Turbo basics represents ground we have covered many times in this magazine. However, from the questions we get and some of the articles about turbos we read, the basics still need to be addressed. A turbo is an exhaust-driven compressor that compresses the intake charge, resulting in a denser mixture with more oxygen molecules compared with an un-compressed mixture. Exhaust gases spin the turbine wheel, which is connected by a common shaft to the compressor wheel. The inducer portion of the compressor wheel draws ambient air into the turbo from the intake tract. Once in the compressor housing, the exducer portion of the wheel compresses the air and moves it out the nozzle. A byproduct of compressing air is heat, which is why intercoolers are an integral part of the turbo system. For more on intercooling, check out the tech piece in the November '99 issue. Only when the compressed air is met with the proper ratio of fuel and effectively ignited will more power be generated. The more fuel an engine burns, the more power it will make.
Turbos cannot be rated by horsepower. There can't be a 230 hp turbo; instead, everything depends on which engine the turbo is paired with. A street Honda would do well to make 230 hp, but place this turbo on a V8 that already makes 345 hp and there is a problem. Manufacturers often say that a given turbo will "support" 500-550 hp. This figure is based on airflow, efficiency and in most cases, takes into account general rules of brake specific fuel consumption.
There is no real way to rate a turbo. The best way would be to measure airflow at peak efficiency. Airflow is measured in cfm and peak efficiency is the compressor wheel speed (rpm) that the turbo is compressing the air with the least amount of heat generation. Even with all this data, there is still a range of flow (called the peak efficiency island on a compressor map) where the changes in efficiency are so slight that no singular number can be affixed to a turbo. Since a compressor map that illustrates the efficiency of a given turbo is a bit over the head of most enthusiasts (due to establishing and understanding the proper pressure ratio), the "supports blank horsepower" statement is a good place to narrow the field.
Sizing is critical. Can you turn up the boost of a turbo and see no power gain? Yes, you can. A "T-2-small" turbo can max out its flow or compress the air so violently that any increase provided by the additional boost is negated by the coinciding increase in heat.
One can also see no real increase of power over stock on a turbo set at 25 psi. If it is too big and does not spool up until, say 8500 rpm, an engine that maxes out a 6500 rpm will see little useable boost. Turbo selection is a compromise-a smaller turbo will spool up quicker than a large turbo, but provide less flow at the top end. A larger turbo will flow more air, but take longer to provide useable boost. So the intended usage of the vehicle and the power curve of the engine play a key roll in sizing. Sizing comes down to two factors: housing size and wheel selection. A housing's A/R ratio is a value based on the area and radius of the housing. The higher the A/R value, the more top-end flow the housing will have. A lower A/R results in a more responsive housing with less top-end. When it comes down to it, wheel size (diameter), trim (contours of the blades) and even the number of blades impact a wheel's efficiency and backpressure characteristics.