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.