The turbocharger, or a just simply the turbo, has been around now for more than a century. It was invented by Swiss engineer named Alfred Buchi in 1905 and was first used on the diesel engines of ships and locomotives from the 1920s. It was used on the engines of production airplanes from the 1930s and on truck engines from the late 1940s. But it only found its way onto the car engine of a production vehicle in 1962 when it was used on the Oldsmobile Cutlass Jetfire.
As a forced induction system, a turbo is nothing more than an air pump that is driven by the exhaust gasses of a car engine. It consists of a compressor-wheel and a turbine-wheel that are connected by a common shaft. The compressor increases the density of the air that enters the intake manifold by forcing more air into the intake manifold than what the car would normally ingest. This higher intake air density contains more air molecules and produces more power when combined with the correct amount of fuel. This is similar to the way NOS allows more fuel to be burned by providing extra Oxygen as explained by Ian. The major difference between NOS and a turbo is that the turbo provides a constant supply of extra Oxygen to the car engine while NOS only provides a limited supply.
You've got three options when it comes to turbocharging a car:
- You can simply buy an OEM turbocharged car such as a Mitsubishi Lancer Evolution, a Nissan GT-R, a Nissan 300ZX, a Nissan Silvia spec-R, a Toyota Supra, etc.
- You can buy an aftermarket turbo kit for your car engine. Here there are many options to choose from. There are Garrett turbo kits, STS turbo kits, Turbonetics turbo kits, and so much more.
- You can also build your own turbo system, which could be the best approach to car engine turbocharging as it gives you the option to build a system that meets your performance requirements and your objectives.
A complete turbo kit consists of the turbocharger as well as the necessary parts required to bolt the turbocharger onto the car engine. This includes an exhaust manifold, intake runners (plumbing to connect the turbo to the intake manifold), and can include an intercooler as well as cooling and lubrication feed lines for the turbo. When building your own turbo system, selecting the perfect turbo for a particular application can be a real challenge as no one turbo is best suited to all applications.
There are a number of things you need to consider when selecting a turbo. These include:
- The capacity of your engine.
- The number of valves.
- At what RPM to you want the turbo to come in.
- The type of fuel you plan on using.
- The turbo boost you plan on running.
- The amount of horsepower you want.
In this custom-car.us turbo guide, we'll thoroughly explain the mechanics of turbochargers and turbo systems and show you how to design and install your own turbo system. As always, the DIY route is not for everyone and if you'd rather install a turbo kit, we cover that too! For now, we'll start with the turbocharger basics ...
Approximately a ⅓ of the energy produced by an internal combustion engine is lost as thermal energy that is fed out the exhaust manifold. It is this energy that is used to drive a turbocharger. When the exhaust gases are forced through the turbine-wheel, the turbine-wheel becomes a reduced-flow area in the exhaust system and causes some back pressure, which causes some loss in engine power. Of course, back pressure increases as the size of the turbo decreases and inversely, back pressure decreases as the size of the turbo increases. So a larger turbo causes a smaller loss in power, but it also requires more air-flow, and hence more RPM, to spin up or spool up and produce boost pressure (i.e. above-atmospheric pressure). This is referred to as turbo lag. So a larger turbo produces less back pressure but has more turbo lag while a smaller turbo produces more back pressure but has less turbo lag. So what is better? The answer to that depends on what you're looking for — low-end torque, top-end power, or a bit of both.
A Garret Turbocharger
TURBO LAG
Later on in the series we'll look at turbo sizes, but for now, let's get back to turbo lag. Turbo lag is defined as the time between the point when you hit the accelerator and the point at which the turbo produces enough boost to create boost pressure. This may sound like a bad thing but what would happen if you didn't have a turbo? You'd get no boost! So it's either no turbo lag or no boost. A simple choice, I think, especially when you consider that the loss of power due to back pressure caused by the turbine-wheel is hardly noticeable. Provided you haven't done something silly like lower your compression ratio! In years gone by car manufacturers built production turbo motors with low compression ratios to counter the thermodynamic effect of compressing air. Any time air is compressed, the temperature of the air increases. This affects the internal combustion temperatures in the engine. But when a suitable intercooler is used to cool the intake air, normal compression ratios can be used. With normal compression ratios, you're still getting close to normal aspirated performance until you get boost and then you're flying with an up to 50% increase in bhp, depending on the boost you're running! But let's not get too excited just yet, we'll go back turbo boost first.
BOOST PRESSURE
We've said that turbo lag is the time between the point when you hit the accelerator and the point at which the turbo produces enough boost to create above-atmospheric pressure in the intake manifold. The boost level at which the turbo produces enough boost to create above-atmospheric pressure in the intake manifold is called the boost threshold. This is the point at which the exhaust gas flow over the turbine is high enough to overcome inertia and spin the turbine-wheel fast enough so that the compressor-wheel can begin creating boost pressure. From that point on boost will increase but it is important to remember that the quality of the fuel you run and the temperature of the air pumped into the intake manifold will influence the amount of boost you can run. With normal pump fuel, a stock engine and an intercooler, you can safely run at 7-12 psi boost. A wastegate regulates the boost pressure by allowing exhaust gases to pass around the turbine-wheel so as to limit the exhaust gas flow that drives the turbine-wheel.
But more about wastegates at a later stage; here's something to ponder on for now: A properly installed and tuned turbo operating at 10 psi can reduce the 0-60 mph time by a third, despite turbo lag! Yes, you read right a 10 second car will do 6.66 seconds if the turbo is done right!
There are a number of factors, such as turbo lag, boost threshold, heat, back-pressure, low-end torque, and top-end power, that you must take into account when selecting a turbo. A large turbo will suffer from turbo lag and won't produce much low-end torque but it also won't put too much heat to the intake charge, won't have much back-pressure, and will produce loads of top-end power. A small turbo, on the other hand, won't have much turbo lag and will produce loads of low-end torque but will also have lots of back-pressure and will add lots of heat to the intake charge. You can't have the best of both worlds but you can select the best turbo to suit your needs.
Deciding which turbocharger best suits your needs in a bit complicated. You need to know what your objectives are — street car, a purpose built ¼ miler, a race car, or a rally sprint car. Once you know what you want, you should have a better idea of at what rev range you want your power band to be. Once you know that, then it becomes easier as you can select a compressor-wheel to match your rev range.
SELECTING THE COMPRESSOR WHEEL
The compressor-wheel is most efficient at a particular boost pressure or pressure ratio (PR) and air flow (cfm). At this point the turbo will put the least amount of heat into the intake charge; anywhere else, including at lower boost pressures or revs, it will put more heat into the intake charge. The idea id that the point of efficiency should coincide with your most useful rev range. So it's a matter of determining the bore diameter of the compressor wheel that is most efficient at your most useful rev range; and by most efficient, I mean at least 60% efficient. Each compressor-wheel has a compressor map that maps efficiency at various pressure ratios and air flow rates but you need to calculate the air flow rate for your engine. You can use the following formula to calculate the air flow rate:
In this formula, PR is the Pressure Ratio. This is the absolute pressure produced by the turbo divided by atmospheric pressure. Atmospheric pressure is 14.7 psi at sea level. If you're running 7 psi of boost, your absolute boost pressure is 21,7 psi (7 psi + atmospheric pressure). This will give you a PR of 1,47 (21,7 ÷ 14,7), which means that approximately 47% more air/fuel mixture is being forced into each cylinder.
We halve the RPM because a four stroke internal combustion engine requires two revolutions to complete one power cycle
CC is engine capacity but in cubic feet and not in cubic inches. Why cubic feet? Because cfm is cubic feet per minute. You can convert engine capacity to cubic feet by dividing cubic inches by 1728.
VE is volumetric efficiency. This is the total amount of air/fuel mixture that each cylinder ingests during the intake stroke and is expressed as a percentage of the actual volume of the cylinder. You can calculate the VE as follows:
air density × swept volume × RPM
Yes, I know, it's getting a bit complicated! Fortunately we can use a rule of thumb that states that modern engines have a VE of 80-90% while older engines like the Datsun L-series engine have a VE of 60-70%!
SELECTING THE TURBINE WHEEL
The turbine-wheel uses exhaust gas energy to spin the compressor-wheel fast enough to produce the required air flow rates at the desired boost pressure. A larger turbine-wheel will produce more power to spin the compressor-wheel at the required air flow rates, although s smaller turbine-wheel will spin faster. A smaller turbine-wheel will also offer greater restriction to the exhaust gas flow, causing back pressure between the turbine-wheel and the combustion chamber. So the basic size of the turbine wheel will be determined by the air flow required from the compressor-wheel. The important element here is the extruder bore size, i.e., the inner diameter of the turbine outlet. An extruder bore with a 2 inch diameter will be sufficient for a compressor-wheel air flow of 250 cfm to 400 cfm; an extruder bore with a 2½ inch diameter will be sufficient for a compressor-wheel air flow of 400 cfm to 500 cfm; an extruder bore with a 2¾ inch diameter will be sufficient for a compressor-wheel air flow of 500 cfm to 600 cfm; an extruder bore with a 2⅞ inch diameter will be sufficient for a compressor-wheel air flow of 600 cfm to 800 cfm; and an extruder bore with a 3 inch diameter will be sufficient for a compressor-wheel air flow of over 700 cfm.
CONSIDERING THE A/R RATIO
The A/R ratio is another important consideration in choosing the turbine-wheel. The A/R ratio is the ratio between the cross-sectional area (A) of the turbine scroll at any one point and the distance or radius (R) from that point to the center of the turbine-wheel. This ratio is always constant so each point along the turbine scroll will have the same A/R ratio. A turbo with a smaller A/R ratio will tend to create more torque while a turbo with a larger A/R ratio will provide more power because more exhaust gas energy will be acting on the turbine-wheel. Generally, an A/R ratio of 0.7 will provide better low-end response, while an A/R ratio of 1.4 will provide more top-end power.
