This guide isn't meant to be an in-depth look at turbo sizing, but rather to give the reader a working knowledge of how to read maps. In other words, to decipher what all those swirling lines mean when choosing a turbo compressor.
Terminology
Compressor - This is the "cold" side of the turbo that sucks in intake air and compresses it for the engine to later combust with fuel.
Turbine - This is the "hot" side of the turbo. Hot exhaust gasses pass through it, expanding and cooling. This expansion spins a turbine that the compressor wheel via a shaft. Unfortunately turbo manufacturers don't make turbine maps available to the general public.
Absolute Pressure - This is pressure referenced from a pure vacuum. Most calculations done involving compressors use absolute pressure. Note - 1 atmosphere = ~14.7 psia (Absolute pressure in pounds per square inch) = 0 psig (gage pressure in pounds per square inch). Your boost gauge reads in psig, referenced to local atmospheric pressure.
ONTO THE MAPS
Surge - This is lowest amount of airflow a compressor can supply at a given pressure ratio(getting to that). Any pressure above this at this airflow, the compressor will "gulp" air. This is not good for your turbo, or your power output. Fortunately you have to saddle a pretty huge compressor with a small turbine to really worry about this effect.
Here is a compressor map with the surge line highlighted in red.
http://www.60-130.com/images/importe...07/surge-1.jpg
On the X-axis(horizontal) you'll notice the mass airflow of the compressor in lbs/min. On the Y-axis there is the
Pressure Ratio. Pressure ratio is defined as follows:
Atmospheric Pressure + Boost Pressure = Pressure Ratio
Atmospheric Pressure
So the astute reader will notice a pressure ratio of 1.0 is the exact same as atmospheric. A pressure ratio of 2.0 is equivalent to 1 atmosphere or ~14.7 psig in your intake manifold. Without concrete data proving otherwise, it is always the best course of action to assume the pressure is ambient at the compressor inlet and make note of the pressure drops of the system will in the end cause less to be produced than the mass flowrate of the turbo would suggest.
The oval shaped rings on the compressor map are
efficiency islands. These are regions where the compressor has approximately the same efficiency at compressing the air. Of course, the higher the efficiency the better since the compressor will be introducing less unneeded heat into the charge air. Note that as you go away from the maximum efficiency island, you always go down in efficiency. By the time you're off the map you're usually in the <60% range, which is not a good thing.
The lines that slope from the surge line to the right and down across the efficiency islands are constant speed lines. This would be really useful if you could match up the speed of the compressor to the speed of the turbine and find out its efficiency and mass flowrate for that shaft speed, but since we don't have turbine maps we're kind of at a disadvantage there for picking the ultimate turbo match. The maps used here out of Garrett's publicly available catalog aren't too detailed. Some maps will have much more data like putting RPM values on the speed lines, more efficiency islands etc.
I won't go into the hard equation to calculate the mass airflow of the engine, as it really doesn't gain anybody any further insight into the turbo selection process. The only important things to understand that the big factors in how much mass airflow an engine is consuming are:
- Engine Displacement
- Volumetric Efficiency(how well the engine breathes)
- Pressure at the inlet valves(BOOST!)
- RPM
By altering these things(more displacement, cams to increase VE, more boost, more RPMs) you can make the engine combust more air and make more power. I'll be attaching a spreadsheet that makes calculating the airflow of an engine an easy matter. It does over simplify things since it doesn't vary VE by RPM and whatnot, but it is a reasonably close approximation. I use a VE of 90% in most my calculations. It is pretty close to what a modern 4 valve engine gets in high RPMs, and tends to be conservative on less modified engines.
So go ahead and download the spreadsheet and we can look at a compressor map.
Here I'll look at a GT30R turbo on an S52B32 engine with a VE of 90%, displacement of 3.2L and maximum RPM of 7000. For the first go, we'll see what happens at a modest boost level of ~8.7 psi(pressure ratio of 1.6).
http://www.60-130.com/images/importe...T30RPR16-1.jpg
How I evaluate compressor maps is to note the airflow at 2000 RPM. Find that on the X-axis and draw a straight line from that point at a PR of 1 to the airflow at 3000 RPM at your desired PR(1.6 in this case). This gives you an idea of how a typical turbo will look when spooling up, and let you know if it's at a risk of surging. From there, the line should stay at that PR all the way to the airflow at redline ~39 lb/min here.
As you can see, surge is not a problem here, but this turbo sure does look a bit too small for this sized engine! It goes off the map just before redline, so that means it is very inefficient at higher revs.
Let's see what happens when we up the boost to ~17.4 psi(PR of 2.2).
http://www.60-130.com/images/importe...T30RPR22-1.jpg
No risk of surge due to this being a large engine for the turbo, but boy does it ever get REALLY inefficient at higher revs. Past 6000 RPM it is again off the map.
Let's go to a slightly larger turbo, a GT35R, to see the difference. Same boost of 17.4 psi(PR of 2.2).
http://www.60-130.com/images/importe...T35RPR22-1.jpg
Now that is more like it! Notice how the engine spends a good amount of time in the really efficient islands, and the turbo is still at 72% efficiency all the way to redline. I'd think this turbo would be putting out in the 450-500rwhp range at this boost on this engine, and that's probably being a bit conservative. If the VE of the engine was even higher(which it can be), this turbo could still put out even more power. The compressor map also suggests it has a bit more headroom on this particular engine.
I hope that was helpful to everybody, and gives people a start in the right direction on reading compressor maps themselves. If you want to modify the Excel airflow chart I attached, just extend the RPM row and copy the formula in the airflow cells for CFM and lb/min over below the RPM and it should fill in correctly. You can change the displacement, VE and pressure ratio in the parameters section to get an idea of how these change things.
Enjoy boost junkies!
