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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