Turbo FAQ Version 1.12 - 27.2.2002

So, what is turbocharger anyways?
On internal-combustion engines, turbochargers (and superchargers) work by compressing the intake air used to support the fuel's combustion. The more you can cram air (and fuel) into a cylinder, the more it produces power!

1. Compressor housing
2. Compressor wheel
3. Shaft
4. Rotor
5. Bearing housing
6. Exhaust-gas intake
7. Exhaust-gas out
8. Atmosphereic air intake
9. Compressed air out
10. Oil supply
11. Oil return


The exhaust-gas turbocharger consists of the turbine and the compressor on a common shaft. The turbine uses the exhaust-gas flow energy to drive the compressor. The compressor draws in fresh air and supplies it to the cylinders in compressed form.


What is a hybrid-turbo?
A hybrid turbo is build from two different turbochargers to meet a specific need. Usually the turbine side is taken from smaller turbo and the compressor from larger turbo.


What is this NA engine people talk about?
NA = Naturally Aspirated


What is a MAS and MAP sensors?
MAS = Mass Air-flow Sensor.
MAP = Manifold Absolute Pressure sensor.


What is the A/R ratio?
The smaller the area (A) is, the faster the flow at the exhaust gases enter the turbine blades.

Smaller A/R makes turbo more easy to spool when accelerating, in other words it means less turbolag. On the other hand too small A/R ratio causes exhaust gas packpressure to rise which is fatal to the engine.

The correct A/R ratio for the motor is always a compromise between good throttle response and high output power. Small A/r turbines cannot handle hign flow rates like bigger ones.


What is a wastegate?
Wastegate is a bypass valve tied into the exhaust manifold which opens at a pre-determined pressure. This slows down the turbo and regulates the boost level, preventing damage to the engine from too much pressure.


How does the wastegate actually work?
Wastegate; which is actually an exhaust bypass valve is connected to the turbine just like the turbo is connected to the engine, only by the flow of the exhaust gases. The wastegate controls the gas flow through/by the turbine and that way controls the MAP. Wastegate's valve is controlled with the actuator, which actually is a pneumatic cylinder. When the MAP rises to the desired level, the actuator starts to open the wastegate's valve allowing a significant part of the exhaust gas flow to bypass the turbine, which causes the rev speed of the turbo to drop.
As the MAP is dropped slightly the actuator starts to close the wastegate forcing a bigger part of the exhaust gas through the turbine forcing the turbo to rev faster. This all happens in very short period of time and continues on and on, so the pressure seems to be constant. The actuator is designed to keep the pressure about constant, but there is an unfortunate fact that the actuator opens slightly in much lower pressures than desired. This phenomenal is called creeping. Creeping can be avoided with the usage of electronic control valves but they require sophisticated electronics to operate properly.


What is a blow off valve (BOV)?
Blow off valve (Also known as dump valve) is a valve designed to relieve any pressure build-up between the compressor and the throttle body that occurs when the throttle is sharply closed.


What is that giggling sound during gearshifts?
It's NOT a wastegate, believe me.

Sound is produced when the compressor rotates rapidly and the throttle valve is suddenly shut. Compressor keeps pumping air during the gearshift with its rotational energy. Because there is no major exhaust gas flow through the turbine, the rev speed of the turbo decreases constantly. This leads to the pressure drop right after the compressor. When all this happens in a fraction of a second the air mass rushes against the throttle valve with its kinetic energy. This causes temporary pressure peak right next to the butterfly valve. Because of the pressure drop mentioned above, the air starts to flow backwards through the charge air piping colliding with the rapidly rotating impeller wheel. The surging sound is produced when the wheel kicks the air back through the piping all again against the throttle valve. This is repeated as long as there is enough rev speed at the turbo or the throttle is opened again. Because the rev speed decreases constantly, the frequency of the surging sound tends to drop as it's heard. By technical terms the compressor surges or reaches the surge limit when the giggling sound is heard. Although it sounds cool it's actually hazardous to the axial bearing of the turbo. Surging can be avoided with the blow off valve.


Why do I have to decrease the compression ratio within the turbo installation?
This is a related to the gasoline/air-mixture's tendency to detonate (knock) when it's compressed enough. 95E (regular) gas easily handles effective compression ratios up to 15:1 and 98E (super) even a little higher. Maximum compression is determined by various factors like the shape of the combustion chamber, location of a spark plug, ambient air temperature etc. Unwise the NA engine the effective compression ratio is determined by the geometrical compression and the compression done by the charger. Due to the fact that Ottoengine's mixture is better served with pressure than naturally aspirated, the turbocharged engine can produce significantly higher hp's/litre up to even 500bhp/litre, while even the most effective NA engines produce less than 200 hp/litre.


Why an intercooler?
An intercooler is used to increase the density of the charge air by cooling it. When a turbo compresses air, the air is heated up, and it's density drops. This means that it weighs less. Since more density means more oxygen, and more oxygen means more power, it is a good idea to get the temperature down.


Temperature of the air rises not depending to how it is done. In other word this has nothing to do with the heat of the turbocharger. Increase of the temperature happens by the fact that thermal energy actually is kinetic energy of the gas atoms.

The second problem with an engine breathing warm air is that the likelihood of detonation is increased. This is also one of the reasons why you can run higher boost levels with intercooler than without it.


What are the intercooler's pros and cons?
Pros of the intercooler are e.g. better efficiency, lower NOx -emissions and better reliability.

Cons are the fact that the intercooler and it's pipings requires a lot of space, most commonly there aren't any in the engine compartment. Big IC doesn't increase the lag as it's commonly supposed; actually too small IC does, especially with the help of narrow piping.

Intercooler also drop slightly the boost pressure. This comes from the restriction to flow caused by the intercooler's core. Some restriction is unavoidable because the flow through an efficient intercooler core needs to be turbulent if a lot of the air is to come in contact with the heat exchanger surfaces. However, if the pressure drop is too high, power will suffer. A pressure drop of 1-2 psi can be considered acceptable if it is accompanied by good intercooler efficiency.


How can I adjust the boost pressure?
There are many ways. For example, when question about integral wastegate you can adjust the actuator's pull/pushrod's length and that way adjust the opening of the wastegates valve. There are commonly threads designed for this purpose as OEM. Otherwise it's often seen this bar to be sawed shorter and then welded back together.

You can also tighten or change the spring inside the actuator. This isn't possible in most integrated wastegates, but in poppit-type wastegates it's usually very easy.

Furthermore you can adjust or drop the pressure that operates the actuator by simply attaching a pressure regulator or similar controller to the hose that connects the actuator to the MAP. You can use BIT-valve or a simple throttling device sold e.g. in Hi-flex.

In some cases there is a possibility to set pressure to the counter side of the foil of the wastegate. This is very accurate way to adjust the boost pressure, but is possible only with some poppitt-type wastegates.


Written by Jyrki Repo & Tuomas Kuisma © 2000-2002



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