Lag was one of the biggest demons in a turbocharged set up. Back in the old days, when turbo charging was just finding its way into production vehicles, lag was at its all time ugliest. Low compression internals, coupled with relatively high boost and limited, primitive tuning ability created extremely peaky, temperamental and unreliable engines. They were all or nothing affairs, with two distinct personalities and neither of them actually getting the job done very well.

These days turbo engines direct from the factory are becoming more and more seamless, with the transition from off boost to on boost an almost sublime affair. More complex computer systems and electronic fuel injection means that compression ratios can be reasonable for off boost performance and therefore lower boost can be run in order to produce meaningful power.

Generally speaking, turbo lag is the term which is used to describe the time that elapses from when the accelerator is open to full throttle to when the turbo charger comes onto boost. Turbo chargers are designed to operate at their optimum through a specific RPM range and with certain airflow conditions. This means that turbo lag is going to be inherent in any turbo system.

Sizing of a turbo will dramatically change the performance and characteristics of an engine. A small turbo will operate at its best low in the rev range because it has a smaller amount of rotational inertia. A larger turbo will buffer the mid range due to having potential to flow more air but with an increased rotational inertia, and a monster turbo will provide its magic at the top end as the needle is approaching the red. Bigger cams, a performance manifold or worked heads are similar in that you can't have peak performance in all RPM ranges.

One thing that needs to be kept in mind when considering force induction is that revs per minute is not the only contributing factor in making boost. The airflow through an engine is quite different at 6,000rpm in neutral, at 6,000 rpm in first gear at part throttle, and at 6,000 rpm in a high gear at full throttle. Turbochargers are extremely sensitive to these airflow characteristics, and even more sensitive to load differences.

Looking at load is another area which affects how a turbo operates. In lower gears the load is less than in higher gears. Tyres also affect the load. A car with lower gears such as 4.88:1 and short tyres will have a lower engine load as opposed to a tall tyre with 3.08:1 gears. Weight also affects load, the heavier the car the bigger the load. A given turbocharged vehicle may produce full boost at 3,500 rpm in first gear. The same car may produce full boost at 2,300 rpm in fourth gear, and when traveling up a steep hill, may produce full boost by 1,800rpm even in first gear.

Manual transmissions will also have more of an effect here. Boost needs to rebuild between gear changes because the throttle is shut while the driver is shifting. Autos are a different kettle of fish because they can be stalled up to create enough load to build boost before the vehicle is launched. During shifts the throttle is not closed, which can cause the boost to spike. When the change occurs, the RPM will drop which also increases the load, so the boost will still be high. An auto equipped turbo car that is accelerating in first gear with low boost can go instantly to full boost when shifting into second.

One method to eliminate lag is being used extensively with rally cars. It is commonly known as anti lag. When the driver takes his foot off the accelerator ignition timing is altered up to 40 degrees retarding the spark. Intake air and fuel supply becomes richer. An air injector is added or the throttle is left partly open to maintain air supply to the engine. This allows air to still get into the engine even when the driver is not accelerating. Because the ignition timing has been retarded the air/fuel mixture reaches the exhaust tubing predominantly unburnt. The heat of the piping coupled with the spark plug firing as the exhaust valves are opening explodes the air/fuel mixture spinning the turbo up ready for action. Due to this set up, even at idle these cars are creating some serious boost.