RE: Turbo CFM- calculations for the archives

[QSHIPQ@aol.com]

Mr. Fawcett writes:
>Hey everyone.  I looked in teh archives for this, but had no luck.  My
>question is, what amount of cfm is the k26 turbo capable of producing?  Or
>is it a better question to ask, with 20psi in the intake manifold, how
>many cfm is the k26 producing?  This is on a 10v I5.  Any idea where I
>could find this out?  
>Thanks, as usual.
>James Fawcett
>86 5kTQ

James:
I thought I posted this once, but thanks for searching first.  Here's how you 
can figure engine cfm and turbo map stuff (and I'm not giving away my store 
here, basic information only).

2.2liters = 134ci
134 * rpm/1728 * 2 = 134*5500/3456
= 213CFM @ 5500 rpm NORMALLY ASPIRATED
213 * PR = AIRFLOW RATE (approximate, but maximum, figured at 100% Volumetric 
Efficiency - I usually use 80% for the 10v cars, 85-87% for the 20v cars.  
The actual calculation to get to the above:
cid * rpm*0.5* VE/1728    
**  Note: most tuners choosing turbos use the above calculation without VE to 
help them account for altitude compensation)
 
PR = Po + P1/Po, let's assume sea level
First convert psi to bar:
20psi + 14.7/14.7= 2.36bar absolute so,
2.36bar - 1.0 bar (atmospheric) = 1.36 boost pressure

PR = 1.0bar + 1.36bar/1.0bar

Pressure Ratio = 2.36 (not a futile exercise for those above sea level:)

Back to AIRFLOW RATE
2.36 * 213 = 502.68CFM

PROBLEMS:
*  A two pass IC can only flow about 400CFM thru it's core in *single* pass 
trim
*  A 2.36PR on a k26 turbo is outside both the k26 turbo map in terms of 
speed (that's 120,000rpm+++) - fyi, KKK doesn't list speeds higher than that 
on just about any turbo they make
*  You are just at 60% compressor efficiency (peak in stock trim is 72%)
*  A stock 10vt k26 doesn't put out 500cfm
*  RULE TO REREAD:  Turbo speed alone doesn't increase flow, it increases the 
pressure ratio
*  Bottom Line:  Wrong turbo to be running 2.36PR
*  Next rule of thumb on I5 10vt:  Density rules over boost pressure.  Take a 
ride in a 5ktq with a big effiecient intercooler @ 15psi boost, you'll forget 
about more boost.

To begin your journey into the black magic of turbochargers get:  Maximum 
Boost by Corky Bell, it's a great baseline, and some insurance against doing 
something silly.

For those so inclined to do the math, figure 2.0 maximum PR on a stock k26 
audi 10vt turbo car, 5700 maximum rpm cfm, 80-85% VE depending on mods, 65% 
minimum compressor efficiency .  


HTH

Scott Justusson
QSHIPQ Performance Tuning
QSHIPQ@aol.com