Bit of a history to start with....
The first exhaust-driven supercharger was developed by Dr. Alfred J. Buchi of Switzerland between 1909 and 1912, long before Garrett products entered the turbocharger picture. Dr. Buchi was Chief Engineer of Sulzer Brothers Research Department and in 1915 proposed the first prototype of a turbocharged diesel engine, but his ideas gained little or no acceptance at that time.
Now...lts see the theory behind it
A turbo charger is basically an exhaust gas driven air compressor and can be best understood if it is divided into its two basic parts
1)Exhaust gas driven turbine and its housing
2)Compressor and its housing
I did say divided didn't I.Each of them perform different functions but, because they are joined together at the hip via a common shaft, the function of one impacts the function of the other. How? Take a perfectly set up compressor section and mate it with an incorrect turbine section, or visa versa, and you end up with with our Siamese twins trying to go in different directions. The result is that our Siamese twins end up wasting all of their energy fighting each other and go nowhere.
When considering a turbo charger most folks tend to look at the maximum CFM rating of the compressor and ignore everything else under the assumption that the compressor and the exhaust turbine are perfectly matched out of the box. I will grant you that in stock factory applications that is probably close to the truth but, in all out performance applications, nothing could be further from the truth because of the extremes of operation in a performance application.
The goal in a performance application is to get the exhaust turbine up to speed as quickly as possible however, it must be mated to a compressor wheel that will generate as much pressure as it can as soon as possible. This is a contradiction because the exhaust turbine generates the drive power and the compressor consumes that power. The larger the compressor and the higher the pressure (boost) we want, the quicker the power from the exhaust turbine is used up. Put in a larger exhaust turbine and it will take the engine longer to develop enough hot expanding exhaust gas to spin it, slowing down the compressor and causing turbo lag. At this point I am going to repeat something stated earlier, do not think of a turbo charger as a bolt on piece of equipment, think of it as a system.
The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better. I am sure many of you have seen pictures of turbo charged engines with cherry red hot exhaust systems and turbo housings. The captions under most of these types of pictures proclaim outstanding horse power numbers. What most of the articles related to these pictures do not tell you is that the engine was under an extreme load. A load so heavy that the engine was almost at its stall point for a prolonged period of time. A condition that most turbo charged engines will never see.
The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load. There is where the selection of transmission gear ratios and the ring and pinion ratio play a critical part. The fact that the engine must be under a load is the reason why, no matter how high you rev a turbo charged engine with no load on it, you will not see the boost gauge move.
This is also where the term 'turbo lag' came from. Turbo lag is basically the amount of time it takes from the time you place a load on the engine (stomp the gas peddle to the floor and dump the clutch or, get full converter lock up with your automatic trans) until the time the engine develops enough hot expanding exhaust gas to spin the turbine fast enough for the compressor to do its job.
Effectively, a turbo charged engine is a normally aspirated engine until the turbine and compressor spin up. To minimize turbo lag, it is imperative that the turbine and the compressor are properly matched to the engine as well as the engine being properly matched to the transmission gears, the ring and pinion gears, and the tires.
