Reading Compressor Maps
We'll start off by looking at a map and looking at the x(horizontal) and
- The x-axis shows Air Flow Rate. The Air flow rate of these
particular maps is in metric units and I think it's in m^3/sec (My tech rep
wasn't sure either), but I do know for sure that to convert to lbs/min(more traditional),
multiply that number by 154.66. Or if you figure out your Mass air flow rate in lbs/min,
divide your number by 154.66 to get the corresponding chart number. To convert to
CFM, multiply the chart number by 2118. Keep in mind you will be calculating air flow rate at
different operating points along your RPM range.
- The y-axis shows your Pressure Ratio. The Pressure ratio is simply the absolute
pressure coming out of the turbo, divided by atmospheric pressure. Boost gauges show
relative pressure, meaning that they show pressure above absolute pressure (atmospheric).
For example, you're tooling along at 120mph with your foot to floor, and your boost gauge
shows 14 psi (or .97 bar). The pressure ratio is:
[(your boost gauge reading)+(atmospheric)]/(atmospheric) = pressure ratio(PR)
Now you can find a pressure ratio for any boost level you want and you'll use this in
finding your operating points.
[14psi+14.7psi]/14.7psi = 1.95 PR
Now comes the part where you will plot your operating points on the compressor map.
Print out a map because it'll be a pain to do on screen and you'll lose track of where you
are. You'll want to pick at least 3 major events in your RPM range that you will plot:
- The point where the boost threshold is reached, the point where the boost
starts to rise (1 psi)
- The point where your turbo starts to make maximum boost.
- The max HP point. Max HP is where your engine is flowing the max amount of
air/time. Max torque is where the engine is flowing max amount of air/cycle.
Your operating points will be based on Air flow rate, and pressure ratio.
To calculate your air flow rate you need to use equations, or you can get a rough estimate
by using this free program:
So for each boost level you will need to figure out how much air is going through your
engine (remember that the volumetric efficiency is constantly changing). Your max VE is
usually somewhere close to 95+% and that's the point where max torque is made. Max
HP is made at a lower VE somewhere from 80%-90% depending on the setup. This is kind
of tricky as in reality turbocharged engines operate above 100% VE because you have air
at higher than atmospheric pressure in the combustion chamber. Here again
I claim not to be an expert on this subject, but I'm assuming the TurboCalc program is
asking for VE without taking the turbo into account. Anyway before I de-rail..., find the
air flow rates at the different points, using corresponding BOOST, RPM, and VE. As a
very rough guide if you don't want to use the program or do some number crunching,
divide your horsepower by 10.85 to get airflow rate in lbs/min. Of course you will have
to know your HP level at the different RPM, boost points which you can get from a dyno
Once you plot your points you will see where the compressor is operating. What looks
like a topographical map, is really the effiency islands for the compressor. The peak
island efficiency for a 14G for example is 77% This means that 23% of the work is
heating the air. As a general rule of thumb, you dont' want your compressor operating
below 70% efficiency. You also do not want to operate your turbo to the left of the surge
line, the surge line the left most line. This only happens when you have a big PR at low
airflow (big boost at low engine RPM). Surge is a condition similiar to stall over an
airplane wing, the flow becomes turbulent over the compressor vanes and the compressor
surges in and out of boost and all sorts of weird things happen. You'll definetly know if
you're operating in the surge area. Notice that too high a PR also makes the turbo spin
faster, those big number on the right hand side are compressor speed in RPM's.
If you ploted your airflow and PR over your whole RPM range, you will basically chart a
line on the map(or get the 3 critical op points and interpolate) of where your compressor
is operating. As long as it's staying in the higher efficiency areas in your upper
HP(airflow) range then you're ok.
Text and maps courtesy of Martin Musial at Automotosports