Turbo lag is the lead time between mashing the throttle and feeling the rush of torque from a turbocharged engine.
The lag comes from the time it takes the engine to create enough exhaust pressure to spin the turbo and pump compressed intake air into the engine and is longest when the engine is in a low-rpm, low-load cruising situation.
Turbo lag was the biggest problem preventing the early turbo cars from being accepted as practical.
The Problem
Contemporary turbines were large and heavy and could not start spinning until about 3,500 rpm. As a result, low-speed output remained weak.
since contemporary turbocharging required compression ratio to be decreased to about 6.5:1 to avoid overheating to cylinder heads, the pre-charged output was even weaker than a normally aspirated engine of the same capacity!
The Solution
The secret was a recirculation pipe and valve: before the exhaust gas attains enough pressure for driving the turbine, a recirculation path is established between the inlet and outlet of fresh air turbine, then the latter can spin freely without being slowed down to boost pressure.
When the amount of exhaust gas becomes sufficient to work, a valve will close the recirculating path, then the already-spinning turbine will get into operation quickly. Therefore, turbo lag is greatly reduced while power transition becomes smoother.
The first practical turbo appeared in The Porsche 911 Turbo 3.0 in 1975. Engineers had designed a mechanism allowing the turbine too “pre-spin” before boosting
