How to read a compressor map
 

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!!:D

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.

TC- Do you really feel that the belt is slippng that much? Or just a guess? If belt was not slipping than the boost at the compressor would be greater? With resistance in system being greater. Correct?

Keep it comming.

CFM
 

tomcat..
i get it, with my twin, naturally aspirated, 350's...but how can we put it into layman's terms...so ALL enthusiasts can apply that knowledge...my buddy has a 24' Outlaw with a 454 Mag...can we help him?
C

Turbojack - I'm just being conservative and guessing that there is some belt slippage. If there is no slippage, then compressor RPM is higher and that higher RPM curve is going to cross the system resistance curve at an operating point with slightly higher discharge pressure and CFM. The resistance curve of the system does not change, you're just forcing a little more air through the system, so discharge pressure has to be higher.

CG - High flow flame arresters, bigger carbs, high rise intakes, ported cylinder heads, big valves, big cams, better exhaust...in other words, the whole high performance engine industry, is aimed at reducing the system resistance that the air moving device (piston and cylinder) have to work against. What the average enthusiast can learn from our discussion is how to decipher some of the claims made for some of these products.

For example, if I tell you that a certain flame arrester flows 33% more air than your stock flame arrester, what does this mean, and how will it help? First of all, I assume that this claim means that the flame arrester flows 33% more air at a given pressure. Sounds great, but it doesn't mean that the engine is going to get 33% more air because the flame arrester is only one of many components in the intake pathway that present resistance to air flow. It's the total resistance that counts, since that is what the piston and cylinder is working against.

So if the flame arrester is only a small part of the total resistance, even a large improvement in its ability to flow air will not make much of a reduction in the total system resistance. This is why it's common to hear of people changing a single component and seeing no change in performance.

If reducing system resistance is so good, how do you do it?

There are two ways to reduce system resistance so that the compressor will flow more air at the same compressor RPM. The first way is to change cylinder heads, cams and exhaust so that the engine breathes better. If you have ever looked closely at Procharger's specs you would see that the HP500 kits take less boost to achieve the same HP as the stock 502 kit. That's because the engine breathes better to begin with. But one of the main attractions of supercharger kits is not having to go into the engine. So what can you do with the supercharger kit components to reduce their contribution to total system resistance?

First of all we have to know how much of the total system resistance comes from the supercharger kit components as opposed to the engine. In the post above we calculated for the Vortech kit that the compressor was delivering 1100 CFM @ 10 psi, but intake manifold pressure was only 7 psi. So the ducting, elbows, carb box and carb had a combined resistance or pressure loss of 3 psi. There's not a lot you can do to the carb to reduce it's resistance, and you need the pressure drop through the venturis to pull the fuel, so that leaves the carb box, elbows and ducting.

I have written a little spreadsheet computer program that calculates the resistance or pressure losses in these components. The key things are the cross-sectional area of the component (and therefore the air velocity through that component), and the loss coefficient for that type of component. I made educated guesses for the losses and then revised them slightly based on Vortech, Spearco and my own flow bench data.

Here's how the losses at 1100 CFM stack up:

Duct - 0.073
Elbow - 0.418
Elbow - 0.418
Box - 0.783
Carb - 1.259

Total - 2.951 psi (~3 psi as noted above)

I happen to have spent a lot of time developing a new compressor to carb arrangement for my improved intercooler setup. Here's how the losses at 1100 CFM look for the improved setup (no intercooler):

Duct - 0.049
Plenum - 0.865
Box - 0.019
Carb - 1.259

Total - 2.192 psi

The improved arrangement has 0.759 psi less resistance. We haven't changed the engine so 7 psi in the intake manifold will still push 1100 CFM of air through the engine. But the compressor doesn't have to work as hard to get that air to the intake manifold.

The compressor discharge pressure only has to be

(7 + 2.192) = 9.192 psi,

not 10 psi, to move 1100 CFM through the system.

No change was made in the carb; all this improvement occurs between the compressor and the carb. The key was to get away from the small cross-sectional area of the duct and into the large cross-sectional area of the plenum as soon as possible.

Now we can draw the new system resistance curve on the compressor map. We find the 1100 CFM line and mark it's intersection with the new pressure ratio.

New pressure ratio =

(9.192 psi + 14.7 psi)/14.7 psi = 1.625

The new blue curve drawn through this point is lower than the red curve based on the Vortech magazine test. Since we have not changed the pulley on the compressor, we extend this new curve to the 40,000 RPM line to find the new operating point for the compressor and system.

The new operating point looks to be about 81 lb/min of air flow vs. 79 lb/min, maybe an extra 20 HP on top of 780 HP. Such a small gain isn't worth the effort, but the difference between the two systems changes drastically when you add an intercooler.

Next time, reducing system resistance on an intercooled setup.

Three Steps Forward, Two Steps Back

Something very interesting happens when you add the extra elbows, expansions and contractions to the standard ducting arrangement when you install an intercooler. All of these additional components, including the intercooler core itself, add resistance to air flow between the compressor and the carb. You don't think about this because the effect of cooling the charge air is so beneficial, but it can be calculated and drawn on the compressor map.

The computer spreadsheet, confirmed by flow bench testing says that to push 1100 CFM of air through the standard intercooled setup takes 4.662 psi. So the new compressor discharge pressure is:

7 psi + 4.662 psi = 11.662 psi

The new pressure ratio is:

(14.7 psi + 11.662 psi)/14.7 psi = 1.793

Locating this point on the map and drawing a curve through it we discover that the curve passes through the 40,000 RPM line at less than 1100 CFM. This is our new operating point; about 1040 CFM. The higher resistance has reduced air flow and would probably cost about 40 HP if you tested without running water through the intercooler. Of course running water through the intercooler gets this HP back and more, but to understand the difference between the standard intercooler setup and the improved setup, we compare their resistance to air flow alone.

20hp in a cat mabe = 1 mph.

Did you take in account the larger pulley thus less HP used to run blower? While I am thing of it, do you know how much HP it take to turn a centrifical blower verse say screw or ?? blower?

Now how does our reduced resistance setup fare when we add the intercooler? Because the plenum has already increased the cross-sectional area for air flow to the size of the intercooler, and the modified carb box retains that same cross-sectional area after the intercooler, we really only add the resistance of the core itself.

The computer spreadsheet says to push 1100 CFM worth of air through the low resistance setup takes only 2.319 psi. So the new compressor discharge pressure is:

7 psi + 2.319 psi = 9.319 psi

The new pressure ratio is:

(9.319 psi + 14.7 psi)/14.7 psi = 1.634

Locating this point on the compressor map and drawing a curve through it we find that we have to extend the curve to reach the 40,000 RPM line. Dropping down to the CFM scale we see that we have about 1120 CFM. Compared to the standard intercooler setup we have improved air flow by:

1120/1040 = 7.7%

which is good for about 60 HP.

Now adding water to both intercooler setups will add even more power, but the 60 HP advantage will remain as long as the intercoolers do the same job of heat removal in both cases. There is some cause to believe that the improved intercooler setup may actually work better since air flow is distributed more evenly across the face of the intercooler by the plenum as compared to the ducted intercooler, which tends to concentrate flow in the center of the intercooler. But that isn't something we can figure out on a compressor map.

Next time, what happens when you turn on the water?

Turbojack - All comparisons are being made at a compressor RPM of 40,000 so I am ignoring differences in HP to drive the blower. I don't know how much HP a centrifugal compressor needs to move this much air. I'm guessing around 50 HP.

When you look at Vortech's data and see how much HP was made with how many lb/min of air, and assume a BSFC of 0.5, there's always less HP than the air flow would suggest. The difference is the HP to drive the blower, but without BSFC numbers at each point, it's just a guess how much.

The best way to measure this would be on the test bench when they're making a compressor map by spinning the compressor with an electric motor. Measure the amps and you've got it.

I'm trying to be conservative on my estimates of improved air flow and HP. What I learned on the flow bench with a crude mockup suggested that it was worth pursuing. The next test with the real thing will be the crucial one. As you know I'm hoping for an overall improvement of 10% more HP, but we'll see.

Also considering the midrange boost levels and the slightly lower by about 50% boost/pressure ratio, it looks that the V7 YS blower is a little small and you need a larger blower and move the working area into the most efficient island. By moving into the more efficient areas of the blower you will also pick up HP.

Vortech's web site doesn't have the curve for the V-4 J or X trim. These might fit the application better. I might have the curves somewhere, I'll check.

I'm wondering that myself Marty. When the intercooler is added the system curve will shift even farther to the right.

I have some data from an old magazine test in which Vortech added an intercooler and picked up HP without increasing compressor RPM, even though the boost went down slightly and even though we know the resistance to air flow had to be higher. I worked this out in another thread a while back. The intercooler drops the pressure in the intake manifold, because as temperature is reduced, pressure X volume must go down (Boyle's Law). But since the engine, a positive displacement pump, controls the volume, pressure must go down.

This is seen by the compressor as a reduction in total system resistance, and at the lower compressor discharge pressure, the compressor flows more air. Since the intercooler adds power, the compressor must be moving more air.

This means that both the standard intercooler curve and the improved intercooler curve will shift over to the right. In that magazine article HP increased from 449 HP to 481 HP so let's say the compressor delivered about 7% more air. If we apply that number to the 1100 CFM of the standard Vortech system with the V-7 YS compressor, we would have a new operating point of about 1175 CFM @ 9.7 psi, (the green curve).

That's a nice gain over the non-intercooled setup, but the main reason for intercooling is of course to get the temperature down so you don't have detonation.

The big revelation is how the sins of higher resistance are covered up by the cooling effect of the intercooler. That's why we never think about it; the overall result is good.

So if turning on the water in the standard intercooler setup got us from 1040 CFM to 1175 CFM, we can assume that turning on the water in the lower resistance intercooler setup will add a similar % in air flow. But the starting point of the improved setup is much better; 1120 CFM.

(1175 CFM/1040 CFM) = ~13 %

1120 CFM X 1.13 = 1265 CFM

The new operating point will be about 1265 CFM @ 9.1 psi (compressor discharge pressure). For the first time we have strayed over the 72% efficiency line on the compressor map (yellow curve). We had also better hope that our intercooler does work better in the improved setup because we are asking it to cool 7.7% more air (1265 CFM/1175 CFM) than the standard setup.

We are starting to get into an area where a larger compressor might be helpful. We are also starting to approach ridiculous HP levels. This compressor spinning at 40,000 RPM made 780 HP @ 10 psi in the Car Craft test. By going to lower system resistance and intercooling we are predicting 890 HP. I can't see that living for long in the marine environment. I think that I would be more inclined to run the compressor at 35,000 RPM (5000 engine RPM)and make 765 HP @ 6.6 psi compressor discharge pressure. Intake manifold boost with the intercooler and lower resistance components might only be 4 psi but who cares, it's air flow that counts.

This operating point would be right on the 72% efficiency line. That's not bad, but a larger compressor running just inside the 74% island would be even better. The only caution is that a larger compressor may not have as much midrange. At 3500 engine RPM the compressor is spinning at 25,000 RPM, and if you suddenly opened the throttles the compressor discharge pressure would be just under 3 psi; intake manifold pressure only 2 psi. This should be enough since the compressor/system would be handling 50 lb/min of air, enough to instantly apply 500 HP.