Deciphering All Those Curving Lines

Words And Photos: Jeff Smith

Any pirate movie worth its pieces of eight offers the usual plot involving a ridiculously valuable buried treasure cleverly hidden by the ruthless Captain Black. Our handsome, windblown hero must best the evil Captain by deciphering the location of the treasure from the map’s hidden references. If you’re into high performance engines and power adders that involve boost, you may have heard of something called a compressor map. To the majority of engine freaks, those odd lines might just as well be some ancient pirate’s treasure map – and that’s exactly what it is. This modern day diagram points to the buried treasure of horsepower. We’re going to show you how to read Captain Black’s chart and literally find where X marks the spot!

[Ed. Note: This story is an introduction to compressor maps. In an attempt to be brief, we are omitting reams of information about how to use a compressor map to help select a turbo or supercharger. This is an important point – we are well aware of the huge volume of information we’re omitting. But to include even a portion of these details would make this story near book length. More in-depth topics can be found on several turbocharger websites should you want to dive deeper into this subject – which we highly encourage you to do.]

A compressor map is a sophisticated graph that tells you just how efficient a supercharger or turbocharger is at creating pressure. The whole reason for these devices is to move extra air into the engine than it would normally consume by employing atmospheric pressure. It’s this simple – pushing more air into the engine will make more power. While most maps refer to turbochargers, similar maps are available for centrifugal blowers, positive displacement superchargers like the venerable 6-71 GMC blowers, as well as the latest generation Eaton and Lysholm screw superchargers.

Increased air density is what we are seeking. You can think of air density in terms of weight since air has mass. Think of this as a column of air extending from sea level all the way to the edge of space. This column has enough mass to create pressure – 14.7 psi at the bottom of the column at sea level. Normally aspirated engines use this atmospheric pressure to push air into the cylinders. In order to make more power, we need a way to move more air than what the engine can take in without help.

The key to a good turbo or blower is not just how much air it moves, but how efficiently it compresses the air. We measure this efficiency in terms of adding the least amount of heat. We already know that as a pump creates pressure, the compressed gas (air in this case) will naturally increase in temperature. Just touch the housing of a shop air compressor to understand this concept.

In a perfect world, air temperature will increase a minimum amount with a given increase in pressure. This is called 100 percent adiabatic efficiency. The physics geeks have defined the term adiabatic efficiency as the expression of how efficiently a compressor operates at increasing pressure. Since our world is far from perfect, a turbo – or supercharger adds heat to the air.

Since 100 percent efficiency isn’t possible, good compressors come in roughly around the high 70 percent range. The heated and pressurized discharge air affects what is referred to as charge density. A great example of charge density would be high pressure (10 psig of manifold pressure for example) with a very low temperature like 70 degrees F.

Air density is defined as the weight per unit volume (cubic feet) of an air mass. This weight is directly affected by both the temperature and pressure of the air, with the obvious goal being denser air per volume. Pressure increases the density because the molecules are pushed closer together, but doing so increases temperature. As the temperature rises, the air becomes less dense, so we have two factors working against each other and no free lunch.

Suffice to say, there is a point of diminishing return for increased pressure since additional heat added to the air by the compressor eventually negates further gains in pressure. The evidence of this on the compressor map is expressed in percentages of the efficiency islands that we will get to shortly.

With the basic physics of air density under our belt, let’s get into actually defining the map parameters. All compressor maps use a horizontal scale (x-axis) and a vertical (y-axis) scale to present the information. The x-axis displays mass flow of air in pounds per minute (lbs/min). We could get into the math of how that’s calculated, but the important point here is mass flow – not volumetric (cfm). The numbers along this horizontal scale represent the actual mass of air exiting the compressor.

Here’s a trick the engineers showed us that will help you with these numbers. Generally speaking, engines produce approximately 100 horsepower for every 10 pounds of mass flow pushed through the engine. Let’s say we’re looking at a compressor map with an x-axis mass flow range of 10 to 70 lbs/min. If we choose 60 lbs/min, moving this mass of air, our engine can potentially make 600 horsepower or more. This is a big step toward making the map easier to read.

Next is the vertical, or y-axis scale. These numbers represent the pressure ratio, which is the difference of the ambient inlet pressure versus the compressor’s discharge pressure. As you can probably guess, not all superchargers operate at sea level – only heroic car magazine engines do that!

You might think, “Simple, all I have to do is multiply sea level pressure times the ratio to get boost, right?” The short answer is – you’re close. We won’t get into the complex reasons why, except to say that within this story we will be exclusively dealing with standard temperature and pressure (STP) – which is 60 degrees F with a pressure of 14.7 psia (we’ll define what psia means in a moment).

Happily, the math to convert pressure ratio to boost pressure is simple. But first we have to talk about absolute pressure versus gauge pressure. If we park our supercharged Chevelle on the beach with the engine shut off, our boost gauge reads zero (0). That is gauge pressure or psig. But we also need to account for the atmospheric pressure that pushes air into the supercharger. This atmospheric pressure is created by the mass of the air pushing down from that long column of air we spoke of earlier. At sea level, this atmospheric pressure is 14.7 psi absolute or psia. So the pressure ratio is the difference between absolute pressure (psia) and the intake manifold pressure (psig) created by the supercharger. If our supercharger creates 10 psig at sea level, the equation looks like this:

Pressure Ratio = (10 psig + 14.7 psia) / 14.7 psia

Pressure Ratio = 24.7 / 14.7 = 1.68:1

If we need to know the boost pressure in psig from a given pressure ratio on the compressor map – the formula looks like this:

PSI = (Pressure Ratio x 14.7) – 14.7

PSI = (1.68 x 14.7) – 14.7

PSI = 24.696 – 14.7 = 9.996 rounded to 10 psi

The reason that pressure ratio is used instead of just gauge pressure is because often the inlet pressure at the compressor could be either more or less than atmospheric. As an example, with a killer 200 mph street/drag ’69 Camaro with a forward-facing turbo inlet in the grille, the at-speed pressure feeding the turbo inlet will be higher than atmospheric. Or, filter ducting on a normal street car could reduce the pressure at the inlet. Positive inlet pressure will improve the turbo’s efficiency (increasing boost) while reduced pressure would obviously negatively affect the boost.

We now have a map with the basic inputs of the amount of air the supercharger can move plotted against the pressure ratio. We’ve already stated that as we compress air, it gains heat, so one key to comparing compressors is to determine how well they create boost.

There will always be a specific area for any compressor where it is most efficient at creating pressure. These areas are called efficiency islands and are represented by oblong circles in the compressor map. These circles look very much like a topological map that indicates height for hills or mountains. For example, in the Garrett compressor map (Caption 2), you’ll notice a red X at the intersection of 80 lbs/min. of corrected mass airflow with a pressure ratio of 2.85. The X sits just on the inside envelope of the 74 percent island for this turbocharger.

The islands created on the compressor map are defined by the compressor’s ability to move air under pressure while adding the least amount of heat to the discharge air. Expressed as a percentage, they show how efficiently a given mass of air (in lbs/min.) is compressed at a given pressure ratio. As an example, a 70 percent island means that 70 percent of the energy put into driving the compressor increased the air pressure, while the remaining 30 percent was converted into heat that raises the discharge temperature. It might seem that operating the engine in the highest efficiency island would be the best plan for the most power, but most enthusiasts find themselves pushing their compressors toward the upper right corner of the map.

This means that operating at sea level with no inlet restrictions, a 2.85 pressure ratio would be around 27 psig of boost moving 80 lbs/min, or roughly 800 horsepower, worth of air. The discharge temperature exiting the turbocharger would be the least we could expect from this turbo given how hard it is squeezing the air. If we’re going to run this engine at the drag strip, will this turbocharger still work at a lower launch speed? To determine that, we need more information than the compressor map reveals.

Another important series of lines on the map are called speed lines. On the aforementioned map, these are a series of elliptical lines running laterally across the face of the map. These are turbocharger rpm lines that indicate the turbo’s shaft rpm. Our X is located between 90,000 and 100,000 rpm speed lines. These are not excessive speeds for a turbo, but every turbo does have a speed limit.

Note there are also blank areas located on the map. These are blank by design. The upper left hand side is what is called the surge area bordered on the right by the linear diagonal line that defines the left side of the compressor efficiency curves. The surge area is defined as the low-side boundary of the turbo or supercharger’s flow stability.

The most common situation that might put a centrifugal supercharger or turbo into this area would be at the conclusion of a high-rpm, high-boost pass when the throttle is closed downstream of the compressor. Mass flow is radically reduced by the closed throttle, but the blower is still spinning at high speed. This buildup of pressure on the discharge side of the turbo can damage the compressor wheel. This is the reason for blow-off valves placed in between the compressor and the throttle blades to dump the excess pressure, which minimizes the chance of compressor surge.

Surge can also be created by attempting to run a larger compressor at very low speeds – way below and to the left of the surge line. This causes compressor instability and loss of power. The surge line is something to avoid – don’t cross it if you want to be kind to your turbo or centrifugal supercharger.

The second area of concern lives in the upper right hand corner of the map defined by the outer curve of the last efficiency island. This outer boundary is called the Choke Line. This is the area where compressor efficiency trails off. The cut-off point is not the same for all turbos or superchargers – compressor manufacturers use varying no-man’s-land percentages.

Operating the supercharger beyond this line results in high discharge temperatures because the compressor is operating well outside its efficiency area. This means the discharge temperature will spike rapidly, which kills air density. Oftentimes, rules requirements limit a compressor’s size, forcing racers to push their turbocharger or supercharger into this area. This also means that the discharge temperature of the air will be very high – a perfect combination for creating detonation. Sometimes the gains in the midrange offset these small top end losses.

That’s about as far as our attention span will allow us to go, and likely your eyes are about to roll back into your head. If you’ve come with us this far, take a break – you deserve it! Don’t be afraid to run through this again after a couple of WOT blasts in your hot rod. Likely all the factors will begin to make more sense once you take some time to study these maps. The treasure is certainly buried there – all you have to do is expend a little effort to dig it up!

Sources

Blower Drive Service (BDS)
blowerdriveservice.com

BorgWarner
turbodriven.com

Magnuson Products
magnacharger.com

Precision Turbo & Engine
precisionturbo.net

ProCharger
procharger.com

The Blower Shop
theblowershop.com

Turbonetics
turboneticsinc.com

Vortech Engineering
vortechsuperchargers.com