Frequently Asked Questions

Here are a few of our most common asked questions:


If you are unable to find the answer to your question on our FAQ page, please don't hesitate to contact us, a member of our experienced staff should be able to further assist you.


Q. What is Turbo Lag?

A. Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust back pressure, etc.


Q.What is Boost Threshold?

A. Boost threshold is the engine speed at which there is sufficient exhaust gas flow to generate positive manifold pressure, or boost.


Q.What is a boost leak?

A. A boost leak means that somewhere in the turbo or intake, there is an area where the air (boost) is escaping. Typically a boost leak is caused by a loose or bad seal, cracked housing, etc. When a boost leak is present, the turbo will be able to generate boost, but it may not be able to hold it at a constant level and pressure will drop off proportionally to the size of the leak.


Q.What is compressor surge?

A. The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side.

Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are:

- A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
- The outlet plumbing for the bypass valve is too small or restrictive
- The turbo is too big for the application


Q.How does a Wastegate work?

A. A Wastegate is simply a turbine bypass valve. It works by diverting some portion of the exhaust gas around, instead of through, the turbine. This limits the amount of power that the turbine can deliver to the compressor, thereby limiting the turbo speed and boost level that the compressor provides.

The Wastegate valve can be "internal" or "external". For internal Wastegates, the valve itself is integrated into the turbine housing and is opened by a turbo-mounted boost-referenced actuator.

An external Wastegate is a self-contained valve and actuator unit that is completely separate from the turbocharger.

In either case, the actuator is calibrated (or set electronically with an electronic boost controller) by internal spring pressure to begin opening the Wastegate valve at a predetermined boost level.

When this boost level is reached, the valve will open and begin to bypass exhaust gas, preventing boost from increasing.


Q.How should I break in a turbo?

A. A properly assembled and balanced turbo requires no specific break-in procedure. However, for new installations a close inspection is recommended to insure proper installation and function. Common problems are generally associated with leaks (oil, water, inlet or exhaust).


Q.What is/causes Shaft Play?

A. Shaft play is caused by the bearings in the center section of the turbo wearing out over time. When a bearing is worn, shaft play, a side to side wiggling motion of the shaft occurs. This in turn causes the shaft to scrape against the inside of the turbo and often produces a high-pitched whine or whizzing noise. This is a potentially serious condition that can lead to internal damage or complete failure of the turbine wheel or the turbo itself.


Q.What is intercooler heat soak?

A. Heat soak is when the intercooler can't dissipate the heat that it absorbs from the turbo fast enough. When an intercooler can't cool the charge air by removing the heat from it, it loses its effectiveness. This explains why turbo cars tend to run slower or have slightly less power when the weather is warm.


Q.What is the purpose of an Intercooler?

A. An intercooler's primary function is to cool the charge air after it has been heated due to boosting and the heat that is produced by the turbo before sending the air into the engine. As the air is cooled, it becomes denser, and denser air makes for better combustion (more power). Additionally, the denser, cooler air helps reduce the chances of knock.


Q.Will an intercooler help make more horsepower?

A. Yes, although it is only indirectly responsible for helping make more power. Since the intercooler increases the charge air density, an intercooled engine will typically make more power than a non-intercooled engine with the same setup by allowing more air to be crammed into each cylinder.


Q.What is the difference between an air-to-air intercooler and a water-to-air (liquid-to-air) intercooler?

A. An Air-to-Air intercooler uses ambient air flowing over the fins to cool the charge air, while an Air-to-Water intercooler uses coolant (water) with a system similar to that of a radiator\'s cooling system. Traditionally, air-to-air intercoolers are used for street applications because of their lower cost and reduced complexity, while air-to-water intercoolers are used in race and packaging-constrained applications.


Q.What other systems are affected by turbocharging? (Fuel, Oil, Cooling, Drive train, etc)

A. There are several factors that must be addressed when deciding to turbocharge a previously naturally aspirated engine, such as: Is the current fuel delivery system capable of providing increased, adequate amounts of fuel? Is the cooling/oiling system capable of handling the extra power and consequently, extra heat that is generated by the turbo? Is the clutch/transmission/drive train up to the task of handling the extra power? Etc


Q.How is boost measured? (Bar, mmHg, PSI) and How do you convert from one to another?

A. Boost is measured as the pressure that the turbo creates above atmospheric pressure.

Normal Atmospheric Pressure (1 atm) = 14.7 psi = 760 mm Hg

1 Bar is not actually equal to 14.7 psi, but rather it is equal to 14.5 psi, = 0.9869 atm = 750.062 mm Hg


Q.What is Knock/Detonation?

A. Knock is a condition caused by abnormal combustion of the air/fuel mixture and can result in damage to an engine.

The three factors that result in engine knock are: 1) knock resistance characteristics (knock limit) of the engine, 2) ambient air conditions, and 3) octane rating of the fuel being used.

Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber shape, spark plug location, bore size and compression ratio affects the knock characteristics of an engine. In addition, engine calibration of fuel and spark plays an enormous role in dictating knock behaviour.

For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases thus increasing the tendency to knock. Charge air cooling (e.g. an inter-cooler) addresses this concern by cooling the compressed air produced by the turbocharger.

The octane rating of fuel is a measure of a fuel\'s ability to resist knock. The octane rating for pump gas ranges from 85 to 94 while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant it is to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for your application. Generally speaking, the more boost you run, the higher the octane requirement.


Q.Do I really need the cool down procedure on my turbo?

A. The need for a cool down procedure depends on how hard the turbo and engine is used, and whether or not the turbo is water-cooled. All Garrett turbochargers must pass a heat soak test and the introduction of water-cooling has virtually eliminated the need for a cool down procedure. Garrett is one of the few turbocharger manufactures that subjects their turbos to several OE qualification tests. When you buy a Garrett turbo you can be sure it\'s a reliable one!


Q.What additional maintenance is required for the turbo?

A. Good, clean oil is extremely important to the turbocharger. It is best to change the oil and filter at least as often as the  manufacturer recommends. We would always recommend the correct grade and amount of oil confirmed by your manufacturer.

Turbo performance is sensitive to turbo inlet conditions. A clogged air filter can drastically affect the turbo inlet. Air filters should be inspected at every oil change and replaced at 12,000 to 15,000 mile intervals. 

NOTE: Never exceed the vehicle manufacturer's recommended filter change intervals.


Q.What should I look out for when buying a turbo?

A. Condition of the turbine housing - inspect for cracks on the exterior and inside the inlet of the housing. If the housing has cracks then the housing needs to be replaced.

Condition of the turbine and compressor wheels - inspect for cracks and damaged blades. If either of the wheels are damaged then the wheel (s) need to be replaced and the center section balanced.

Condition of the bearings - spin the turbocharger shaft and check for roughness. If roughness is detected then the turbocharger needs to be disassembled and the internal components inspected and replaced if necessary.

The most important factor is to make sure the turbo is the proper one for your application. A properly matched turbo will provide better performance and more reliable operation. A properly matched turbo includes matched turbine and compressor wheel sizes and appropriate housings.


Q.What is the Inducer?

A. Looking at a compressor wheel, the inducer is the "minor" diameter. For a turbine wheel, the inducer is the "major" diameter. The inducer, in either case, is where flow enters the wheel.


Q.What is the Exducer?

A. Looking at a compressor wheel, the exducer is the "major" diameter. For a turbine wheel, the exducer is the "minor" diameter. The exducer, in either case, is where flow exits the wheel.

Compressor wheel exducer diameter is incorporated into Garrett GT-series nomenclature: The "60" in the GT2860RS for example, has a 60mm compressor wheel exducer diameter.