This is some thing that I wrote for a different MSG Board. I don't know it all and maybe others can fill in info, but I hope it helps:
To start slow, because I’m not sure how much people know out there. A turbo blows compressed air. The pressure at which the turbo is blowing air into the engine above ambient pressure is called boost. Boost is measured in PSI (lbs per square inch) and bar (which is in atm). One bar = 14.7 PSI.
However, pressure (boost) is not the only thing to talk about, when trying to see how much air is going into your engine. The amount of air going into the engine is better termed weight flow rate, or something like lbs per min. The weight flow rate is a function of both boost and airflow (CFM, cubic ft. per min).
weight flow rate = boost * airflow
CFM is a function of the compressor size (intake) of the turbo. Under the same boost, a large compressor will have more weight flow rate then a smaller turbo. However, as you can see from the equation above, it is also possible for a smaller turbo to have a greater weight flow rate, if it is making more boost.
So, why choose one turbo over the other if you can just trade boost for airflow? Well, this is where the compressor efficacy comes into play. Whenever you compress air or any fluid for that matter, it heats up. The more compression, the greater the increase in temp. This is why a lot of turbo/supercharged cars have intercoolers. As in my car, in addition to my intercooler, I spray alky to cool down the intake temp.
Since on a turbo car you can control the density of the air by the level of boost (more boost=greater density) that the car is running, why reduce air temp? The hotter the air temp going into the engine is, the more likely knock / detonation is to occur. Knock / detonation, is when you get premature ignition. During premature ignition the mixture of gas and air lights before the spark plug fires. When the spark plug does fire, that makes for a second exposition. When these two meet, they make a shockwave. Not only does this decrease power, but it can also do engine damage. Therefore, it is necessary to prevent knock / detonation, both to create more power and to protect your engine.
This can be done by upgrading the cooling system. The more aluminum an engine has in it, the cooler the intake temp will be. Therefore, it will be better able to prevent knock / detonation. This is also what octane does in gas. The higher the octane, the more resistant it is to premature ignition. This is why even non-blown performance engines require 93 octane, because they run higher compression, the non performance engines.
Well, with that out of the way, back to talking about turbo selection, big turbos vs. small ones. Compressor efficiency describes then amount of heat a turbo will be producing. A smaller turbo will reach a point at higher boost levels where it can no longer force air into an engine without having knock / detonation. This is a good reason to choose a bigger turbo.
One might then ask, why not go straight for the biggest turbo on the market? If a big turbo is used in the wrong application, then the engine will not be able to drive the turbo and that engine will loss power. The ability to drive a turbo is a function of engine speed, engine displacement and cylinder heads / exhaust efficiency.
This is finally when compressor maps come into the conversion. Compressor maps are a tool used to help in the selection of a turbo for a given application. After reading what I wrote and answering questions, I will talk about how to make use of compressor maps.
Jason
.
