Tech File: Understanding compressor maps

[GT40FIED]

One great source of frustration amongst turbo shoppers is the turbo's compressor map. To many it looks like some sort of strange mathematical graph from outer space. Quite the contrary. If you know a few basic things they can be reasonably easy to read and understand. Let's start with the compressor map from a T-66 turbo (I know a T-66 turbo is generally a bit large for most sport compacts, but the fundamentals are the same for any turbo map).



Now the first thing we have to do is understand the axis. The X axis shows us airflow through the turbo. Now, while some companies show airflow in cfm, many others (like Garrett) choose to show the flow in pounds per minute. It makes it a little harder to understand, but we can convert lb/min to cfm by understanding that for every 10lb/min in airflow, it relates to about 145cfm.

The Y axis represents the ratio of air pressure leaving the turbo to pressure entering the turbo. Since, at sea level, atmospheric air pressure (or 1 bar of pressure) is 14.7psi, if you were to run 29.4psi it would be represented on the Y axis as 2.

Now onto understanding the rest of the map.

The oblong ovals on the chart (or "islands", as they are sometimes refered to) give you an idea of the turbo's efficiency in that range. Without going to much into adiabatic efficiency since it has already been covered in a seperate article, we can see that this turbo's maximum adiabatic efficiency is 76%. That would make it among the better turbos in regard to efficiency.

The wheel speed is just that...the rpm of the compressor wheel. This is represented by the lines that intersect the islands...seemingly from right to left. By that we can see that the maximum compressor speed of this particular turbo is 102,400rpm. In this case, they've been nice enough to tell us up front the compressor's max speed, maximum efficiency, and maximum flow in the top left hand corner. This is generally not the case.

An important thing to mention is that most turbo maps do not list the turbo's "choke point". The choke point is the flow rating the turbo is capable of regardless of efficiency. I have not marked it here since it is seldom listed. This particular turbo will reach it's choke point at 1160cfm at right around 41psi. A bit extreme and probably unheard of for street applications...this is why manufacturers generally do not list them. If you run 41psi on a street motor for longer than a week, I'd like to shake your hand.

The last important factor to look at is the surge point that is plotted at the left. Beyond that point, compressor surge occurs. Simply put, exceeding that limit would cause backpressure in the turbo housing which almost always leads to compressor wheel damage or total turbo failure. This can usually be avoided by the use of a quality blow off valve to vent excessive pressure away from the turbine housing and into the atmosphere. Even low boost turbo application seekers should consider the use of a good blow off valve. It's fairly low cost insurance if you figure a new turbo costs anywhere between $500-$1300 for a street turbo just for the turbo itself.

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