Re: RS2 turbo

[Dan Bocek <dan@di.com>]

> Does anyone know if there is a system for cataloging turbo dimensions & 
> performance?
> 
> Are turbo characteristics something precise & numeric like bore & stroke?
> Or are they more artistic like combustion chamber design and intake port
> shape?  Are they categorized primarily by the measurement of their physical
> components (like displacement) or by their bench tested and real life
> measured performance?

Turbo's are normally characterized by something known as a compressor map.
This is a graph of the pressure ratio vs. airflow.  Pressure ratio is 
defined as the ratio of compressor total outlet pressure to total inlet 
pressure.  Plotted on the map are things called efficiency curves, a lot 
like the "hills" on a topological map.  Efficiency increases from right 
to left, until it reaches a peak efficiency "island", which is usually 
somewhere around 70%.  Plotted more or less horrizontally accross the 
efficiancy "islands" are turbocharger constant rpm "load lines".  These 
lines are curved, and where they intersect the efficiency "islands" is 
an indication of how much airflow and pressure ratio you can expect at that 
particular turbine rpm at that particular efficiency. 

Let's say, for example, that we want to know what efficiency and what
turbine speed we could expect from a Rayjay model 370E turbo (just happen
to have the map in front of me) flowing 400 CFM at a pressure ratio of 
1.9.  From the map, you'd look along the horizontal AIR FLOW axis until 
you hit 400 CFM, and follow that up the vertical PRESSURE RATIO axis 
until you hit 1.9.  Where those values intersected, you'd look to see 
which efficincy "island" you'd landed in (in this case, 65%), and you'd 
also look at the turbine rpm curves and see that the turbine would need 
to spin at 80,000 rpm to accomplish this.

>From the compressor map and a healthy knowledge of thermodynamics, you can 
get a pretty good idea of what that turbo will be like on your engine,
but to get an exact idea of what it will be like, you have to know things 
like the volumetric efficiency of your engine at various RPMs, 
intake, and exhaust flow characteristics.

Of course this is all a moot point when it comes to KKK turbos on Audis, 
because KKK won't release compressor maps to anyone but major auto 
manufacturers (believe me, I've tried!).  Oh well, I guess every power 
upgrade eventually boils down to "seat-of-the-pants-i-think-it-feels-more-
powerful-now-than-before-because-i've-spent-last-weeks-rent-check-on-this-
new-upgrade" anyway.

                                                         Dan Bocek
                                                         dan@di.com