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Technical Charts

FINAL COMPRESSION RATIO CHART

Blower Boost

COMP RATIO

2

4

6

8

10

12

14

16

18

20

22

24

6.5

7.4

8.3

9.2

10

10.9

11.8

12.7

13.6

14.5

15.3

16.2

17.0

7

8

8.9

9.9

10.8

11.8

12.7

13.6

14.5

15.3

16.2

17.0

17.9

7.5

8.5

9.5

10.6

11.6

12.6

13.6

14.6

15.7

16.7

17.8

18.6

19.5

8

9.1

10.2

11.3

12.4

13.4

14.5

15.6

16.7

17.8

18.9

19.8

20.9

8.5

9.7

10.8

12

13.1

14.3

15.4

16.6

17.8

18.9

19.8

20.9

21.9

9

10.2

11.4

12.7

13.9

15.1

16.3

17.6

18.8

20.0

21.2

22.4

23.6

9.5

10.8

12.1

13.4

14.7

16

17.3

18.5

19.8

21.1

22.4

23.6

24.8

10

11.4

12.7

14.1

15.4

16.8

18.2

19.5

20.9

22.2

23.6

24.8

26.0

10.5

11.9

13.4

14.8

16.2

17.6

19.1

20.5

21.9

23.4

24.8

26.2

27.6

11

12.5

14.0

15.5

17.0

18.5

20.0

21.5

22.9

24.5

26.0

27.5

28.9

 

  • Final Compression Ratio Chart
  • BDS Stage 1 & 2 Blowers Only 
         - download Engine Belt Size Chart
         - download Drive Chart for 1/2" Pitch Pulleys
         - download Drive Ratio Chart for 8mm Pulleys
         - download Drive Ratio Chart for 13.9mm & 14mm Pulleys
Final Compression Ratio Chart

This chart shows the final compression ratio combining the static compression ratio of an engine and the maximum blower boost from the blower system. It is to be used as a guideline in determining the proper maximum boost level for a specific application. Final compression ratios in excess of 12.4:1 are not recommended for use with "pump gas." The higher the final compression ratio, the higher the octane rating of the fuel must be in order to help prevent detonation and serious engine damage.
The formula for calculating your exact final compression ratio is as follows:

Final Compression Ratio (FCR) = [ (Boost÷14.7) + 1 ] x CR

Boost = Maximum blower boost
14.7 = psi at sea level
CR = engine compression ratio

Altitude plays an important role in determining compression ratios. If the altitude in the area where you normally drive is significantly higher than sea level, then your compression ratios will also vary. To determine the effects of the altitude on a calculated compression ratio, use the following formula:

Corrected Compression Ratio = FCR - [ (altitude÷1000) x 0.2 ]

 = Pump Gas = Good Gas
 
BDS Street Blower Boost Charts

The boost levels listed in our charts are based upon an average that was determined by 39 years of experience testing superchargers. These boost readings were measured at wide open throttle and with the engine under load at 6000 RPM using a large bore blower unless otherwise specified.

There are two different 671 blowers commonly used. They are the larger diameter blower and the small diameter blower. The large bore blower (case thickness 1/2"-) is approximately 11 percent larger in volume than the small bore. The small bore blower (case thickness 3/4"+) is recommended for 370 CID engines or less for street applications. The larger bore blower is recommended for 371-500 CID engines in high performance, racing applications.

The actual boost readings from your system may vary greatly from the figures shown here. Aside from the possible differencein blower diameters, there are numerous things that will determine maximum boost levels.

  1. Cubic inches of the engine - more cubic inches means less boost.
  2. Cam Shaft specifications - lift, duration, and lobe center play a big part in boost levels.
  3. Size of the exhaust system - a small, restrictive exhaust can cause high boost readings.
  4. CFM available to the blower - carbs that are too small will keep the blower from making maximum boost.
  5. Blower size and drive ratio - larger blowers and higher drive ratios can create more boost.
  6. Blower efficiency - blower efficiency is determined by the materials used and the methods of assembly.

BDS Stage 1 & 2 Blowers Only


471 Blower

Cubic
Inches

231

262

289

350

400

-20%

N/A

N/A

N/A

N/A

N/A

-15%

N/A

N/A

N/A

N/A

N/A

-10%

11.5 lbs

9.5 lbs

5 lbs

3 lbs

1 lbs

-5%

13 lbs

11 lbs

7 lbs

4 lbs

2 lbs

1:1

14.5 lbs

12.5 lbs

8 lbs

6 lbs

4 lbs

+5%

15 lbs

13 lbs

9 lbs

7 lbs

5 lbs

+10%

16 lbs

14 lbs

10 lbs

8 lbs

6 lbs

+15%

17 lbs

15 lbs

11 lbs

9 lbs

7 lbs

+20%

18 lbs

16 lbs

13 lbs

11 lbs

9 lbs

+25%

N/A

N/A

N/A

N/A

N/A

+30%

N/A

N/A

N/A

N/A

N/A

+35%

N/A

N/A

N/A

N/A

N/A

+40%

N/A

N/A

N/A

N/A

N/A

 

671 Blower (Small Bore)

Cubic
Inches

289

327

350

400

427

454

500

-20%

3 lbs

1 lbs

N/A

N/A

N/A

N/A

N/A

-15%

5 lbs

3 lbs

1 lbs

N/A

N/A

N/A

N/A

-10%

7 lbs

5 lbs

3 lbs

1 lbs

N/A

N/A

N/A

-5%

9 lbs

7 lbs

5 lbs

3 lbs

2 lbs

N/A

N/A

1:1

11 lbs

8.5 lbs

7 lbs

4 lbs

3 lbs

2 lbs

N/A

+5%

13 lbs

10 lbs

9 lbs

6 lbs

5 lbs

4 lbs

2 lbs

+10%

15 lbs

12 lbs

11 lbs

8 lbs

7 lbs

5 lbs

4 lbs

+15%

17 lbs

14 lbs

12 lbs

10 lbs

8.5 lbs

7 lbs

6 lbs

+20%

N/A

15 lbs

14 lbs

12 lbs

10 lbs

8 lbs

7 lbs

+25%

N/A

17 lbs

15 lbs

14 lbs

11 lbs

9 lbs

8 lbs

+30%

N/A

N/A

17 lbs

16 lbs

13 lbs

10 lbs

9 lbs

+35%

N/A

N/A

N/A

N/A

N/A

N/A

N/A

+40%

N/A

N/A

N/A

N/A

N/A

N/A

N/A

  

671 Blower (Large Bore)

Cubic
Inches

289

327

350

400

427

454

500

550

-20%

9 lbs

7 lbs

5 lbs

3 lbs

2 lbs

1 lbs

N/A

N/A

-15%

11 lbs

9 lbs

7 lbs

5 lbs

4 lbs

2 lbs

N/A

N/A

-10%

15 lbs

11 lbs

9 lbs

7 lbs

6 lbs

4 lbs

1 lbs

N/A

-5%

17 lbs

13 lbs

11 lbs

8 lbs

7 lbs

6 lbs

2 lbs

N/A

1:1

21 lbs

15 lbs

12 lbs

10 lbs

9 lbs

8 lbs

4 lbs

N/A

+5%

23 lbs

16 lbs

14 lbs

12 lbs

11 lbs

10 lbs

6 lbs

2 lbs

+10%

25 lbs

18 lbs

16 lbs

14 lbs

13 lbs

12 lbs

8 lbs

4 lbs

+15%

27 lbs

21 lbs

18 lbs

16 lbs

15 lbs

14 lbs

10 lbs

6 lbs

+20%

N/A

24 lbs

20 lbs

19 lbs

17 lbs

16 lbs

12 lbs

8 lbs

+25%

N/A

N/A

22 lbs

22 lbs

19 lbs

18 lbs

14 lbs

10 lbs

+30%

N/A

N/A

N/A

24 lbs

21 lbs

20 lbs

18 lbs

14 lbs

+35%

N/A

N/A

N/A

N/A

24 lbs

22 lbs

20 lbs

16 lbs

+40%

N/A

N/A

N/A

N/A

N/A

24 lbs

22 lbs

18 lbs

871 Blower

Cubic
Inches

289

327

350

400

427

454

500

550

-20%

11 lbs

8 lbs

6 lbs

5 lbs

4 lbs

2 lbs

N/A

N/A

-15%

13 lbs

10 lbs

8 lbs

7 lbs

6 lbs

4 lbs

2 lbs

N/A

-10%

15 lbs

12 lbs

10 lbs

9 lbs

8 lbs

6 lbs

4 lbs

2 lbs

-5%

17 lbs

14 lbs

12 lbs

11 lbs

10 lbs

8 lbs

6 lbs

4 lbs

1:1

19 lbs

16 lbs

14 lbs

13 lbs

12 lbs

10 lbs

8 lbs

6 lbs

+5%

21 lbs

18 lbs

16 lbs

15 lbs

14 lbs

12 lbs

10 lbs

8 lbs

+10%

24 lbs

20 lbs

18 lbs

17 lbs

16 lbs

14 lbs

12 lbs

10 lbs

+15%

26 lbs

22 lbs

20 lbs

19 lbs

18 lbs

16 lbs

14 lbs

12 lbs

+20%

N/A

25 lbs

23 lbs

21 lbs

20 lbs

18 lbs

16 lbs

14 lbs

+25%

N/A

N/A

26 lbs

23 lbs

22 lbs

20 lbs

18 lbs

16 lbs

+30%

N/A

N/A

29 lbs

25 lbs

24 lbs

22 lbs

20 lbs

18 lbs

+35%

N/A

N/A

N/A

27 lbs

26 lbs

24 lbs

22 lbs

20 lbs

+40%

N/A

N/A

N/A

29 lbs

28 lbs

26 lbs

24 lbs

22 lbs

  

871 Blower (Stage 2)

Cubic
Inches

289

327

350

400

427

454

500

550

-20%

13 lbs

10 lbs

8 lbs

7 lbs

6 lbs

4 lbs

N/A

N/A

-15%

15 lbs

12 lbs

10 lbs

9 lbs

8 lbs

6 lbs

4 lbs

N/A

-10%

17 lbs

14 lbs

12 lbs

11 lbs

10 lbs

8 lbs

6 lbs

4 lbs

-5%

19 lbs

16 lbs

14 lbs

13 lbs

12 lbs

10 lbs

8 lbs

6 lbs

1:1

21 lbs

18 lbs

16 lbs

15 lbs

14 lbs

12 lbs

10 lbs

8 lbs

+5%

23 lbs

20 lbs

18 lbs

17 lbs

16 lbs

14 lbs

12 lbs

10 lbs

+10%

26 lbs

22 lbs

20 lbs

19 lbs

18 lbs

16 lbs

14 lbs

12 lbs

+15%

28 lbs

24 lbs

22 lbs

21 lbs

20 lbs

18 lbs

16 lbs

14 lbs

+20%

N/A

27 lbs

25 lbs

23 lbs

22 lbs

20 lbs

18 lbs

16 lbs

+25%

N/A

N/A

28 lbs

25 lbs

24 lbs

22 lbs

20 lbs

18 lbs

+30%

N/A

N/A

31 lbs

27 lbs

26 lbs

24 lbs

22 lbs

20 lbs

+35%

N/A

N/A

N/A

29 lbs

28 lbs

26 lbs

24 lbs

22 lbs

+40%

N/A

N/A

N/A

31 lbs

30 lbs

28 lbs

26 lbs

24 lbs