Compressor Maps
 

I was reading falcon's compound charging thread and it made me wonder about the compressor maps.

You get compressor maps in both volume flow and mass flow. since the relationship between volume and mass depends on density, that has to mean that one of those depends on a specific intake air density.

I assume that it is mass flow that would stay constant for the compressor maps and that if you are dealing with a volume based compressor map with a different intake temperature/pressure the figure would need to be adjusted?

can anyone confirm this for me?

Volume based maps are nearly useless, and can actually be misleading in some circumstances. Even mass flow based maps need to be corrected for temperature and pressure, as should your engine airflow needs when doing matching work. Granted, aftermarket matching like we do here doesn't need to be as exact as OEM stuff does, but it still makes a difference if you do it properly.

It's hard to know where you are going to be on the map without engine/intake flow data and how efficient everything is.

For example, I was running a 105mm pulley on the Rotrex before and stock intake manifold and made 14PSI. I bumped it down to a 95mm pulley and a GotPSI? intake and only gained 1PSI. I know it's flowing way more but because a lot of intake restriction is gone the actual boost level didn't increase much.


For my application I need to find a compressor that is efficient when flowing about 8PSI on it's own and then when the Rotrex is sucking way more air through it (the turbo is still blowing 8PSI) the effieciency moves on a parallel line to the right all the way to whatever CFM the Rotrex+turbo are pulling/pushing through the compressor. The pressure ratio will remain the same however the flow on the map will move to the right when the Rotrex starts overpowering the turbo.

This is a really good thread to read -> http://www.rx7club.com/showthread.php?t=472529

Which then points you to this thread which teaches you how to interpret a compressor map.

-> http://www.rx7club.com/showthread.php?t=478915

This thread is what helped me decide to go with the C30-94 over the C30-74.

I'm happy with using compressor maps, Just wasn't sure whether it was the CFM maps that were a special case or the lb/min maps.

I guess we just get maps in CFM or M^3/s because it saves calculating the mass flow when you are working from the point of view of engine air consumption and they work for the common case.

falcon, for a compound 22psi you're going to need a turbo that can flow ~35lb/min regardless of what the ratio of boost is between the turbo and the supercharger.

I guess it also shouldn't matter whether the turbo is before or after the SC, at least it wouldn't change the math.