Some Turbo Basics

[aj510]

I thought it might be nice to add a few basics about turbos as a first off.

Please feel free to add more in replies.


What are the main tuning problems when dealing with Turbos?

Engine calibration - fueling and ignition timing. It is extremely important that an engine under boost isn’t suffering from detonation (pinging). To achieve this the air/fuel ratio need to be tuned (left a little rich helps to cool the combustion gas) and also thru tuning the ignition advance curve to make sure the combustion chamber pressures do not reach a level that causes unburned fuel to ignit ahead of the advancing flame front.

What is Turbo Lag?

Turbo lag is the delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo Lag is affected by an array of different factors, such as the size of the turbo compared to the engine size, condition of the turbo and engine, the efficiency of the turbine in the turbo, intake plumbin losses, exhaust pressure… the list goes on.

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?

This simply means that somewhere in your turbo plumbing there is a leak (not sealed correctly). There are numerous reasons why a boost leak can occur, most common would be a loose fitting on incorrectly fitted seal, cracks in the plumbing or turbo housing etc. If a turbo setup is suffering boost leak then it will still be able to generate boost but it will not be able to maintain a conssitent boost level with the leak.

How can I adjust the turbo boost?

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

1. 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.
2. For mechanical boost control systems, adjustments may involve changing the setting on a regulator valve(s).
3. For electronic boost control systems, adjustments may need to be made to the vehicle’s engine management system.
4. 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 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 used as a bypass for the turbine valve. Put simply, a wastegate diverts some of the exhaust gases past or around the exhaust turbine so as to limit turbine speed and the boost level the compressor can reach.

* A Wastegate valve can be either “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, which inturn prevents boost from increasing.

How should I break in a turbo?

There should be no need to conduct any specific break in procedure for your new turbo. You should however closely inspect the turbo looking for leaks (oil, water, exhaust or inlet).

What is/causes Shaft Play?

Shaft play occurs when the bearings of the turbo shaft wear out and becom worn and loose. Due to wear of the bearing, shaft play occurs causing the shaft to be able to move from side to side. If the shaft play becomes quite severe, it can cause the shaft to touch the side of the turbo and inturn cause your turbo to make a high pitched whine. 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.

more turbo related info HERE

Anything else you think is missing - please add(reply) as this is just a quick list I put together.

cheers,
AJ 510