I noticed a few errors in that. Overall it is a pretty good explaination though.
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Originally Posted by greasemonkey0018
The size of the compressor turbine determines the maximum amount of boost that the turbo charger can produce. It also effects the spool-up time of the turbo. The type of compressor wheel is usually designated as its "trim", which is a value that describes the inducer and exducer sizes. Typically, the exducer is significantly larger than the inducer on a compressor turbine.
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The compressor wheel determines the maximum flow (at a certain boost level) for the turbo at any given efficiency.
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Originally Posted by greasemonkey0018
The exhaust turbine design is a balance between absorbing as much energy from the exhaust gasses as possible and allowing the gasses to flow as easily as possible. This is closely related to the size of the exhaust housing. A larger turbine can absorb more energy from the gasses and spin the shaft with more torque and speed, but too large a turbine will restrict the flow of exhaust such that engine performance is greatly reduced. Typically, the inducer is only slightly larger than the exducer on the exhaust turbine. Generally, you would want to stick with the stock turbine because it's size is not nearly as important as the compressor turbine's size. If you want to reduce restriction through a smaller housing, you can have the turbine "clipped", which reduces the size of the fins and allows more air to flow around the turbine.
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This is off a bit. A larger turbine does not absorb more energy from the engine, it is actually less. It also does not spin the shaft with more torque and speed. If that were true a larger turbine wheel would have less lag than a smaller one. The reason a larger turbine wheel produces more power is because there is less restriction in the exhaust, so the engine breathes easier.
Also, where he says that more energy is transfered into the wheel because the exhaust gases cool down when it goes from a small space to a larger one, that is not the reason why it helps. The reason is that going to a larger place will create a drop in pressure and help suck the exhaust through the wheel.