How to read Compressor Maps

[BOOSTD]

here is how to begin analyzing a turbo compressor map, this will help you learn what makes turbo cars move, and why it is that some cars can run on stock turbos during elevated boost, and others are barely running in stock form on the turbo they have.

1) pressure ratio. This number represents the boost you intend to run, it makes up the Y axis of the graph. The numeric pressure ratio value is calculated by taking your target psi (15 psi for example) then adding 1 Atmosphere(14.7psi) and dividing the total by 1 bar(14.7 psi). The equation looks like this for our example:

15 + 14.7 = 29.7

29.7 / 14.7 = 2.02

2.02 is the refrence number that you would look at on the Y axis.

2) The X axis of the graph represents airflow in pounds per minute. As a general rule, each pound of air generated represents 10hp. This is flywheel hp and not wheel hp, so don't confuse the two. When looking at this be sure to account for driveline losses when you estimate your power output.

3) The third area represents the efficiency island. This is the target area you want to keep your boost at. This is where the turbo is at it's peak efficiency and thus runs the best.

4) These are the outer rings of efficicncy or wheel efficiency. As you can see from the numbers, each ring drops in efficiency and thus drops in power output. The percentage of efficiency lost and power loss vary from turbo to turbo, but as a rule of thumb you want to remain as close to your efficiency island(center island) as possible.

5) This dashed line is the surge limit. Any points to the left of this line indicate that the compressor wheel in quesation is too big for the expected boost and power output you are planning for. There would not be enough exhaust gas volume to spin the wheel fast enough to make useable boost.

6) This area to the right of the graph plots overspin choke. This means that the compressor wheel is too small and will have to spin too quickly to make the target boost/power. At these extreme speeds, efficiency goes out the door because the wheel chops the air so badly, this will likely cause a dramatic loss of power.

7) This is the compressor wheel speed in rpm. This measures the shaft speed of the compressor wheel. The faster it is spinning the hotter the outlet charge will be, and thus the lower your power output becomes. This is where a good intercooler comes into play.

Turbo Matching:

From your newly (or reviewed) skill on reading a turbo compressor map, you'll now need to be able to decode it into how it fits your engine, this will require numerous equations, all of which can be done simply without the use of advanced mathematics and will bring a great deal of illumination to the problem of perfect boost vs. monumental lag. This part of the process requires a sacrifice, but it's not so bad if you have a stock engine, or at least stock compression. A person with the 2.3T or 2.4T (and now 2.5T) engine is going to be @ a relatively high compression ratio, the hpt engines run a 8.5:1 CR and the lpt run 9.0:1 CR which means at elevated boost, especially over 20 psi, you're going to need either to reduce your compression ratio, or install a water injection kit, and even then it is not advised to go much over 20 psi at all on stock compression in the hpt engine, the lpt engine with it's 9.0:1 compression and less beefy cylander walls shouldn't exceed 15 psi without water injection and should be advised to be cautious above this level.

The main point of using a compressor flow map is to determine if the compressor part of a turbocharger is sized properly for your engine. In order to do this, you need to know how much air the engine flows. The volume air flow (VAF), measured here in cubic feet per minute (cfm), is a linear function of engine displacement (here in CI), compression cycles per minute (RPM/2), and volumetric efficiency (VE). After doing some searching online, it seems that our engines operate at approximately 19.4 lb/min, knowing that every 10 lb/min is equal to 144.7178 cfm, our CFM equals roughly 266.28 ft^3/m. THIS IS AT 10 psi, if you increase your boost levels, you can expect to see:

Stock:

18.4 lb/m @ 10 psi: 266.28 cfm

Modified Boost:

23.0 lb/m @ 16 psi: 332.85 cfm

24.2 lb/m @ 18 psi: 350.22 cfm

27.1 lb/m @ 20 psi: 392.20 cfm

If seen on the 15G map provided above, you'll notice that we're on that very narrow island of efficiency at 10 psi, and that once you're at 15 psi (around 319 cfm) you're really starting to heat things up over stock. However, you'll also notice how TALL the efficiency range goes, and how (compared to others shown below) much more efficient the midrange of this turbocharger is, especially compared to the obviously lightweight and heavyweight turbos we'll see soon.