Compressor map data points

[90AWDTalon]

I was looking through some back issue's of Sport Compact Car and they had formula's for figuring out the data points to plug into the compressor maps. I decided to plug these into an excell spread sheet so I don't have to do the math every time I want a new point. While doing this I found two things that I have questions about.

Input for the formula's is boost, displacement in cubic inches, rpm, volumetric efficiency, and ambient temp. Output is CMF and pressure ratio.

First, these formula's where in two different issues. They were identical except for finding Mass flow rate. In one the formula was Di*displacement in cubic inches*(rpm/2)*Volumetric Efficiency. In the other formula it was (Di*Displacement in cubic inches*rpm)/(2*volumetric efficiency). I think the first one is right, because I used the second one at first and I noticed that as VE goes down CMF went up. I figured this was wrong and tried the other formula and found as VE goes up CFM goes up. Am I right in assuming the first formula listed is correct?

Second, according to these formula's as ambient temp goes up CMF goes up. This seems wrong to me, shouldn't cooler incomming temp mean more flow? Not less.

 

[90AWDTalon]

I'm starting to think their both wrong. Anyone know the right formula?

 

[WhatsADSM]

www.stealth316.com

its all there, in fact he has a bunch of calculators that you can use that will give you what you need. At least that is how I do my compressor maps

 

[90AWDTalon]

Originally posted by WhatsADSM
www.stealth316.com

its all there, in fact he has a bunch of calculators that you can use that will give you what you need. At least that is how I do my compressor maps

Great site, alot of very good info. He does his calculations alot differenct than Sport Compact Car. I need to finish reading it all.

After looking at his some more and the ones I'm using some more. The temp issue is in the conversion to Corrected Mass Flow Rate garrett uses on there compressor maps. The formula is ( SQRT(inlet temp/545)/(14.7/13.95) ), square root of inlet temp divided by 545 degrees rankin(85degrees F), all that is divided by inlet pressure divided by 13.95, garrett says this is the average pressure drop across an air filter. Playing with just this part of the equation will show as temp goes up this correction factor, which gets multiplied by mass air flow, goes up. As this multiplier goes up mass air flow goes up. This still doesn't seem right to me but I'm not a turbo engineer and the people at garrett are. If some one could explain why this happens I would really appriciate it.