What Are the Causes of Turbo Lag?by Patrick Gleeson, Ph. D.,
Turbo lag is the hesitation, before acceleration, when you step on the accelerator of a car with a turbocharged engine. To some extent, turbo lag has physical causes inherent in turbocharger technology. However, different turbocharger designs and different conditions will affect the degree of lag.
A turbocharger uses engine exhaust to power a spinning rotor in a chamber above the engine intake. The air-fuel mixture flows through this chamber; the rotor compresses it and delivers a denser air-fuel mixture with higher potential energy to the cylinders.
The Key to Turbo Lag
How quickly the rotor in the turbocharge accelerates--how quickly it can increase pressure in the intake manifold--depends on the pressure in the exhaust manifold. An idling engine generates relatively small amounts of exhaust gas; the engine first has to accelerate in order to increase the amount of exhaust gas, which increases exhaust gas pressure. The exhaust gas pressure has to increase before the exhaust gas can power the turbocharger, and the turbocharger has to rev up before it can increase pressure in the intake system. From the beginning of this process to the end takes time. The time it takes is "turbo lag."
It takes more energy to push an object from rest to walking speed than it does to continue pushing an object that's already at walking speed. This force is called "inertia." It takes more inertial force to push a 200 lb. object from rest to walking speed than it takes to push a 100 lb. object.
Inertia and Turbo Lag
The weight of the moving parts in a turbocharger affects the force needed to accelerate the turbo. A turbo rotor (sometimes called "vein" or "wheel") made of very lightweight alloys will produce less turbo lag than a heavier rotor because it takes less inertial force to accelerate it--there's less mass. Also, a compact rotor design generally requires less centrifugal force and will therefore accelerate faster and with less lag than a larger diameter rotor.
Driving conditions and different transmission designs will also affect turbo lag. An engine already revving above 3,000 RPM has more energy in the system than an idling engine; systems with greater internal energy will always overcome turbo lag faster than lower energy systems. Similarly, transmission designs that keep engine revs high will produce less turbo lag than designs that require sudden engine accelerations and decelerations at shift points.
- STS Turbo: What is Turbo Lag?
- Autozine Technical School: Forced Induction, Turbocharging
- "Maximum Boost: Designing, Testing and Installing Turbocharger Systems;" Corky Bell; 1997
- engine image by goce risteski from Fotolia.com