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.The 310 horsepower rating of a 1973 Pontiac Trans Am SD 455 may seem quite dismal if you're comparing it to something like the 370 horsepower rating of the 1970 Pontiac Trans Am RAIV 400, but the actual difference in the power output of these two engines is much smaller than the 60 horsepower discrepancy indicates. Net horsepower was attained from engine dynos with the accessories, OE air cleaner, and factory exhaust system present on the test subject engine. To make horsepower numbers more accurate to what you would actually see from your engine when installed in your car, for the 1972 model year, American car manufacturers started using "net" horsepower figures. Between the lack of power-robbing accessories and restrictive exhaust systems, the advertised horsepower numbers were quite inflated when compared to today's power figures. Also these power figures were typically attained with a very unrestrictive open exhaust system, along with the lack of restrictive OE air cleaner assemblies. These gross horsepower figures came from engine dyno results in which the tested engine had no real-world accessories attached such as power steering pumps, smog pumps, and or A/C compressors. "Net" HorsepowerĪs for American carmakers, previous to the 1972 model year, the horsepower figures you typically saw in car manufacturer literature and car magazines were "gross" horsepower figures. One horsepower is equal to 745.7 watts, watts being a unit of power that James Watt also came up with, and the same unit you see everyday associated with things like light bulbs and electrical devices. One horsepower is equivalent to the amount of force needed to lift 550 lbs of weight within a period of 1 second. Watt would call this new unit of measurement "horsepower". Back in the 18th century James Watt was developing steam engines and wanted to come up with a unit of measurement that would indicate the amount of workhorses the steam engine would replace. Horsepower is a measure of the ability to move a certain amount of weight a certain distance, within a certain amount of time.
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