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

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

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

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.

Six most basic items that affect boost are listed below:
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

BDS Stage 2 Blower