FW: [s-cars] Could it be the GP RS2 turbos are "over" spec?

[Lewis, Gary M]

This got bounced bacause of the attachment.  I removed it. This is a resend

Hi Bob,
You are never screwed up, Professor, usually it is me.
 
But you said...

	The only difference I see between the two turbos is that one may be operating at a lower rpm than the other.  They are both putting out air at essentially the same pressure and at essentially the same flow rate.  The only possible significant difference might be that the more efficient turbo doesn't heat the air as much as the less efficient turbo and that difference would have to come from heat transfer from the hot side of the turbo to the cold side (by conduction, I presume) and the into the air flowing through the cold side.  I'm having difficulty seeing why this might be the case.  The two turbos are basically similar in structure and should transfer heat similarly as well.  So, why does a more efficient turbo produce cooler air?  I don't see it. 
	 

Why are all turbo types (K24 vs. RS2 for instance) rated at different efficiencies based on the same mass of air moved?  Efficiency factors into the amount of heat created during compression.   And pressure really isn't important, it's mass.  A k24 and a K27 at say 20 psi put on our 2.21 motors at 5,000 rpm put out wildly differing amounts of mass, due to the difference in heat.  Same PSI, different mass.  Hence a MAF sensor, not a VAF sensor.  Hap runs 25 psi, yet I bet his turbo flows much more mass than an RS2.
 
A K24 on a 2.2l at say 5,000 rpm at 26 psi makes REALLY hot air.  A bigger turbo is more efficient at moving mass than a smaller one, all things being equal.  Why you want a small one is for early response.  The argument is always the "Right Size" (not too hot, not too cold), i.e., low down torque and high rpm staying power.
 
I hope this comes out.  It is from my "Turbocharger Sizing Worksheet", an excel file that I have also attached.  Note the air temps between a K24 and the RS2++ from my car.  It isn't perfect, but close.  Great for illustration purposes.  Also good for backing into the efficiency of a custom turbo if you know the outlet temp, psi and atmospheric.
 
  
	Turbocharger Sizing Worksheet	
Inputs	 			
				
Engine Displacement  (in3)	 	135.0877	 135.0877	
RPM	 	5000	 5000	
Engine Volumetric Efficiency  (%)	 	0.85	 0.85	
Boost  (lb/in2)	 	26	 26	
Compressor Efficiency  (%)	 	0.85	 0.50	
Inlet Temperature  (oF)   	 	70	 70	
Atmospheric Pressure  (Lb / in2)   (P1)	 	14.7	 14.7	
				
				
Outputs	 Equation	 		
				
Absolute Inlet Temp  (oR)   	 Inlet Temperature + 460	 530	 530	
Ideal Unpressurized Engine Airflow  (CFM)	 in3 x rpm / 2 / 123	 195	 195	
Actual Unpressurized Engine Airflow  (CFM)	 Ideal Flow x Vol. Eff.	 166	 166	
Absolute Boost  (Lb / in2)   (P2)	 Boost+Atm	 40.70	 40.70	
Pressure Ratio	 (Boost+Atm) / Atm	 2.77	 2.77	
Adiabatic Outlet Temperature  (oR)   (T2)	 T2 = T1 (P2 / P1)0.283	 707.0	 707.0	
Adiabatic Outlet Temperature  (oF)	 oR-460	 247.0	 247.0	
Adiabatic Temperature Rise   (oF)  	 Adb Outlet Temp - T1	 177.0	 177.0	
Actual Temperature Rise  (oF)	 Adb Temp Rise / Comp Eff	 208.3	 354.1	
Actual Outlet Temperature  (oF)	 T1 + Actual Temp Rise	 278.3	 424.1	
Density Ratio	 (Inlet oR / Outlet oR)(P2 / P1)	 1.988	 1.660	
Compressor Inlet Flow  (CFM)	 Outlet Flow x Density Ratio	 330	 276	
     Compressor Inlet Flow  (LBM / Min)	 Inlet CFM x 0.069 lb / ft3	 22.78	 19.03	
Compressor Inlet Flow  (CMS)	 Outlet Flow x Density Ratio	 0.1558	 0.1301