A
turbocharger on an engine is not directly linked to the engine drivetrain. It is connected, if that is the word, to the rest of the engine and more importantly the engine's
motive force by a flow of
exhaust gases. In addition, a
turbocharger impeller set and shaft have mass (not much, hopefully, but some). Thus, when the speed of the engine is increased, the turbo will not reach its
optimum or desired level of
boost (rate of spin) for the new engine speed immediately. Rather, there is a small but perceptible delay between the engine speeding up and the turbo speeding up to match the new
exhaust flow. This delay is called
turbo lag.
Turbo lag is a bad thing to some degree. First of all, it lowers the driver's control of the car; when the gas pedal is pressed, at some point in the future, there will be a second surge or increase in power as the turbo spins up. Unless expected and compensated for, this surge might result in loss of control if the vehicle is operating near its performance limits. This lag will tend to complicate manual gearshifting, since the higher revs attained just before the shift point will result in a turbo surge perhaps during the period of declutch, or perhaps just after. The former will cause the engine to race, possibly slowing the shift or inducing excessive wear on the clutch depending on whether the driver compensates. The latter will apply additional power to a more heavily-loaded engine (at lower revs and a smaller gear ratio). This sort of unrealized force induces stress and wear on the drivetrain and engine itself.
There are a few ways to combat turbo lag. The most significant methods involve the design of the turbocharger system. The shorter the path from exhaust manifold to turbo, the less distance an increase in gas pressure must travel (as a wave). Thus, the lower the lag.
Also, forming the turbo mechanism out of lower mass materials means that it will more swiftly reach 'matching' speed, reducing lag as well.