We have all read that engines are air pumps, in particular, that the piston and cylinder is an air pump. At a given RPM and cylinder volume, the amount of air that this pump can move is affected primarily by the restriction of the intake and exhaust pathway. So we already know the first things about air flow; there is always an air moving device and there is always an air pathway.

In any discussion about air moving devices and air pathways two variables are most important - Pressure and Flow. It turns out that air moving devices and air pathways have different and distinctive curves when plotted on a graph of pressure vs. flow.

Air pathways present resistance to air flow due to friction and turbulence. This resistance creates pressure that the air moving device must overcome. This resistance has a very important characteristic: it increases in proportion to the square of the air's velocity through the pathway.

The curve looks like this:

The air pathway is usually referred to as the "system" and the resistance through that pathway, the system resistance or system pressure loss. The blue curve is for a lower resistance, better flowing system. You can think of this as very similar to what happens when you put on a better set of (ported) cylinder heads. There are two ways to benefit from the blue curve, Either you get more flow at the same pressure pushing air through the system, or you can get the same flow as the red curve at a lower pressure.

Now to the air moving device. I started off discussing the engine itself as an air moving device, since that is something we are all familiar with. But forget about the engine now and we will switch to the air moving device of interest, the centrifugal supercharger or compressor. The centrifugal compressor is a rotating blade air moving device. Rotating blade devices for moving air, (as opposed to positive displacement devices such as engines and Roots blowers) all have a typical pressure versus flow curve. They are often referred to as fan curves.

I have plotted two fan curves on the same chart as before. The green curve is the pressure vs. flow performance of the fan at low RPM. The yellow curve is the performance of the same fan spinning at a higher RPM.

Now here's where it gets interesting. When you connect the fan to the system the operating point of the two together can only be where the curves cross.

Say you were using the green fan curve (compressor) on the red system resistance(engine). Together these two curves have an operating point that delivers 500 CFM and a proportional amount of horsepower. You, of course, would like more power; you would like to alter things to deliver 600 CFM. So you change pulleys and increase compressor RPM to achieve the yellow curve. The yellow curve crosses the red curve at 600 CFM, but wait, there's another way, isn't there?

What if you could lower the resistance of the system (all components in the air pathway from the compressor to the cylinder)? You could achieve 600 CFM without increasing compressor speed and pressure (boost). The advantage would be reduced intake air temperature.

What if you did both? It looks like the yellow curve would cross the blue line at about 700 CFM. Yahoo!!

Now we're ready to look at a compressor map. A compressor map is a series of curves for the same compressor running at different RPMs. You will see Flow on the bottom, expressed in CFM or lb/min of air, and Pressure on the side, expressed as a ratio:

Pressure Ratio =

Compressor Discharge Pressure + Atmospheric Pressure
Atmospheric Pressure

You can see each fan curve labelled with its RPM. You don't see any system resistance curves since they are a characteristic of the intake pathway (ducting, intake manifold, cylinder heads) of the engine.

You also see efficiency lines and a definite island of highest efficiency. Whenever air is compressed, its temperature increases. This is unavoidable. However, the compressor adds to this heat because it is not 100% efficient. To build pressure the compressor accelerates the air a lot and beats it up pretty good. This increases air temperature even more.

When a compressor map is made, the compressor is run at different RPMs and forced to work against different pressures by opening and closing a valve on the test bench. The resulting air temperatures are used to calculate the actual efficiency the compressor achieved at different pressures and flows. Obviously it's a good idea to stay near the high efficiency area of the map.

Now let's plot some actual test data for an HP500 using this compressor. By plotting the compressor discharge pressure and air flow at different engine RPMs (and therefore different compressor RPMS), we get the system resistance curve (the red line) for everything downstream of the compressor, including the engine "air pump". Looks familiar doesn't it?

With this curve and the compressor map we can now predict what compressor RPM and discharge pressure we would need to achieve higher air flows. With the predicted pressure ratio we could calculate the air temperature leaving the compressor.

Once again, suppose it was possible to reduce the system resistance to that shown by the blue curve.

Next time...the difference between compressor discharge pressure and intake manifold boost.

How do you move the system curves from the 72% island to the 74% island??
And which way is north on that map??
Thanks Tomcat!!

TC-Great Reading. Keep it comming.

GO4BROKE - If your system had the blue resistance curve and you wanted a higher efficiency compressor, you would not modify your system to have higher resistance like the red curve just so you could pick up a couple % in compressor efficiency. You would lose more than you would gain. What you would do is select a different compressor that had its high efficiency island farther to the right on the CFM scale.

The Difference between Boost and Compressor Discharge Pressure

We often focus on the boost level of a supercharged engine. Boost is usually measured at the intake manifold under the carb. Boost is the amount of pressure needed to overcome the resistance to air flow presented by the intake manifold and cylinder heads. But this is not the same as the compressor discharge pressure.

The compressor must develop enough pressure to overcome the resistance to air flow presented by ducting, elbows, carb box and carb, as well as the resistance of the intake manifold and cylinder heads. Therefore compressor discharge pressure is always larger than boost. Those of you who have measured pressure in the carb box know that it is greater than intake manifold pressure, and if you measured the pressure at different points in the ducting from the carb box to the compressor, you would see it rising all the way.

To use a compressor map, you must know the compressor discharge pressure. If you are aiming for a particular boost level (intake manifold pressure) you must add a few psi to that amount to estimate the compressor discharge pressure and caculate the pressure ratio. Otherwise you will be looking at the wrong part of the compressor map.

For example, a recent Car Craft article described a dyno test of the Vortech carb kit on a 454 using the V-7 YS trim compressor. The article quoted the boost level at 7 psi. But the article also said that a 3.47 " diameter upper pulley was used and peak power was achieved at 6100 RPM. What was the true operating point of this system?

Compressor RPM =

Engine RPM X Bottom pulley (7") X Compressor gear ratio (3.45)
Top pulley (3.47")

= 42,454 RPM

Allow 5% belt slip for 40,331 RPM

The dyno test of this system showed 779 HP @ 6100 RPM. This requires about 79 lb/min of air flow or about 1100 CFM.

This operating point is shown on the attached compressor map. Look at the pressure ratio. It's about 1.68, so the compressor discharge pressure under these conditions was:

Compressor discharge pressure =

(1.68 X 14.7 psi) - 14.7 psi

= 10 psi (14.7 psi is atmospheric pressure)

Boost (intake manifold pressure) was 7 psi, so the ducting, elbows, carb box and carb have a combined resistance or pressure loss of 3 psi. An additional loss of this magnitude is unavoidable, unless the compressor blows directly into the intake manifold without any ducting or throttle body of any kind, (like some diesel engines).

So the operating point of the compressor in the Car Craft article was 1100 CFM @ 10 psi, ~ 40,000 RPM.