted energy from the exhaust stream to do work, i.e., compress the intake charge to increase its density, energy that is normally just dumped out the exhaust pipe. Since the four-stroke, gasoline-fueled, spark-ignition engine found in your typical automobile, release (and thereby wastes) over 30 percent of the energy contained in the fuel right out of the tailpipe, any device that can use this energy to perform a useful function is a godsend.

This use of waste energy is where a turbocharger really shines over other types of forced induction, like supercharging. Being an engineer by trade, I dig efficiency and minimizing waste in all aspects of life. This is probably the prime reason for my neurotic love of turbochargers. Because of this use of wasted energy, the turbocharger is capable of producing more power, more efficiently than any other form of mechanical power adder yet devised for a four-stroke engine.

Conversely, superchargers are compressors driven by the engine's crankshaft. Since they are powered directly off the crankshaft, this means that superchargers are a parasitic loss, using energy that could otherwise be driving the car forward. Even a low-boost, 6 to 10psi, four-cylinder street supercharger takes between 12 and 20hp from the crank to turn at its rated boost. A Top Fuel dragster bleeds off between 600 and 800 hp to turn its huge Roots blower hard enough to pressurize its monstrous maw. If the compressor on a Quick Class racer churning at 25psi had to be driven off the crankshaft, it would suck down about 65 hp. Because the winning Quick Class racers are all turbocharged, this 65 hp is largely recovered from the exhaust stream instead of the crankshaft.

Prime examples of the awesome potential of turbocharging are engines from Formula One's pre-restriction turbo era. At this point of racing history, before they were banned to reduce speeds, the development of the four-stroke engine was at its peak. A typical 1500cc (this was the FIA displacement limit for turbocharged engines at the time) engine in qualifying trim made more than 1,500 hp. This power level is a confirmed fact from contacts in the racing industry, from engineers working for a smaller, less successful F-1 team. If a backmarker team's qualifying engine made 1,580 hp, how much power did the all-conquering Honda engines of that era make? To put that in perspective, imagine your D15-powered Honda Civic DX making 1,500 hp!

In the early 80's, the domestic performance turbo guru, Gale Banks tried to develop a turbocharged Top Fuel Dragster. Even with only 20 percent nitromethane in the fuel (a 90 percent mixture of this powerful, nearly explosive oxidizer was typical at the time for Top Fuel racers), the engine developed so much explosive power that the drivetrains at that time could not contain the power. The team lacked the money to fully develop the concept and soon the NHRA banned turbochargers from Top Fuel before a real, well-funded turbocharged team could come upon the scene.

Turbocharging's power advantage has lead to turbos being banned or heavily restricted in nearly every class of traditional racing. Only FIA rally racing, and CART Indy racers now allow turbos, with heavy restrictions on boost or inlet diameter. Only USAC (the sanctioning body behind the Pikes Peak hillclimb), the IDRC and NIRA sanctioning bodies of import drag racing allow turbos to run unrestricted in their various classes. Isn't it ironic that the once grassroots sport of import drag racing now has some of the most interesting engine development in the world of racing?

DO TURBOS HAVE ANY DISADVANTAGES?
As awesome as they are, turbos are not the perfect device. Turbos do have some disadvantages to them. Some of the issues are turbo lag, long-term durability, cold start pollution and backpressure. most of the issues can be addressed or have been addressed through good sizing of the turbo for the application and through advancing technology in turbo design. only cold start pollution has yet to be addressed by turbo manufacturers. The details of how the problems can be or are addressed are covered below.

TURBO LAG
Being a turbine-powered compressor driven from the exhaust stream, the turbocharger suffers from what is known as boost lag. This is the delay between the throttle being pushed down and the engine kicking out enough hot exhaust gas to spin the turbine and compressor fast enough for boost pressure to be made. Typically this lag is only a problem below a critical rpm limit where there is not enough exhaust gas volume and energy to spool the turbo. This can be controlled, to a large degree, by the design of the turbocharger and how it is installed in the engine.

When operating above a certain rpm, the engine produces enough exhaust gas volume for a turbo to spool almost instantaneously. Since the typical performance turbo spins from around 80,000 rpm when making full boost, to as high as 180,000 rpm for some small-frame turbos, it is no wonder that the lag time to spin the turbo to these speeds must be strongly considered when designing a turbo system. Most people think that turbo lag is solely due to the inertia of a turbo's rotating parts, but lag is caused by a combination of insufficient gas flow at low rpm, inertia and mechanical losses. Insufficient gas flow, however, is the biggest contributor to lag.

Early turbochargers suffered from a great deal of lag and had pretty poor throttle response, limiting their use mostly to superspeedway racing in USAC Indy cars. With modern turbos, huge advances in compressor and turbine aerodynamics have resulted in greatly improved efficiency. New, computerized design techniques have been used to optimize wheel mass and reduce its lag-producing inertia. These design improvements have largely overcome the lag problem in the last few years. Other recent innovations such as ball bearing center sections, lightweight compressor and turbine wheels have been used to reduce lag. Low-mass compressor and turbine wheels made from exotic materials like carbon-reinforced plastic, ceramics and titanium aluminide are currently in limited production or are being experimented with.

Advanced materials such as super-plastics for compressor wheels and ceramics for turbines have been tried, but are often subject to reliability problems. Nissan has applied these technologies successfully in production applications in the Japanese-market Skyline GT-R and the Cima sedan. The main problem with lightweight ceramic turbine wheels is that the slightest contact with a foreign object, such as a disintegrating spark plug electrode, casting sand from the exhaust manifold or even a big hung of dislodged carbon from the engine can cause the ceramic to shatter. Nissan's ultra-light plastic compressor wheels tend to lose their blades when the boost is turned up beyond 12 psi or so. Having driven two vehicles with these turbos (the ceramic-turbine-equipped R32 Skyline GT-R and a Maxima equipped with a Japanese-market Cima turbo), I can vouch for the fact that lag is very minimal. In the future, as materials science progresses, look to see high performance turbos with these features hitting the market.

Another technological advance that helps greatly reduce turbo lag is the VNT turbo. VNT stands for Variable Nozzle Turbine. A VNT turbo has a ring of movable vanes that change both the incidence angle at which the exhaust gas hits the turbine and the A/R of the turbine housing (we'll explain A/R in a bit). This variability gives the best of both worlds: a free-flowing, low backpressure big turbine housing at high rpm and a quick spooling high-gas-velocity small turbine housing at low rpm. We will explain the mechanics of how the exhaust housing affects turbo spool in more detail later, but for now, let's say the VNT can act like two differently sized turbine housings. VNT turbos are just beginning to see production in small diesel engines in Europe where they are helpful in reducing lag in a diesel's low-energy exhaust stream. If turbos start making a comeback in passenger cars, look for wider use of VNT turbos in the future. An early, less durable version of the VNT was used in the late 80's Shelby CSX and GLH.

With these engineering advances in turbocharger design and with proper matching of the turbocharger to the engine, boost lag can be largely eliminated. Late-model factory-turbocharged cars like the VW Golf, certain Saabs and the Audi A4 have so little lag that the turbocharger is almost unnoticeable except for the unusually high power output. Others like the Japanese-market Nissan Silvia have larger turbos making a healthy amount of power. Although the Silvia has more turbo lag than the nearly lag-less Audi, it uses a ball bearing center section to reduce friction, there by making the lag still feel minimal, especially considering the power output of its 250 hp, 2.0 litre engine.
Generally, given the same engine, small turbochargers spool faster than big ones, due to their ability to extract more energy from the exhaust stream at low exhaust flow levels, and their lower inertia. There are a few other factors to spool and turbo design that we will get into later, but this is a good generalization. Larger turbos can pump more air and have less backpressure, therefore allowing them to make more power than smaller turbos. Big turbos have a harder time collecting energy from a low volume exhaust flow; additionally, their bigger parts have more inertia.

A useful analogy is a teapot. If you boil water and put a big pinwheel over the spout, so the pinwheel will be driven by the stream coming out of the tea pot, it will spin slowly. If you put a small pinwheel in front of the spout and put a small nozzle on it so the steam shoots out faster, the small pinwheel will spin faster.

LESS LAG THAN SUPERCHARGERS?
The supercharger's major claim to fame is that, because the supercharger is directly engine driven, they don't have any lag. This is really only partially true. Some low inertia, low friction, small turbochargers are capable of spooling faster and producing more boost at a lower rpm than even a direct-acting Roots supercharger. The Roots is a common type of supercharger found in everything from Top Fuel dragsters to Jackson Racing Supercharger kits, including many OE applications. A Roots blower has two rotating, intermeshing lobes that pump air simply by trapping it on one side of the supercharger between their lobes and the supercharger housing and carrying it to the other side of the supercharger. The trick to making this simple device a performance enhancer is to size it and drive it such that it pumps more air than the engine does by itself. The air is packed and compressed mostly in the intake tract after the supercharger. The main advantage that a Roots-type blower has over a turbocharger or a centrifugal supercharger is immediate and proportional response to the throttle since the blower is always spinning and its pumping ability increases very linearly with engine speed. Because of its constant pumping ability, the Roots blower is capable of making more boost at low rpm than any other form of mechanical supercharging. This makes a Roots blown engine feel like a big displacement version of the same engine.

Since the turbocharger is a free-floating device, not directly coupled to the crankshaft, it is possible to design a turbo that actually allows more boost at a lower rpm than even a Roots blower, but doing so will usually result in reduced high-rpm power. Even with such a small turbo, the Roots blower still maintains a slight driveability advantage because of its linear throttle response due to its direct coupling to the engine.

The other common supercharger found in the automotive aftermarket is the centrifugal supercharger. Centrifugal superchargers, like the Vortech, are basically compressor sections of a turbocharger, driven by the crank through a step-up gearbox. The gearbox is necessary because centrifugal compressors must spin very fast, on the order of 30,000 to 60,000 rpm, to pump effectively. One of the disadvantages of the centrifugal supercharger is that the centrifugal compressor works best over a rather small rpm range.

To prevent overspeeding the compressor into a choke condition, (where the air velocity inside the compressor reaches the speed of sound and stops flowing) at high-rpm means selecting a step up gear ratio that will not spin the compressor too fast at the engine's maximum rpm. Because of this, the compressor is spinning nowhere near its optimal boost-producing rpm when the engine is at lower speeds. Thus, even though the centrifugal compressor is connected directly to the crankshaft, it still suffers from power lag, non-linear power delivery sometimes worse than that of even a fairly aggressive, large-sized turbo. On a traction-limited, front-wheel-drive vehicle, this sort of delivery can sometimes be advantageous for traction though. Centrifugal compressors are, by design, more thermally efficient than the Roots blower, which is the main reason why the centrifugal supercharger usually makes more peak power than the Roots-type. Efficient or not, due to its direct-coupled positive displacement design, the Roots supercharger is the king of drivability and throttle response.

BOOST CONTROL
Although a turbocharger is a centrifugal compressor with a narrow operating speed range just like a centrifugal supercharger, since it is not directly coupled to the crankshaft, boost pressure and turbo speed can be regulated independently of crank rpm. This makes it possible to keep the turbo in its most effective speed range more of the time. To control compressor speeds and boost pressure produced by a turbocharger, a device called a wastegate is used. A wastegate is a valve that allows hot exhaust gasses to bypass the turbine when the set level of boost pressure is reached., The wastegate allows the turbo's compressor to remain close to its most efficient operating speed over a wide range of engine speeds. With a wastegate, the turbo can be sized to arrive at the set boost pressure rapidly, then the wastegate opens, allowing the turbo to stay at a constant efficient speed even as the engine's rpm climbs. The wastegate prevent the turbo from over-speeding and thereby creating too much boost, moving into an inefficient area of the compressor map or even exploding due to too much dynamic stress. The regulation provided by the wastegate is one of the reasons why a turbocharger works so well over a broad range of engine speeds.

DURABILITY ISSUES
Some older types (read old farts) may make comments about the issue of durability associated with turbocharging. Previously, this was partially true. With recent advances in materials technology and turbo design, durability is a problem of the past. A major problem that had to be solved was coking of the oil in the turbo's center section. When a turbocharged engine was run hard and then suddenly turned off, the oil remaining in the turbo bearings was heated to the carbonization point buy the heat soak.
Because the turbocharger spins at such a high speed, it could also spin for several minutes when the engine was shut off after a hard pull. Without any lubrication, this was not the greatest thing for the bearings. After many cycles of this sort of abuse, the coked oil would eventually block oil passages within the turbo causing bearing and shaft failure by lack of lubrication. This most common automotive failure mode has been eliminated with water-cooled center sections. The aftermarket has solved this by manufacturing turbo timers that allow the engine to idle after key removal so that the rapidly spinning turbine can slow and the internal parts cool, dramatically helping turbo life. Low-ash synthetic oils also greatly improve the lifespan of turbos to equal that of the rest of the engine.

Early turbos also had a much shorter service life because of rotating assembly harmonics. All rotating assemblies have a natural frequency, which creates what is called a critical speed. A critical speed is usually a harmonic period where various minor vibrational frequencies can build to create a significant force capable of doing lots of damage. To solve this problem, semi-floating bearings were developed. These are oil-film bearings that have an oil space on both the inside and the outside of the bearing. Since there is oil on both sides of the bearing, the bearing itself acts like a damper, helping control turbo-destroying harmonics and reducing the turbocharger's sensitivity to minor imperfections in balance. Semi-floating bearings were a significant breakthrough to improve a turbo's life expectancy. We will discuss the construction of semi-floating bearings in more detail a little later.

TURBOS AND POLLUTION, THE FINAL PROBLEM
A difficult problem that technology must overcome in the near future is excessive catalytic converter light off time. A catalytic converter must reach a temperature of at least several hundred degrees before it can start to function, reducing NOx pollutants and oxidizing hydrocarbons. A turbocharger is a massive heat sink in the exhaust system that is normally placed before the catalytic converter to make the most of the exhaust gas energy. Because of the turbo's ability to soak up exhaust heat, adding a turbo can double or triple the time it takes for the catalytic converter to start to function or light off.

Since the pollution controls on a modern engine are so effective, most of the pollution emitted by a modern car is produced in the first few seconds of engine operation before the cat lights off. ANy delay in cat light off time significantly increases the total amount of pollution the car emits. Modern cars are so clean that even a few seconds of delay time makes a big difference in the total amount of pollution emitted. with ever-tightening emission requirement, turbocharged cars have difficulty meeting the latest, most stringent LEV and ULEV standards. Garrett and other OEM turbo manufacturers are currently working full speed to solve these problems, which must be solved if we are to see turbochargers as standard equipment on passenger cars or in CARB-approved turbo kits on the latest generations of low-emission production cars.

VOLUMETRIC EFFICIENCY AND BACKPRESSURE
Although the turbine recovers wasted exhaust gas energy from the expansion of the hot exhaust gas, the kinetic energy of the flowing exhaust gas and the acoustic energy of the exhaust gas, the working turbine also causes an increase in exhaust gas backpressure. This increase in backpressure can reduce the engine's volumetric efficiency. A typical, streetable turbo system has more exhaust backpressure than boost pressure and the power gains from such systems are due to the increase in the density of the intake charger, not due to increases in volumetric efficiency. (Volumetric efficiency, if you don't remember, is the volume of intake charge inhaled during the intake stroke vs. the actual displacement of the cylinder. VE is expressed as a percentage; the larger the VE, the better.) Backpressure is higher than boost pressure because the smaller turbine housings and turbine wheels used to ensure a quick spool-up time also, by nature, restrict the exhaust flow. We will explain the mechanics of this in more detail a little later. Racing turbos, the latest generation of medium-sized turbos and turbochargers for engines where throttle response is not much of an issue (like fixed industrial engines, long haul trucks and aircraft), have free-flowing turbines that have less exhaust pressure than intake pressure. Engines using these turbos often do have improved volumetric efficiency. This condition, where boost is higher than backpressure, is called crossover and crossover is what ever turbo system designer strives for. In crossover, VE percentages as high as 110 percent are not unheard of. Unfortunately, some of the design features that can create a free-flowing turbo can also contribute to turbo lag, something that is not desirable in a street-driven car that needs a wide dynamic power band.

Excessive backpressure is hard to manage in a boosted four-stroke engine. Excess backpressure causes what is known as reversion. Reversion is when hot exhaust gas gets pumped backwards into the engine during the overlap period. Reversion can cause the engines internals to get excessively hot as cross flow of the cool intake charge during overlap is one of the ways an engine cools its self internally. Hot internal parts can trigger uncontrolled combustion and engine-destroying detonation. Because of this, it is sometimes not a good idea to really crank the boost on an engine that has a small, high-backpressure turbo - in other words, the kind of turbo that usually comes on a factory turbo car.

This is a good reason not to go crazy with a boost controller on a factory-equipped turbo car. A little more boost, perhaps 4 to 5 psi might be tolerated, but trying for 20psi could be flirting with disaster. On small turbo cars with a lot of backpressure, camshaft overlap should be kept to a minimum. This means that the stock cam usually will work best. To deal with the problems associated with backpressure and reversion, the engine's tuning must also be compromised with richer mixtures and more retarded timing than what would normally be optimal for the best power. Even on full race turbocharged cars with low backpressure turbos, camshaft overlap should be several degrees less with more lobe separation angle than on an equivalent naturally aspirated engine, unless physical measurements indicate that the engine is in crossover in the engine's operational range.

Because of backpressure and VE issues, the correct turbo size for the application is very important when designing a turbo system. A small, quick-spooling turbo can be restrictive, causing a great deal of backpressure and reducing VE at higher rpm. This means that small turbos should be limited to lower boost levels. A big, free-flowing turbo can be laggy and unresponsive, making it unpleasant for street driving but producing awesome power at higher rpm. To combat high backpressure and possible reversion, the compromised tuning needed to prevent destruction with an overboosted small turbo will also reduce power. If a small turbocharger is running backpressure to boost ratio of more than about 1.8:1, a supercharger has a good chance of performing better. Fortunately, it is easy to design a reasonable responsive, powerful turbo system with a ratio of less than this.

In future installments, we will delve deeper into turbo tech, discuss some more advantages and disadvantages of turbo vs. superchargers and give some guidelines and parameters so you can figure out what it takes to build a system that meets your needs.