Turbo FAQ's

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Turbo

What are the main tuning problems when dealing with Turbos?

What are the main differences between a Single and Twin Turbo setup?

What is Turbo Lag?

What is Boost Threshold?

What is a boost leak?

How can I adjust the turbo boost?

What is boost spike?

What is boost creep?

What is compressor surge?

How does a Wastegate work?

What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?

How should I break in a turbo?

What is/causes Shaft Play?

What is causing my turbo to sound like a sewing machine's whistle?

I want to make x horsepower, which turbo kit should I get? or Which turbo is best?

Does my turbo require an oil restrictor?

What are the main tuning problems when dealing with Turbos?

Engine calibration - fueling and ignition timing. Under boost, it is crucial that there is no engine-killing detonation occurring within the cylinder. This is done by fine tuning the air/fuel ratio a bit rich to help cool the combustion gas, and by tuning the ignition advance curve to ensure that combustion chamber pressures stay below the level that causes unburned fuel to ignite ahead of the advancing flame front.

What are the main differences between a Single and Twin Turbo setup?

A single turbo receives exhaust flow from and supplies air to all cylinders.

The most common type of twin turbo setup is the parallel system where each turbo is fed by ½ of the engine's cylinders. Here, both compressors supply air to the intake manifold simultaneously.

There are also sequential twin turbo systems, which run on one small turbo at low engine speeds and switch to two parallel turbos at a predetermined engine speed and/or load.

Furthermore, there are series twin turbo systems where one turbo feeds the other turbo. These are primarily used on diesel engines due to the extremely high boost levels that can be generated.

For this FAQ, we will just refer to the first two setups identified above.

Choosing between a single or parallel twin turbo setup is primarily based on packaging constraints in the engine bay, or a personal choice by the tuner. In most cases, for top performance, a single turbo is preferable because larger turbos are generally more efficient than smaller turbos. However, often there is not room for one large single, or the tuner wants the visual impact of twin turbos. The notion that two smaller turbos will build boost faster than one large turbo is not always accurate because even though the turbos are smaller, each one is only getting half of the exhaust flow. Sequential systems seem to have the capacity to support big power. In theory, the sequential twin turbo setup is a potent combination. A few O.E.s have produced systems of this type but control issues have proven significant, making them challenging to function seamlessly. One slight draw back to a sequential twin turbo system is that sometimes during daily driving (specifically, in cornering) if the driver is not constantly aware, the second turbo will spool and result in a lot of unpredicted power.

What is Turbo Lag?

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 backpressure, etc.

What is Boost Threshold?

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

What is a boost leak?

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.

How can I adjust the turbo boost?

Adjusting the boost is straightforward. However, it depends on the type of boost controller.

For a standard Wastegates actuator, simply recalibrate the actuator to open (more or less) for a given pressure. Changing the length of the rod that attaches to the Wastegates lever accomplishes this adjustment.

For mechanical boost control systems, adjustments may involve changing the setting on a regulator valve(s).

For electronic boost control systems, adjustments may need to be made to the vehicle's engine management system.

For an external Wastegates, adjusting the boost often requires turning the adjustment screw (when equipped) to increase/decrease spring load, changing Wastegates springs, or shimming Wastegates springs.

IMPORTANT: WHILE ADJUSTING THE BOOST IS STRAIGHTFORWARD, OFTEN THIS CHANGE REQUIRES MODIFICATIONS TO THE ENGINE FUEL MANAGEMENT SYSTEM!

What is boost spike?

A boost spike is a brief period of uncontrolled boost, usually encountered in lower gears during the onset of boost. Typically spikes occur when the boost controller cannot keep up with the rapidly changing engine conditions.

What is boost creep?

Boost creep is a condition of rising boost levels past what the predetermined level has been set at. Boost creep is caused by a fully opened Wastegates not being able to flow enough exhaust to bypass the housing via the Wastegates itself. For example, if your boost is set to 12psi, and you go into full boost, you will see a quick rise to 12 or 13psi, but as the rpm's increase, the boost levels also increase beyond what the boost controller or stock settings were. Boost creep is typically more pronounced at higher rpm's since there is more exhaust flow present for the Wastegates to bypass.

Effective methods of avoiding or eliminating boost creep include porting the internal Wastegates opening to allow more airflow out of the turbine, or to use an external Wastegates.

What is compressor surge?

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

How does a Wastegate work?

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.

What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?

A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge.

Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

How should I break in a turbo?

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).

What is/causes Shaft Play?

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.

What is causing my turbo to sound like a sewing machine's whistle?

The "sewing machine whistle" is a distinct cyclic noise cause by unstable compressor operating conditions known as compressor surge. This aerodynamic instability is the most noticeable during a rapid lift of the throttle, following operation at full boost.

I want to make x horsepower, which turbo kit should I get? or Which turbo is best?

Select a turbocharger to achieve desired performance. Performance includes boost response, peak power and total area under the power curve. Further decision factors will include the intended application. The best turbo kit dictated by how well it meets your needs. Kits that bolt on without any modification are best if you don't have fabrication capabilities. Less refined kits can be cost effective if you access to fabrication capabilities.

For more information on the right Garrett turbocharger for you, please visit the Turbo Application Search Engine

Does my turbo require an oil restrictor?

Oil requirements depend on the turbo's bearing system type. Garrett has two types of bearing systems; traditional journal bearing; and ball bearing.

The journal bearing system in a turbo functions very similarly to the rod or crank bearings in an engine. These bearings require enough oil pressure to keep the components separated by a hydrodynamic film. If the oil pressure is too low, the metal components will come in contact causing premature wear and ultimately failure. If the oil pressure is too high, leakage may occur from the turbocharger seals. With that as background, an oil restrictor is generally not needed for a journal-bearing turbocharger except for those applications with oil-pressure-induced seal leakage. Remember to address all other potential causes of leakage first (e.g., inadequate/improper oil drain out of the turbocharger, excessive crankcase pressure, turbocharger past its useful service life, etc.) and use a restrictor as a last resort. Garrett distributors can tell you the recommended range of acceptable oil pressures for your particular turbo. Restrictor size will always depend on how much oil pressure your engine is generating-there is no single restrictor size suited for all engines.

Ball-bearing turbochargers can benefit from the addition of an oil restrictor, as most engines deliver more pressure than a ball bearing turbo requires. The benefit is seen in improved boost response due to less windage of oil in the bearing. In addition, lower oil flow further reduces the risk of oil leakage compared to journal-bearing turbochargers. Oil pressure entering a ball-bearing turbocharger needs to be between 40 psi and 45 psi at the maximum engine operating speed. For many common passenger vehicle engines, this generally translates into a restrictor with a minimum of 0.040" diameter orifice upstream of the oil inlet on the turbocharger center section. Again, it is imperative that the restrictor be sized according to the oil pressure characteristics of the engine to which the turbo is attached. Always verify that the appropriate oil pressure is reaching the turbo.

The use of an oil restrictor can (but not always) help ensure that you have the proper oil flow/pressure entering the turbocharger, as well as extract the maximum performance.