With a small-block’s deck height listed as 9.025, this appears to put this piston 0.025-inch below the deck. We looked up a typical 350 piston with a 5.70-inch rod and found compression height listed as 1.560 inches. This is then subtracted from the engine’s deck height to find the deck clearance, which can be positive (above deck) or negative (below deck). The simple math calls for dividing the stroke in half (3.48 / 2 = 1.74 inches) and then adding the connecting rod length and compression height. This leaves the last variable as piston compression height. Our example uses a 9.025-inch deck height, along with a 3.48-inch stroke and a 5.7-inch connecting rod. Let’s use the classic small-block Chevy 350 as a case study. While perhaps not accurate to 0.001-inch, it is an easy way to determine whether the deck is square.Īll of this is important if you want to add stroke to the crankshaft to increase displacement. Blower efficiency - blower efficiency is determined by the materials used and the methods of assembly.A simple way to check deck height variation is to measure the piston position relative to the deck using the same piston and rod on all four corners of the block.Blower size and drive ratio - larger blowers and higher drive ratios can create more boost.CFM available to the blower - carbs that are too small will keep the blower from making maximum boost.Size of the exhaust system - a small, restrictive exhaust can cause high boost readings.Cam Shaft specifications - lift, duration, and lobe center play a big part in boost levels.Cubic inches of the engine - more cubic inches means less boost.Aside from the possible differencein blower diameters, there are numerous things that will determine maximum boost levels. The actual boost readings from your system may vary greatly from the figures shown here. The larger bore blower is recommended for 371-500 CID engines in high performance, racing applications. The small bore blower (case thickness 3/4"+) is recommended for 370 CID engines or less for street applications. The large bore blower (case thickness 1/2"-) is approximately 11 percent larger in volume than the small bore. They are the larger diameter blower and the small diameter blower. There are two different 671 blowers commonly used. 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. The boost levels listed in our charts are based upon an average that was determined by 39 years of experience testing superchargers. To determine the effects of the altitude on a calculated compression ratio, use the following formula:Ĭorrected Compression Ratio = FCR. If the altitude in the area where you normally drive is significantly higher than sea level, then your compression ratios will also vary. The formula for calculating your exact final compression ratio is as follows:įinal Compression Ratio (FCR) = x CRĬR = engine compression ratioAltitude plays an important role in determining compression ratios. 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. It is to be used as a guideline in determining the proper maximum boost level for a specific application. This chart shows the final compression ratio combining the static compression ratio of an engine and the maximum blower boost from the blower system. download Drive Ratio Chart for 13.9mm & 14mm Pulleys download Drive Ratio Chart for 8mm Pulleys download Drive Chart for 1/2" Pitch Pulleys Back Fire Valves and Intake Accessories.
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