Saturday, April 23, 2011

SCALING, PART I: ARE TURBOCHARGERS A REPLACEMENT FOR DISPLACEMENT?

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)
"There's no replacement for displacement." -- Old Car Crafters' saying
Much of the writing on turbocharging is based on the assumption that turbocharging is a "replacement for displacement."
Are turbos really a substitute for cubic inch displacement? The answer is: "it depends."
Most certainly if fuel octane is virtually unlimited and engine life is measured in hours or minutes rather than hundreds of thousands of miles, turbocharged small engines have often proven much more powerful than naturally-aspirated big-cube powerplants.
And when street horsepower outputs are modest (less than 600 h.p.) turbocharging often stands in very well for large cubic inch mills.
But what about when lofty horsepower goals must be accomplished with durability and on widely available pump fuels? Is a small turbocharged engine going to be enough? Or will turbocharging need the added "boost" of extra cubes? And what are the downsides to hauling around a larger turbo 'plant?
These are questions that we need to answer before going much further in the turbo selection process.
One way of looking at the problem is through "scaling." "Scaling" in this context is taking proven test data from another project and extrapolating it to a variety of engine sizes for rough comparisons.

Of course this form of "scaling" is not completely accurate because of many factors. Small-bore engines may be stronger and more detonation resistant than larger-bore lumps. Volumetric efficiencies may significantly vary across engine types and sizes. Comparison engines may not be able to "live" under as much cylinder pressure as the original test engine.
But acknowledging these limitations, scaling can provide a "ballpark" figure about what outputs are realistic.
Nelson and Freiburger's "F-Bomb" Camaro provides a good baseline for a scaling exercise. While the "F-Bomb" engine is hardly a grassroots build, published test data is available for a wide range of r.p.m. and manifold pressures.
The first step in scaling is converting the raw data into a factor that can be multiplied by the displacement of the comparison engines. The process is simple and quick on a computer spreadsheet or calculator. First, divide the horsepower output by test engine size. Then multiply the resulting factor by the comparison engine size.
For simplicity, we'll use horsepower per liter (hp/l) as the factor.
The first published data set for the "F-Bomb" engine is a safe "low-boost" test (6.3-7.0 psi)

RPM HP hp/l
3500 418 62.80

3600 434 65.21

3700 454 68.21

3800 475 71.37

3900 498 74.82

4000 520 78.13

4100 539 80.98

4200 558 83.84

4300 574 86.24

4400 588 88.34

4500 605 90.90

4600 625 93.90

4700 644 96.76

4800 661 99.31

4900 670 100.67

5000 680 102.17

5100 695 104.42

5200 705 105.92

5300 718 107.88

5400 727 109.23

5500 730 109.68

5600 738 110.88

5700 744 111.78

5800 751 112.83

5900 761 114.34

6000 769 115.54
The next data set is what DF suggested was the at the limit for pump gasoline with the "F-Bomb's" charge-cooled EFI engine (11.0-14.4 psi)

RPM HP hp/l

3500 524 78.73

3600 559 83.99

3700 600 90.15

3800 638 95.86

3900 673 101.12

4000 705 105.92

4100 732 109.98

4200 753 113.14

4300 768 115.39

4400 787 118.24

4500 808 121.40

4600 822 123.50

4700 842 126.51

4800 873 131.17

4900 901 135.37

5000 927 139.28

5100 950 142.73

5200 967 145.29

5300 981 147.39

5400 991 148.89

5500 991 148.89

5600 997 149.80

5700 1008 151.45

5800 1025 154.00

5900 1022 153.55

6000 1009 151.60

The next data set is a moderate pull on race gas (12.2-18.3 psi).
RPM HP hp/l

3500 560 84.14

3600 600 90.15

3700 652 97.96

3800 709 106.52

3900 755 113.44

4000 778 116.89

4100 809 121.55

4200 834 125.31

4300 854 128.31

4400 874 131.32

4500 896 134.62

4600 921 138.38

4700 948 142.43

4800 979 147.09

4900 1004 150.85

5000 1028 154.45

5100 1059 159.11

5200 1080 162.27

5300 1087 163.32

5400 1103 165.72

5500 1120 168.28

5600 1139 171.13

5700 1150 172.78

5800 1155 173.53

5900 1161 174.44

6000 1165 175.04

By way of comparison, the turbocharged "Low-Buck" Demon 454 BBC in the February 2011 issue of Car Craft yields the following test factors:

RPM HP hp/l (hp/7.4)

3500 443 59.86

4000 527 71.22

4500 619 83.65

5000 696 94.05

5500 743 100.41

6000 776 104.86

6400 767 103.65

It's clear that the Nelson/Freiburger engine is much more efficient than the non-charge cooled, single-turbo "Low-Buck" 454. This could be the result of several factors including turbo efficiency, ignition timing, intake charge temperature and density, and volumetric efficiency. 

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FIRST PRINCIPLES: A PRIMER ON TURBOCHARGER ENERGY USE

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)
"Energy can be neither created nor destroyed. It can only change forms." -- First Law of Thermodynamics (abridged)

Many Car Crafters first learn about this through the Bernoulli Principle, as applied to carburetion. Speeding up air through a narrower passage (a carburetor's venturi) lowers the pressure of the air stream and allows outside air pressure (through the fuel bowl vents) to force fuel through the metering system into the venturi air stream. In other words, Pressure energy is briefly exchanged for velocity energy.

Changing energy into different forms is also at the core of how turbocharging works. Pressure, velocity and temperature of the gas passing through the compressor and turbine are interrelated and change predictably at different points of the turbocharging process.

For example, the compressor impeller increases the velocity of the intake air by pumping it through its blades at a high r.p.m. This air velocity energy is then tranformed into stable flow and higher pressure through the diffuser section of the compressor housing. Although some energy is converted into heat, the bulk of the energy input has transformed from rotational energy into increased air pressure.

Remember that the difference between the outside air pressure and the post-compressor air pressure is called the pressure ratio.

More energy is, of course, added to the system through the combustion process. Thus, on the turbine side, the hot, pressurized and pulsing exhaust gas is accelerated through a volute in the turbine housing(think: funnel bent around a circle) to a nozzle. The exhaust's expansion from the nozzle through the turbine blades to the low-pressure exhaust rotates the turbine, converting velocity energy and sometimes pulse energy into the rotational force that powers the compressor.

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READING TURBO COMPRESSOR MAPS, PART IV: EFFICIENCY ISLANDS

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"Efficiency is doing things right." -- Peter Drucker

Anyone who has mistakenly put their hand on the shop compressor discharge line will always remember that compressing air results in heat. And NASCAR TV viewers often hear about "starting on low tire pressures" to avoid "pressure build-up."


Both of these are examples of the relationship between air volume, air pressure and air temperature.

Ideal gas law states that:
(air pressure x air volume)/air temperature = remains constant (Miller 19-20) Thus, increases in air pressure of a fixed quantiy of air result in increases in temperature.

When any compressor takes a "gulp" of air and compresses it into a smaller space, heat is necessarily produced. Often, the temp increase is explained as the result of increased friction between air moleules rubbing together when jammed into a smaller space.

Engineers and scientists describe the ideal temperature rise from air compression as "adibatic" -- neither gaining or losing any heat (beyond what the Ideal Gas Law predicts, that is).

However, no air compressor is 100% efficient. Internal air movement, impeller friction, pumping losses, and other inefficiencies add extra heat to the compressed air. This inefficiency is represented on turbo compressor maps and is critical to determining the actual density of the compressed charge.

Looking at the old T66 turbo map, the lines in the middle of the map show zones or "islands" of efficiency. The percentages shown are the calculated efficiency of the compressor, based on measurement of the discharge temperature of the compressed air.

Thus, when someone reports that a particular compressor is operating in the 75% efficient zone at a particular pressure ratio, they're saying that the compressor is heating the air 25% MORE than the Ideal Gas Law adibatic temperature rise formula predicts.


By way of comparison, when a traditional Roots blower is operating in a 50% efficiency island, then it is heating the air 50% more than the ideal temp rise formula predicts.


Extra heat in the compressor discharge air indicates two things. First, the extra heaing means that the dischared air is less dense than under ideal conditions. Second, the extra heat shows that not all of the "work" applied to compressing the air actually resulted in air compression -- some of the energy was "lost" to heating the compressed air. (engineers and scientists refer to this as "isentropic efficiency.")

However, for Car Crafters, the more important things are to determine how much density has been lost and how to recover as much of it as possible.

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WHAT IS "PRESSURE RATIO?" (AND HOW ALTITUDE AND RESTRICTIVE AIR INTAKES AFFECT IT)

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"The course of the flight up and down was exceedingly erratic, partly due to the irregularity of the air . . . . " -- Orville Wright

Some readers have probably wondered why turbo maps use "pressure ratio" instead of "boost" (manifold pressure).
The simple answer is that reductions in compressor inlet pressure without any change in the pressure ratio will lead to reductions in manifold pressure.

Conversely, if manifold pressure stays constant and inlet pressure decreases, the pressure ratio must increase.  formula to "prove" this is found in virtually every turbocharger matching book, from the Hugh MacInnes "classic" Turbochargers to the latest books from Miller, Hartman, and Warner.

Pressure ratio = (boost pressure (gauge pressure) + ambient air pressure)/ambient air pressure.

(See MacInnes at 16, Miller at 39, Warner at 23, A. Graham Bell at 68, Corky Bell at 26)

So how does this really work: If you're testing a car on the beach (at sea level), the average barometric pressure reading should be ~ 14.7 psi (29.92 in. Hg.) Now if the "gauge pressure" indicates 30 psi of "boost" . . .

Pressure Ratio = 30 + 14.7 / 14.7

Pressure Ratio = 44.7/14.7
And when you plug these numbers into your handy calculator or computer spreadsheet, you get 3.04. Thus,

Pressure Ratio = 3.04:1
That means you've crammed three "atmospheres" into the space of one. (You get one for free and two more from the turbo)

Now run the same calculation a little more than 2/3s of the way up Pikes Peak at 10,000 ft.
Average barometric pressure reading: 10.1 psi (20.58 in. Hg.) (A. Graham Bell summarizes you lose ~ 0.5 psi ambient air pressure for each 1,000 ft. increase in altitude.)

Pressure Ratio (10,000 ft) = 30 (gauge pressure) + 10.1/10.1

Pressure Ratio (10,000 ft) = 3.97:1
WHAT! How can that be? To produce 30 psi in the thin air of 10,000 ft., the turbo compressor had to work much harder. The pressure ratio had to increase to keep the "gauge pressure" constant.

But what if the pressure ratio hadn't increased? "Gauge pressure" would have dropped to 20.2 psi.

What should be obvious here is that a chart which equates pressure ratio to "boost" only works at a specific altitude. Most such charts assume sea level ambient air pressure.

This also illustrates a flaw in a fixed linking of compressor speed to engine r.p.m., as belt-driven superchargers do. Recalling the general relationship between pressure ratio and compressor r.p.m., a turbo can speed up to to increase its pressure ratio at altitude. The only way a supercharger can do that is either with a pulley change or variable drive system.

Of course, altitude isn't the only thing that can reduce compressor inlet pressure. A restrictive air intake, an air intake located in a low pressure zone, a restrictive mass air sensor, inadequate inlet ducting, clogged air filter or excessive intake air turbulance and heating (i.e. placing your air inlet behind the radiator) are common mistakes that reduce compressor inlet pressure in the real world.

Kenne Bell makes the following suggestion to its supercharger customers:
Although Kenne Bell's suggestion does not take into account potential pre-compressor intake air pressure increases through vehicle velocity (ram air), the basic test method is reasonable for any form of supercharger or turbo.

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KEEP COMPRESSOR OPERATIONS OUT OF THE "CHOKE ZONE"

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"Don't choke now!" -- Grady Seasons (Keith McCready), "The Color of Money" (1986)

The following excerpt appeared on the forum at Bangshift.com:

"[The January 2011 issue of Auto Enthusiast magazing at pg. 30]has a story about a class project at WyoTech's Blairsville, PA campus. They built a crate 350 SBC with a single [Holset]HX35. It maxed out at a paltry 575 h.p. on 15 psi manifold pressure (charge cooled). This shows that the engine was obviously running well into the choke zone of the HX35. [Jay K.] Miller's map [Turbo: Real World High-Performance Turbocharger Systems ]shows that the HX35 goes into choke at ~ 50 lbs/min at 2.0:1 pressure ratio."
 
Sadly, Holset is tight-fisted with its compressor maps and there isn't a good on-line map of the HX35 available today.

 
Referring back to the old T66 map, every point to the right of the mapped area is in the choke zone of the compressor. Choke means just as it sounds -- the turbo compressor is choking the engine because it is not large enough to flow air efficiently in the quantities the engine COULD be consuming.

A turbo operating in the choke zone is too small for the engine at high r.p.m. It's that simple.

That doesn't necessarily mean that the turbo is too small at every r.p.m. For example, the WyoTech Blairville SBC reportedly has excellent low r.p.m. response, with a boost threshold (on-set of measureable manifold pressure increase) of ~ 2,000 r.p.m.

In another famous example (reported in the June 2001 issue of Hot Rod Magazine), Chris Stewart used a pair of junkyard 1986 Ford Thunderbird Turbo Coupe Garrett T03E turbos on a 472 CID BBF. The boost threshold was a mere 1,700 r.p.m. and it pounded out 714 lbs./ft of torque at the rear wheels through a power-guzzling non-lockup C6 automatic. And Stewart achieved this on a mere 10 psi of "boost."

The torque peak was a diesel-like 2,200 r.p.m.

However, the smallish turbos greatly restricted power at high r.p.m., limiting the 8.2:1 BBF to a weak 355 RWHP at a tractor-like 4,200 r.p.m.

Even though a turbo will still flow some air in the choke zone, the mass air density will be lowered by excessive charge heating. The turbo could also be at risk of overspeeding. And if the turbine (exhaust side) is fairly well matched with the compressor (as expected with factory turbos but not always true with aftermarket "hybrid" turbos) the turbine is likely also in an inefficent range of operation when the compressor is in the choke zone. Excessive backpressure and exhaust heat from a turbine opertaing in choke can hurt power production and exhaust valve longevity. In short, operating in the choke zone is akin to a dog chasing its own tail.
 
It should be clear by now, a properly fitted turbo or turbos must avoid the extremes of surge and choke. The "sweet spot" for turbo compressor operation is in the middle portions of the map where compressor efficiency is the highest.
What does "choke" mean? And why does it matter?

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READING TURBO COMPRESSOR MAPS, PART III

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft forum turbo blog)

"One must strike the right balance between speed and quality."-- The Right Honourable Clare Short (Former MP,Birmingham Ladywood, UK)

The next compressor map elements of note are the series of r.p.m speed lines arcing from the surge limit line on the left over to the right.

Each of these speed lines is a graph of compressor performance at specific compressor r.p.m.
 
Warner (Street Turbocharging) and Hartman (Turbocharging Performance Handbook) both provide detailed explanations of how turbocharger engineers use these speed lines in building a compressor map.

The short version is that the turbos are run up to a particular test r.p.m. on a "test rig" (sort of a turbo dynomometer) and then the outlet flow is restricted with a valve to measure how efficient the compressors are at various mass air flow levels (the bottom axis of the map).

Note that until the compressor becomes a flow restriction (the right side of the map) that pressure ratio and compressor speed are closely linked. The speed lines turn down sharply at the right side of the map because the compressor is simply too small to efficiently supply any more mass air, reducing the pressure ratio.

On this map, the downturn in the speed lines becomes more severe at higher compressor r.p.m. levels. This suggests that increases in mass air flow beyond the efficiency range of the compressor can lead to drastic increases in speed as the compressor struggles to "keep up" with air demands.

The map speed lines also show that for each level of mass air flow, there are many pressure ratios that can supply enough air. Thus, if the pressure ratio increases, and mass air flow does not, then the output is being more restricted. (it works the same way with a garden hose nozzle)

Of course, in the real world, the "restriction" of compressor output isn't a test valve. It's the physical ability of the engine to induct, react, and exhaust air. That means better "breathing" engines require less pressure and compressor speed to obtain a particular level of mass air flow.

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READING TURBO COMPRESSOR MAPS, PART II

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft turbo blog)


Turbo Tip of the Day: "Let it eat" -- Dean Skuza (Former AA/FC racer)


Moving from left to right on our old T66 turbo map, we see the dotted line labled "Surge Limit."

Pressures to the left of the surge limit are not mapped. Why? Because they are both unstable and potentially destructive to the compressor.

Surge amounts to air backing up inside the compressor and fighting to get back out through the entrance (or more properly, the "inducer bore")

To understand surge, imagine air being agitated into a mini-tornado by the compressor impeller (that's the fan blade/meat grinder thing that rotates).


Impeller



When flow out of the turbo is shut off or excessively restricted, the mini-tornado is not properly diffused into steady pressure because it has no place to go. So air being air, it takes the path of least resistance toward a lower pressure -- backing "out through the in door."

These reversals of flow fight the impeller's rotation. The exiting air molecules slam up against other air molecules that the impeller is attempting to induct (draw in). That causes inlet pressures to fluctuate and the impeller's blades to lose efficiency.

Thus, on the left side of the surge limit, the turbo compressor is not doing useful work because the exit flow is too restricted.

Simply put, surge occurs when the attempted pressure ratio is too high for the amount of air consumed by the engine. (Remember, just like with your garden hose or shop compressor, "boost" is not mass air flow. Boost without air flow creates surge)

Surge is most easily found (and heard) when a downstream throttle is slammed shut while the compressor is at speed. Surge sounds like chirping out of the compressor. Blow-off and recirculating valves are often used to combat this form of surge.

Using a turbo that is too large can also produce surge when the boost threshold is lower than an engine's abiliy to induct the compressed charge.

The simple rule is that for your turbo to live, you've got to "let it eat" by avoiding surge.

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READING TURBO COMPRESSOR MAPS, PART I

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft turbo blog)



"Somewhere there is a map of how it can be done."

-- Ben Stein

Turbo manufacturers create compressor and turbine maps to summarize the air flow and density characteristics of turbochargers. Sadly, these maps are often withheld from grassroots Car Crafters (especially turbine maps).

But when you can find them, these maps are invaluable guides in turbo selection . . . if you know how to read them.

The next few [articles]are going to be about reading turbo maps.

To make these tips more relevant to our discussion of turbocharging a big-cube V8, here's a turbo compressor map for an old Garrett T66 (a pair of these would support well over 1,000 h.p.).



The first thing to notice is the left vertical axis of the map. Pressure ratio is simply the relationship between compressor discharge pressure to atmospheric pressure.

However, it is NOT a direct measure of how dense the compressed charge is because of charge heating during compression and atmospheric conditions outside of the turbo. (If it were, we wouldn't need the map!)

1.00 is basically no compression (compressor discharge pressure = ambient (outside) pressure)

2.00 means the discharge pressure is twice as much as ambient pressure.

3.00 means the discharge pressure is three times as much as ambient pressure.

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Wednesday, March 09, 2011

A PROBLEM WITH JUNKYARD TURBOS

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft turbo blog)


"Don't send a child to do a man's job!" -- old Southern proverb


Someday turbos such as the tiny units used on the current Ford EcoBoost V6 engines will become widely available at the junkyard. And like an earler generation lured by JY T2s, T3s and T28s, some budget Car Crafters will undoubtedly buy the current generation of small JY turbos to use on big V8s with hopes of of dirt-cheap boost.

Years ago, more than a few Car Crafters grabbed up another small turbo, the once-ubiquitous Garrett AiResearch T3, and slapped pairs of them on all sorts of V8s, including Ford's 460.

Did it work?

It depends on what you wanted. If the goal is maximum pump gas power, the answer was no. Here's why:

Here's a turbo compressor map for one of the larger T3 variants. Note that the bottom axis is in lbs/min. of mass air flow. The highest flow mapped is ~ 35 lbs/min. Although the turbo doesn't stop flowing beyond that, the efficiency level becomes unacceptable (excessive air heating, potential overspeeding, and choking of the engine)


Recalling the general rule of thumb that one pound of mass air flow will support 9-10 horsepower, it should be abundantly clear -- even if you've got no idea how to read a turbo compresor map -- that a pair of these smallish turbos would be hard pressed to feed a 460 above 4,500 r.p.m. (If it's not, then stay tuned!)

Two of these turbos working together would struggle to supply enough "boost" at any pressure ratio for 630-700 h.p. And that's exactly what the Car Crafters who have tried to use pairs of JY T3s on 460s discovered.(See e.g. Trevor Cornwell's twin-turbo 460 Ford Fairmont)

And even that level would be adversely affected by excessive charge heating due to compressor inefficiency. Best efficiency would be at around a mere 200 to 225 h.p. per turbo -- a level that an "all motor" 460 should be able to easily achive on its own.














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Tuesday, March 08, 2011

CONVERTING CFM TO MASS AIR, PART II

By 460-BBF-Turbo-In-CC (From the legendary Car Craft big-cube turbo thread)


STP isn't very "real world."



Standard temperature and Pressure (STP) provides a means to account for the fact that air density and air volume are not linked.


STP - commonly used in the Imperial and USA system of units - as air at 60 [degrees]F (520 [degrees]R) and 14.696 pounds per square inch absolute (psia) (15.6oC, 1 atm) See, Engineering Toolbox.


Of course, engines in the real world seldom if ever operate at STP. Underhood temperatures, heat radiation, convection, and conduction, humidity and altitude variations help all assure that. That's one of the reasons why the SAE and others have developed more realistic standard references for automotive use.


Recall that we learned turbo manufacturers use 85 degrees F as "standard" (A. Graham Bell).


Is there really much difference between the various methods of converting CFM to lbs/min? Let's compare them:

Assume we're trying to convert 500 CFM to lbs/hr:




Warner method: divide 500 CFM by 13.7 = 36.49 lbs/hr

A. Graham Bell method: multiply 500 CFM by 0.07 = 35 lbs/hr
Miller method: multiply 500 CFM by 0.069 = 34.5 lbs/hr

STP method: multiply 500 CFM by 0.076 = 38 lbs/hr
Note that Jeff Hartman in Turbocharging Performance Handbook (2007) also uses the STP method.

Although the spread among the various methods is not that large, it appears that Miller  and A. Graham Bell's conversion factors produce the most conservative results. Moreover, because they track with turbocharger industry standard practice, we'll use the 0.07 conversion factor.

But don't forget that these all of these factors are based on picking a somewhat arbitrary air temperature and pressure as a measuring point. Actual results may vary.



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