What is "turbo lag", or the turbo's response delay?

Antonio Roncero @roncero_antonio 1 June 2020 5 min.

“Turbo lag”, or the turbo’s response delay, is the time that elapses from when the accelerator is pressed until the power delivery, produced by the boost pressure of the intake air generated by the turbo, becomes effective.

Except in a few cases (small, low-displacement, low-power engines), the turbocharger is today practically a standard for increasing the performance of combustion engines, both petrol and diesel.

And if you have driven a turbocharged engine, you will surely have noticed in certain situations a slight delay between the moment you press the accelerator to demand more power and the moment it becomes effective. This effect is known as “turbo lag”, or the turbo’s response delay.

To better understand what “turbo lag” consists of, it is worth knowing at least the basic principles of an engine supercharging system using a turbocharger.

How a turbocharger works

The turbocharger is one of the types of mechanisms that exist for supercharging, or forcing the entry of air into the intake chamber of an engine. It consists of a compressor and a turbine (the name comes from the combination of the two words) connected by a shaft, which rotate inside a housing.

The exhaust gases generated by the engine itself during its operation drive the turbine, which in turn spins the compressor. The latter is responsible for compressing the intake air up to a maximum pressure that is limited by a wastegate. The compressed air is introduced into the combustion chamber, increasing performance.

Why does turbo lag occur?

The characteristic response delay in some turbocharged engines occurs when the exhaust gases must overcome the inertia of the turbine when it is stationary or turning at low revs. Keep in mind that for the compressor to spin at a speed that allows it to compress the intake air, the turbine must be made to spin fast enough.

If the exhaust gas flow is small, for example when accelerating from low revs, it will struggle to overcome that turbine inertia and move it. Likewise, if a turbocharger is very large in order to generate greater boost pressure, the turbine will be bigger, and it will take more effort to move it.

In the past, when engines did not have the electronic sophistication of today, sports car engines often used large turbochargers to achieve significant power gains. These turbochargers generated a lot of compression—and therefore made the engine produce much more power—when spinning at high revs.

But at low revs—when the exhaust gas flow is small—they took time to come into action, which forced the driver to anticipate when accelerating and caused all the power to appear practically all at once.

How turbo lag is combated

It therefore seems logical that, to combat turbo lag, small turbochargers are used, in which the turbine requires as little effort as possible to move, or lightweight materials are used in its construction.

The problem is that a small turbocharger limits the capacity of the supercharging system to improve performance. On the other hand, the high temperatures and rotational speeds reached by the turbocharger (up to 300,000 rpm) make it necessary to use special materials, such as titanium or ceramic.

Nevertheless, various technologies have made it possible to reduce the response delay of turbocharged engines. The most common are the following:

Variable geometry turbocharger. The turbine vanes are not fixed but can move to change the angle at which the exhaust gases strike them, so that the turbine accelerates more quickly.
Twin-scroll turbocharger. The Twin-Scroll technology, or twin-entry turbos, should not be confused with double supercharging. In a Twin-Scroll turbo, two separate inlets are used to direct the exhaust gases to the turbine, thereby separating the cylinders that supply pressure from those that could cause a reduction in pressure by “back-scavenging” the exhaust gases.
Double supercharging. With this technology, a smaller turbocharger can be used to achieve an immediate response at low revs, and a larger one to provide more pressure and deliver more power when the engine runs at high speeds. These systems may use two (or more) turbochargers of different sizes. Other types of supercharging are also used, such as positive-displacement compressors or electric compressors, which provide a rapid response at low revs, combined with a conventional turbocharger.

“Bang-bang”: the “anti-lag” of old competition cars

Before all these systems, in old competition cars that used enormous turbochargers to achieve large power figures, drivers had to contend with huge delays in throttle response. To address this, engineers developed an anti-lag system that quickly became known as “Bang-bang”, because of its characteristic sound.

It consisted of retarding ignition so that the spark plug fired a little later, in order to slightly delay combustion, so that greater pressure was generated in the exhaust gases due to the intake air entering the cylinders. An ingenious solution that was technically easy to implement, but which would not be possible in a road vehicle because of the high fuel consumption and the reliability problems it would cause for the engine.