CR Turbos :: General Turbo FAQs

General Turbo

What Causes Turbo Lag?

The time required to bring the turbo up to a speed where it can function effectively is called turbo lag. This is noticed as a hesitation in throttle response when coming off idle.This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure.

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 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 Single / 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.