Ok. First of all here's a link that gets into your A/R question. Some of the art and text looks like it came straight from Corky's book - http://www.hotrod.com/techarticles/engine/113_0312_turbo/index1.html
They define A/R pretty well:
HotRod dot com
The turbine housing A/R (area/radius) ratio is the area (A) of any turbine inlet scroll cross-section divided by the distance from the center of that cross-section to the center of the turbine shaft (R). For any given turbine housing, A and R vary in the same proportions, so all As divided by their corresponding Rs yield the same dividend--which is the A/R ratio.
Originally posted by DR1665
? > So if A/R is related to the turbine, does the same sort of ratio apply to compressor wheels? Would a lower A/R (if it exists for compressors) mean a broader power band where output jumps and climbs gradually to peak? Would a higher A/R start more gradually and keep building and building to peak?
The compressor is exacly like a turbine in the sense that there is an A/R. You could substitute the word turbine with the word compressor in that snail caption - "For any given compressor housing, A and R vary in the same proportions, so all As divided by their corresponding Rs yield the same dividend--which is the A/R ratio.". Thats about where the similarities end.
It sounds like you have a descent understanding of how the turbine A/R affects the turbo. Check out the rest of that article for some good info. Seriously - it looks like most of it was just ripped out of the book.
Theres a fundamental difference betwen turbines and compressors though. Turbines generate power and compressors generate boost. IMO The questions about compressor A/R are moot. Whats important on the cold side is the compressor map. Your power is directly related to compressor and intercooler effeciencies, so the trick to getting the power where you want it is understanding how to read a compressor map. The best place that I've found for learning how-to read a compressor map is here - http://www.stealth316.com/2-turboguide.htm Scour that site.
Just one more thing about that last link there. My favorite quote from that page addresses the most common misconception about how a turbo works:
Stealth316 dot com
This bears repeating one more time. Just because a turbo is rated at 650 CFM @ 15 psi (for example) does not mean that the turbo flows that amount of air in our 3L V6 engine at 15 psi plenum pressure. The engine mass airflow is determined by the displacement, the RPM, the volumetric efficiency, and the air density (or plenum air pressure and temperature). At a given RPM and at the same plenum air pressure and temperature, the same amount of airflows regardless of which turbo is used.
Basically - you cannot escape volumetric efficiency. It doesn't matter if you've got forced induction - the intake stroke still has to "suck" what it needs from the plenum. Boost makes the air density higher...but it doesn't "force" anything into the engine.
95 Eclipse RS : 5 speed
15.7 @ 87.9mph
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html