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A Primer On Turbocharging and Intercooling The Ford 2.3 Liter Engine
Turbocharging History
We owe much of the development of the turbocharger to the aircraft industry. The piston engine aircraft developers were dealing with a severe performance problem in the early 1920’s. As an aircraft went higher, the loss in air density caused engine performance to drop appreciably. Aircraft were limited to about 20,000 feet of altitude due to the engine performance loss. The answer to the performance problem was to force air into the engine to offset the loss in air density at altitude. Supercharging was born.
A supercharger is an air compressor driven by gears, belts or chains from the engine crankshaft. The development of the supercharger was not just a simple "throw it together". There is a monument on top of Pikes Peak dedicated to the men that spent a winter at 14,000 feet testing centrifugally supercharged aircraft engines. The work was done by GE under the direction of Dr. Sanford Moss. This was picked up in the mid 20's by the American racing fraternity for the 122 CID (cubic inch displacement) class by Fred and Augie Duesenberg and brought to full development by Harry Miller in his 91 CID class cars.
The superchargers were successful in restoring engine performance at altitude and, by the 1930’s, superchargers were beginning to appear on automobiles. Bentley, Morris Garage (MG) , Bugatti, Mercedes, Auburn, Cord and dozens of others made successful supercharged touring vehicles and race vehicles. Positive displacement superchargers were the rule in Europe while centrifugal superchargers were typical in American practice.
The aircraft industry continued to push the development of superchargers up into the mid 1930’s. The 10 year period prior to World War II saw engine power double and then triple. The Pratt and Whitney Wasp and Super Wasp engines are examples of this amazing power increase. At about this time, someone recognized that the energy in the hot exhaust would drive a turbine. The turbine could be used to drive a compressor mounted on the same shaft. This was the beginning of the "Turbo Supercharger". Sometime in the 1950’s, the "super" was dropped out of the center and the name became simply "turbocharger". Turbo superchargers were used on the B-17’s in service as early as 1935 with turbine wheels nearly a foot across, exhausting directly into the airflow under the wing.
Intercooler Development
As boost levels began to climb, engine failures became more frequent. The problem was detonation. As air is compressed, it heats up. Anyone with a home air compressor can easily prove this to themselves by carefully touching the piping where the air is discharged into the storage tank.
This temperature increase ratio is related to the ratio of boost (absolute discharge pressure divided by absolute inlet pressure) to the .285 power.
The increased air temperature into the engine made detonation much more likely to happen.
The solution was to add a heat exchanger in between the turbocharger discharge and the intake manifold. Again, the technology was pioneered by the aircraft industry with Lockheed and others routinely building intercooled aircraft by the late 1930’s. All high performance aircraft of World War Two were both turbo supercharged and Intercooled.
Most Intercoolers were of the familiar "core" type mounted just below the engine. Other innovative solutions were tried. The P-38 Lightning was an excellent example of this; the leading edge of the wing was used as an Intercooler. If, as happened on early models, the engine backfired it tended to remove the turbocharger from the engine and bubbled the leading edge resulting in simultaneous losses of power and lift! The problem was so severe, and coupled to the fact that the leading edge Intercoolers were easily damaged in combat, that by mid 1941 Lockheed had reverted to the familiar "core" type Intercooler.
The Death of Automobile Turbocharging
If Turbocharging is so great, why wasn’t it pursued in automobiles until recently? The development of turbocharging for aircraft was, in most cases, easier than the implementation on automobile engines. Aircraft engines typically run a steady state throttle process while automobile engines are operated with rapidly changing throttle conditions. The problem was being able to cost effectively control mixture.
When used with a carburetor, the turbocharger can either be placed in front of the carburetor, "Blow Through", or behind the carburetor, "Pull Through". Both methods have advantages and disadvantages.
The "Blow Through" arrangement meant that the carburetor was under pressure. The carburetor had to be specially built to avoid float collapse as well as to keep from blowing the fuel out of the float bowl. Under changing boost (throttle) conditions, the carburetor would go from essentially zero pressure to full pressure within seconds. The advantage of this method was that the Intercooler was positioned in front of the carburetor so that only "dry" air was supplied to the Intercooler. This was the typical method used for aircraft. The disadvantages to this method were the high cost and maintenance of the carburetor and the exceedingly high temperature of the air fed into the carburetor. The high air temperature meant that boost levels were often restricted simply because the parts in the carburetor would not tolerate the heat.
The "Pull Through" arrangement used a standard carburetor. However, the addition of fuel prior to the turbocharger severely impacted turbocharger life because the air was laced with droplets of fuel. The high temperatures after the turbocharger typically vaporized all of the fuel. The placement of an Intercooler after the turbocharger reduced Intercooler performance because of the fuel air mixture. In addition, the long piping runs with an Intercooler significantly affected the ability of the carburetor to control fuel mixture.
Several Turbocharged cars made it into production in the 60’s and 70’s. The late 60’s Corvair is an example as is the Mustang in 1979. None of these versions can be called successes.
The Rebirth Of Automobile Engine Turbocharging
The energy crisis of the late 1970’s fostered a rebirth of the turbocharger in the automobile industry. The turbocharger promised increased performance on small engines without a corresponding fuel economy penalty. The marriage of turbocharging with electronic fuel injection is the reason that turbocharging is now a viable automotive technology. Quite simply, the quest for increased fuel economy and emissions control forced automakers to develop electronic fuel injection and fuel injection eliminated virtually all of the problem areas normally associated with turbocharging.
With fuel injection, the turbo could be supplied with "dry" air. In addition, the dry air allowed the effective use of intercoolers. All of the elements were now in place to cost effectively build turbocharged vehicles.
The Turbocharger
A turbocharger is composed of two fan wheels on a common shaft. Just like in a child’s pinwheel, exhaust air blows across one wheel, the turbine, and causes the shaft to spin. Just like a common blower fan, the other fan wheel, the compressor, connected to the shaft spins, pulls in air, and pushes air out.
Because the displacement of the engine is a physical dimension, the discharge flow volume (cfm) of the turbocharger (except when Intercooled) is equal to the displaced volume of the engine at speed as corrected by volumetric efficiency of the engine. The inlet cfm through the air cleaner is something more than the engine displacement. This has led some people to claim that turbochargers increase the cfm of the engine. In actuality, the cfm of the engine hasn’t changed, but the DENSITY of the air has.
This is an important principle. Turbocharging increases the DENSITY of the air going to the engine. It doesn’t change engine displacement. Since turbocharging changes density, the cfm flowrate from the inlet to the compressor to the cylinder head of the engine is no longer constant. Attempting to talk about the cfm flow of a turbocharged engine provokes all kinds of inconsistent concepts. The one thing that is consistent about turbocharging is the MASS FLOWRATE of the air as measured in pounds per minute. This is why turbocharger maps show lb/min versus pressure ratio rather than cfm versus psi boost. Trying to work in either cfm or psi boost on a compressor map generates a map that is only correct at one place and at one time. A mass flow – pressure ratio map can be applied anywhere on earth at any time.
One of the real problems with turbocharging is that the pressurization of the air generates a significant amount of heat. The heat causes the density of the air leaving the turbo to be lower than it would be if you could just pressurize the air at constant temperature. The key to producing consistent high power levels with a turbocharger is to remove that heat from the engine air stream. We do that with an INTERCOOLER.
The Intercooler
An intercooler is simply a heat exchanger. It transfers heat from the engine air stream to someplace else. That someplace else can be the air flowing around the car or it could be to a water loop in the car. Eventually even the water loop has to reject the heat to the air around the car.
Intercooling is a DENSITY RECOVERY technique. By removing heat from the engine air stream, the engine air becomes more dense. The advantages are twofold. First, the power output of the engine increases because the same boost level now has a larger lb/minute mass flow rate. Second, the boost level can safely be raised since detonation is less likely to occur with cooler engine air.
How Intercooling Works
The Intercooler can be thought of as a tube within a tube. Let’s say the inner tube has engine air flowing through it and the outer tube has ambient air flowing through it. As the engine air "scrubs" along the dividing wall, it transfers heat to the wall and then to the other air stream. The amount of heat transferred is dependent on several things.
The first item that influences heat transfer is how different the temperatures are between the two air streams. The larger the temperature difference, the more easily heat is transferred. For an analog idea, the temperature difference is equivalent to voltage in a simple DC resistor circuit. The higher the voltage, the larger the current flow. The same rationale holds for the Intercooler, the higher the temperature difference, the more heat transferred. As the temperature of the two air streams approach one another (voltage goes to zero), heat transfer stops.
The next item that controls heat transfer is the "wetted" area of the heat exchanger. In the analogy above, the wetted area is the perimeter of the pipe multiplied by the length of the pipe. The more area, the more heat transferred.
The third thing that controls heat transfer is a difficult concept called the heat transfer coefficient. This is related to the turbulence of the air or water across the metal tube wall as well as to the physical properties of air or water. The simplest way to look at this is that the heat transfer coefficient for a water to air unit is double that of an air to air unit.
Heat transfer rate is equivalent to current flow in the electrical analogy. The product of the area and heat transfer coefficient is equivalent to the resistance in the electrical circuit.
The Turbocharged Engine Air Flow Process
Imagine that a molecule of air is about to be pulled into the air cleaner of a turbocharged Ford 2.3 liter engine that is spinning at about 5000 rpm and making 175 Horse Power (stock XR4 5 speed). It’s a nice day outside of the engine compartment – about 80 degrees and the ocean is just a mile or so down the road. These conditions are what is called "Standard Temperature and Pressure (STP)". That’s 80 degrees F and 14.7 psi absolute.
As the molecule of air enters into the compressor inlet bell, the conditions are still essentially STP. The air flow at this point is about 250 cfm. The molecule of air is caught by the compressor blades that whip around at about 125,000 rpm and pushed out of the discharge of the compressor housing. The air molecule is now under 15 psi of pressure (boost ratio of 2.02) and its temperature is now almost 250 degrees F. The cfm flowrate at this point is about 190 cfm but the mass flow of the air in lb/minutes is unchanged.
The air is blown out of the compressor and into the Intercooler. At the entering point, the condition of the air is for all practical purposes identical to what it was when the air left the Turbocharger. Inside the Intercooler, the cooler metal of the unit removes heat from the air. As the air cools, its pressure is essentially unchanged, but its density again increases. As the air exits the Intercooler, the cfm flow rate has dropped to about 148 cfm , the displaced volume of the engine cylinders at speed.
The turbocharger/intercooler system has widely changing cfm values through the system. That’s why talking about cfm in this case merely confuses what is happening. Mass flow has remained constant, but density and therefore cfm has changed because the temperature and pressure of the air is different in the various parts of the system.
Air to Air Versus Air To Water Intercooler Performance Issues
Intercoolers can not remove the heat from the air stream unless they have someplace to dump the heat. Air to Air intercoolers transfer the heat to the ambient air directly while air to liquid Intercoolers transfer the heat first to liquid and then to the ambient atmosphere in a second heat exchanger.
At first glance, an air to air unit appears to be simpler and more capable than the air to liquid unit. In practice however, the liquid to air unit has its own set of unique advantages.
The air to air unit must have air flowing through the unit. Getting airflow to the intercooler may require fabrication of ducts, hood scoops, or elimination of air conditioning.
The liquid to air unit will require a second loop with another heat exchanger to finally transfer the heat to the ambient air. The liquid to air system often has problems with providing enough heat transfer capacity due to the necessity of the secondary heat reject loop.
Given an equal sized heat exchanger core and equal heat reject temperatures, the liquid to air unit will transfer almost double the heat of the air to air unit. In addition, the bulk of water stored in the expansion tank provides a "cushion" to the system. Since turbocharging is a surge demand, the heat rejection equipment can be smaller if instantaneous demand can be meet with a stored reserve. Water also has more heat carrying capacity than air. The specific heat of water is 1.0 BTU/lb while it is only 0.24 BTU/lb for air.
Flow and Pressure Losses Of Intercoolers
The concept of an Intercooler performing better because it "Flows" more is inherently flawed. From the above discussion on flow in the system, you can see that the cfm flow rate changes throughout the system and even changes between the inlet and the outlet of the Intercooler. In addition, the flow through the Intercooler, even on a 500 hp 2.3 engine, will never approach the 360 cfm measurement point that is frequently talked about.
After having instrumented several intercoolers to measure pressure drop under various flow conditions, I can say that the pressure drop on ALL currently available production car intercoolers is insignificant on a 2.3 liter Turbo Ford engine.
The SVO Versus Turbocoupe Intercooler Debate
The best example to illustrate this is the SVO versus TurboCoupe Intercooler debate. For several years, owners of SVO’s have replaced their Intercoolers with TurboCoupe Intercoolers. Many have documented performance gains from doing so. The most common explanation is that the SVO Intercooler flows 280 cfm while the Turbocoupe Intercooler flows 360 cfm. The logic is that the additional flow capacity of the TurboCoupe Intercooler made the performance difference.
Reality is quite different. The performance gain is real but "flow" had nothing to do with it. When measured on a flow bench, the pressure drop of the SVO intercooler when having 280 cfm of STP air pushed through it is only about 0.04 psi. The pressure drop of the TurboCoupe Intercooler under the same conditions is about 0.03 psi. Pressure drops this low are insignificant because their percentage of total system pressure is only about 0.1%. The difference between the two units is only 0.03% of the available driving pressure at 15 psi boost.
Well, if pressure drop is not the difference between the units, then what is?
The difference is the heat exchanger area of the intercoolers. The Turbocoupe Intercooler has about 25% more surface area than the SVO Intercooler. While the Turbocoupe Intercooler won’t cool the air 25% more than the SVO Intercooler due to the fact that as more heat is transferred, the driving temperatures get smaller, it will have a significant impact on Intercooler discharge temperature. The cooler the discharge temperature, the more dense the air and the more power produced.
How Much Intercooling Is Enough?
On the grand scale, you can never get an intercooler big enough. Practically speaking, the 80/20 rule holds. You can get 80% of the performance for 20% of the effort. Under high boost (20 ~ 22 psi) compressor discharge temperature will be close to 400 degrees F. Getting an Intercooler that will pull this temperature down to about 150 degrees is at the edge of the 80% result window. A unit to do this is the same approximate size as the Volvo or Saab 9000 Intercooler.
Intercooler Placement
There are two places on cars that are documented high static pressure/ high flow areas. The obvious area is just in front of the radiator. A less obvious area is at the base of the windshield. A third position is in between the radiator and front crossmember positioned so as to allow vertical flow.
A front mounted intercooler has many advantages particularly when combined with a Cosworth style front grill opening. Obviously air flow is plentiful and easily obtained. The major difficulty is that the mounting of the Intercooler often requires relocation of the radiator and may sometimes require that air conditioning be eliminated.
A cowl mounted Intercooler at first seems illogical. However, the next time you see a NASCAR race car on display, look carefully at how air is directed to the air cleaner. In all probability, the air cleaner assembly will have a duct that connects to a cavity at the base of the windshield. For a Merkur, the battery can be relocated to the trunk and the false bulkhead that separates the cowl area from the engine compartment can be removed. The Intercooler can then be mounted in the area where the battery was located. There is excellent air flow, equal to a front mount, through the Intercooler when it is mounted this way. The disadvantage of this is that the ability to draw outside air into the interior through the windshield cowl vents is lost.
Mounting of a transmission cooler horizontally just behind the radiator is a common practice. An Intercooler can also be mounted here. The flow path for this configuration is not the best. Air is normally pulled through the radiator and discharged under the car. There is a strong possibility that the air flow path will be downward pulling hot air from the radiator across the Intercooler reducing its effectiveness.
Verifying Intercooler Performance
Performance of the Intercooler (discharge temperature) needs to be verified. In my opinion installation of a thermocouple or thermister after the Intercooler is MANDATORY. While thermisters can be purchased at the local Radio Shack, a mounting will have to be fabricated.
There are several inexpensive options. Ford used two different thermisters in the EFI systems.
Cars equipped with a Vane Air Meter will have a thermister built into the meter assembly. The thermister can be easily removed and mounted with just a drilled hole in the throttle body. Attached is a chart showing resistance versus temperature for this thermister.
Many Ford cars were equipped with an Air Charge Temperature Sensor. The ACT is a thermister built into a holder with an external thread. The ACT can be easily mounted with a taped hole in the intake manifold. Attached is a chart showing resistance versus temperature for the ACT.
Intercooler Ranking
IC performance has so many factors in it that It is difficult to do ranking. We have to define what engine size, rpm, VE, ambient air temp, location of IC in car, cooling air flow rate over the IC, boost level, compressor efficiency, etc. I have developed computer programs and spreadsheets that WILL calculate the performance of a given IC on a given engine combo.
Given the following,
Engine size 2.3 Ambient Air Temp 80 Deg F
Ambient air pressure 14.7 psi Engine VE 73%
Compressor eff 71% Engine Rpm 5000 rpm
Boost pressure 20 psi gage IC coolant flow 50lb/min
Calculated compressor discharge temp 293 degrees
SVO Isuzu
Pressure Drop 0.05 Psi Pressure Drop 0.12 Psi
Discharge Temperature 191 Degrees F Discharge Temperature 176 Degrees F
Air Mass Flow 21.3 Lb/Min Air Mass Flow 21.8 Lb/Min
Estimated Horsepower 215 Hp Estimated Horsepower 220 Hp
TurboCoupe Saab 900
Pressure Drop 0.03 Psi Pressure Drop 0.23 Psi
Discharge Temperature 175 Degrees F Discharge Temperature 173 Degrees F
Air Mass Flow 21.9 Lb/Min Air Mass Flow 21.9 Lb/Min
Estimated Horsepower 221 Hp Estimated Horsepower 221 Hp
Renault TurboCoupe (Converted to Water)
Pressure Drop 0.31 Psi Pressure Drop 0.03 Psi
Discharge Temperature 163 Degrees F Discharge Temperature 141 Degrees F
Air Mass Flow 22.3 Lb/Min Air Mass Flow 23.1 Lb/Min
Estimated Horsepower 225 Hp Estimated Horsepower 233 Hp
Dual Saab 900 (In Series) Starion/Conquest
Pressure Drop 0.53 Psi Pressure Drop 0.29 Psi
Discharge Temperature 129 Degrees F Discharge Temperature 121 Degrees F
Air Mass Flow 23.5 Lb/Min Air Mass Flow 23.9 Lb/Min
Estimated Horsepower 238 Hp Estimated Horsepower 241 Hp
Saab 9000 Porsche 944
Pressure Drop 0.7 Psi Pressure Drop 0.09 Psi
Discharge Temperature 119 Degrees F Discharge Temperature 119 Degrees F
Air Mass Flow 24 Lb/Min Air Mass Flow 24 Lb/Min
Estimated Horsepower 242 Hp Estimated Horsepower 242 Hp
Cosworth Volvo
Pressure Drop 0.42 Psi Pressure Drop 0.33 Psi
Discharge Temperature 117 Degrees F Discharge Temperature 116 Degrees F
Air Mass Flow 24.05 Lb/Min Air Mass Flow 24.1 Lb/Min
Estimated Horsepower 243 Hp Estimated Horsepower 243 Hp
Dual Volvo (in Parallel)
Pressure Drop 0.1 Psi
Discharge Temperature 91 Degrees F
Air Mass Flow 25.2 Lb/Min
Estimated Horsepower 255 Hp
1. The Intercooler with the lowest pressure drop does NOT provide the highest performance
2. The engine mass air flow CHANGED even though the engine CFM did not. Turbocharging increases density      NOT CFM. The heat generated by the compressor causes the air to be less dense than it could be
3. Intercooling is a DENSITY RECOVERY technique
Degrees Versus Resistance For VAT Thermister Degrees Versus Resistance For ACT Sensor
Degree F Degrees C Resistance
0 -17.78 13102
5 -15.00 11469
10 -12.22 10059
15 -9.44 8841
20 -6.67 7784
25 -3.89 6867
30 -1.11 6069
35 1.67 5374
40 4.44 4767
45 7.22 4236
50 10.00 3770
55 12.78 3361
60 15.56 3002
65 18.33 2685
70 21.11 2405
75 23.89 2158
80 26.67 1940
85 29.44 1746
90 32.22 1573
95 35.00 1420
100 37.78 1284
105 40.56 1162
110 43.33 1053
115 46.11 956
120 48.89 869
125 51.67 790
130 54.44 720
135 57.22 657
140 60.00 600
145 62.78 549
150 65.56 502
155 68.33 460
160 71.11 423
165 73.89 388
170 76.67 357
175 79.44 329
180 82.22 303
185 85.00 279
190 87.78 258
195 90.56 239
200 93.33 221
205 96.11 205
210 98.89 190
215 101.67 176
220 104.44 164
225 107.22 152
230 110.00 141
235 112.78 132
240 115.56 123
245 118.33 115
250 121.11 107
255 123.89 100
260 126.67 94
265 129.44 88
270 132.22 82
275 135.00 77
280 137.78 72
285 140.56 68
290 143.33 64
295 146.11 60
300 148.89 56
305 151.67 53
310 154.44 50
315 157.22 47
320 160.00 45
325 162.78 42
330 165.56 40
Degree F Degrees C Resistance
0 -17.78 239753
5 -15.00 206120
10 -12.22 177646
15 -9.44 153477
20 -6.67 132911
25 -3.89 115367
30 -1.11 100366
35 1.67 87509
40 4.44 76465
45 7.22 66955
50 10.00 58750
55 12.78 51655
60 15.56 45506
65 18.33 40168
70 21.11 35523
75 23.89 31473
80 26.67 27936
85 29.44 24840
90 32.22 22126
95 35.00 19743
100 37.78 17645
105 40.56 15796
110 43.33 14164
115 46.11 12720
120 48.89 11441
125 51.67 10306
130 54.44 9298
135 57.22 8400
140 60.00 7600
145 62.78 6886
150 65.56 6247
155 68.33 5675
160 71.11 5162
165 73.89 4702
170 76.67 4288
175 79.44 3915
180 82.22 3580
185 85.00 3276
190 87.78 3003
195 90.56 2755
200 93.33 2530
205 96.11 2326
210 98.89 2141
215 101.67 1973
220 104.44 1820
225 107.22 1681
230 110.00 1554
235 112.78 1438
240 115.56 1331
245 118.33 1234
250 121.11 1145
255 123.89 1064
260 126.67 989
265 129.44 920
270 132.22 857
275 135.00 799
280 137.78 745
285 140.56 696
290 143.33 650
295 146.11 608
300 148.89 569
305 151.67 533
310 154.44 500
315 157.22 469
320 160.00 440
325 162.78 413
330 165.56 388