This is the best "One Stop" I have found. He keeps the math easy and runs all the variables.
reading a compressor map :: motorgeek.com
Now you’ll notice on the vertical side of the chart is the pressure ratio and then the bottom horizontal line of the chart is the air flow in either LB/min or some display CFM (cubic feet per minute). To convert Lbs/min into CFM you need to consider the temperature of the air. Most compressor maps are taken at 85 degrees Fahrenheit. You can normally tell from the formulas written on the map which will have the number 545 and by subtracting 460 from that number you convert it to Fahrenheit. One cubic foot of air at 85F weighs 0.07282 pounds. So, at 85F, convert pounds per minute to CFM by multiplying by 13.73. This will come in handy in a little while.
To start off reading the compressor map first decide your boost goal. Say you want to run 15 psi on this T04E 40 trim map. You need to find the pressure ratio which is the absolute pressure at the outlet of the turbo. To find the pressure ratio add the boost you want to run (15 psi) to atmospheric pressure (14.7 unless you are at altitude which you can adjust to your numbers accordingly) and then divide by atmospheric pressure. So it looks like:
Pressure Ratio = (15 + 14.7) / 14.7 = 2.02
So if you draw a line horizontally across the compressor map just barely above the number 2 on the pressure ratio side of the map you can see it cuts through the middle of the map. Now if you look at the range from the surge line to the end of the balloon, we have a range from 15 lbs/min to 35lbs/min. Which now if you wanted to convert into CFM by multiplying by 13.73 like earlier mentioned you get a range from 205 CFM to 480 CFM.
So this all sounds pretty sweet , but we need to consider that the engine at a given intake pressure will only be able to ingest so much air. To figure out how much air flow you’ll have in your engine you need to start with the displacement of your engine and an RPM point.
CFM for 4 stroke = Displacement in CI / 3456 * RPM * VE
VE is volumetric efficiency, which is a value indicating how much of the potential air flow volume actually makes it through the engine at a given RPM. Now most of the time unless you know the VE you’ll be guessing, but if you take the hp you know you have on some online engine calculators you can work backwards and adjust the VE number until the HP numbers match up. For this write up I’ll just throw numbers in there. So a stock MC engine is 2226CC or about 136 cu in and at 6000 rpm and 90 percent VE ( .90) it will flow:
136 / 3456 * 6000 * .9 = 212.5 CFM
Now some people might be thinking that this would put you in a good spot between the flow range that was figured out earlier, but the 212.5 CFM is only tell you what the engine will flow if naturally aspirated. So that being said if you do this math for a different engine or a different map and it doesn’t land in that range and looks like it’s in the surge or choke range don’t worry about it too much just yet. To determine what it will do under boost, you have to determine what density ratio of the compressor and intercooling system will give you. To do that we need to take our boost point and determine how hot the compressor is going to make the air at a that boost:
Tout (in F) = (((Tin (in F) + 460) * (Pressure Ratio0.283)) - 460)
For 15psi of boost at sea level at an ambient temp of 85F (85F in this case so that our computed CFM ends up matching that of the compressor map).
Tout = (85 + 460) * 2.020^.283 - 460 = 205F
This assumes an ideal, 100% efficient compressor. The round circles in the compressor map tell us how efficient the compressor is going to be at a given pressure ratio and flow level. Since most of the map is at least 70% (.70) efficient or better (adjust this number for what range you're shooting for), we'll use that figure and double check later to make sure we were either close or underestimating a little. Our real outlet temperature is going to be:
delta T actual = delta T ideal / efficiency
For our example, the delta T ideal is 205F - 85F or 120F:
delta T actual = 120F / 0.70 = 171F
171F is how much the compressor is going to heat the air above the inlet temp, so the real outlet temp is 171 + 85, or 256F. What happens when this air mass hits the intercooler? Two things: first, a pressure drop and second, a temperature drop. The pressure drop is going to be about .5 psi for a smaller intercooler or something like a side mount. Lets assume a 65% efficiency from the smaller intercooler which isn’t the best, but I’ve seen a lot of people pushing things harder then that and a lot of people way better off then that. To figure out the intercooler efficiency and the pressure drop you would use: (IC = intercooler)
T IC drop = (T IC in - T ambient) * IC efficiency
T IC drop = (256 - 85) * 0.65 = 111F
The IC will drop the turbo outlet temp by 111F, turning the 256F air into 145F air and dropping the pressure 0.5psi to 14.5psig. To figure out what this would do to the naturally aspirated engine we have to figure out the density ratio
Density ratio= ((Tin + 460) / (Tout + 460)) * (Pout / Pin)
Density ratio = ((85+460)/(145+460))*(14.5+14.7)/14.7 = 1.79
This density ratio means that you will get 1.79 times as much air flowing through the engine with this compressor and intercooler combination at this pressure point and this ambient temperature than you would in normally aspirated mode. Going back to our 212.5 CFM value, we multiply that by the density ratio to get 380.1 CFM which converts to 27.7 Lbs/min
With all this information you can now draw a line vertical from the 27.7 Lbs/min area on the bottom line of the compressor map and cross through the original horizontal line from the pressure ratio we see that with the MC engine and this 40 trim wheel at 6000 rpm with the intercooler setup we used you can see we’re on the edge of 73 percent efficiency. Now with all the numbers we guessed on like the VE and the intercooler efficiency and if the ambient temperature was different in real life then from the map things could change for the better or worse so this is really like I said before just to give you an idea.
What makes it a little tough to predict what you really are going to get is getting an idea of what the final VE of the system will be (which is not constant, but changes across the RPM/Manifold pressure range) since the turbine housing and wheel themselves are going to have an effect on the VE map. You can have a turbo too small that actually takes VE away from the engine after a certain RPM and can cause choking.
Since we have the numbers calculated we should see whether the compressor will be forced into the surge line. Surge is caused when the engine cannot ingest enough air to keep the compressor inside its map. We saw that at a 2.02 pressure ratio, the surge line is around 15 pounds per minute or 205 CFM. Now, let's assume that the turbine and turbine housing we will choose can power the compressor to reach 15psi by 3500RPMs. We keep the density ratio the same, but we have to re-compute the flow for the engine at 3500RPMs. The VE at this point should be better than at 6000, so we'll use another guessed value of 95%. At 3500RPMs, the engine will be ingesting:
CFM = 136 / 3456 * 3500 * 0.95 = 130.8 CFM
That's in normally aspirated mode. Multiplying the density ratio, we get:
130.8 CFM * 1.79 = 234.1 CFM
This isn’t near the surge limit, but if it were you can fix most of these problems by switching the turbine housing with a larger A/R (aspect ratio). By doing that you’ll slow down the spool up time to bring the compressor up to this pressure ratio when the engine is revving a little faster and ingesting more air.
With all that was guessed during this there is a definite fudge factor so there might still be some trial and error, but at least this will give you a better idea of where you should be with your goals. For all of you reading this I hope it comes out clear I think I pulled my brain into a knot. I’m not a math wiz so if I’d made any mistakes or you see anything not said right just tell me…we all make mistakes. :-)