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.

One of the more popular methods of combating this condition is the use to a pair or more of smaller turbos in lieu of a larger turbo to gain the same amount of boost. While introducing a extra degree of expense and complexity, the fact that a smaller turbocharger has a smaller rotor, and thus less rotational mass makes this method quite effective. Some times it is taken to extremes, as was seen in Ford's GT90, which had a quad turbochaging set up. The resulting amount of exaust and intake manifold tubing was obscene.

There is another method of combating turbo lag, which is noisy, dangerous, hard on equipment, and lots of fun. It involved placing some sort of fuel delivery port upstream of the turbocharger in the exaust manifold, usually some sort of fuel injector. When braking through corners some fuel is dumped through this port, which ignites and keeps the turbo spinning like a top. This method amounts to controlled backfiring of the engine. What fun...

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