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ENGINE TECH TIPS CORRECTION FACTORS, OBSERVED AND CORRECTED HORSEPOWER AND TORQUE THE NEED FOR Load Cell is an electronic device capable of pressure. The conditions for correction are CORRECTED TEST measuring force. 60° F and barometric pressure of 29.92 inches RESULTS from the engine (no other powertrain losses); pressure is calculated and the observed inlet airof mercury. Once the corrected barometric is useful power determinedBrake Horsepower The main reason for a correction factor is the can be observed or corrected. temperature is known, those values are plugged ability to compare testing performed under diff erent atmospheric conditions. The correction BASIC ENGINE into the following formula. The correction factor formula is: factor will contain a temperature, barometric PERFORMANCE C.F. = (29.92/Corrected Barometric Pressure) pressure and an effi ciency percentage. The AND ATMOSPHERIC ^1.2 * {(Observed Inlet Air Temp+460)/ temperature and barometric pressure have CONDITIONS (520)} ^.6 signifi cant impact on the performance of an Engines utilize fuel and air, and apply a form of SUMMARY engine. Also, to a lesser degree, humidity can aff ect the performance. Some dyno facilities Once a correction factor is calculated, the ob-combustion to convert the power stored in fuel have controlled atmospheric chambers to served numbers are multiplied by it. These areinto usable work. The air contains oxygen; this condition air to a desired temperature, humidity the “corrected values.” Undoubtedly, the bestis the element that supports the combustion and barometric pressure. These test cells are scenario is to test under the exact same condi-process. Cool dry air contains more oxygen mol- very sophisticated and usually booked with tions each time. If that is not achievable, a goodecules within a constant volume and pressure. production, emission, cold start and hot test rule of thumb is that engines corrected to theAs barometric pressure increases, additional work. The test cells with these chambers can SAE J607 standard will yield corrected torqueoxygen molecules are present (maintaining a easily cost several hundred thousand. and power numbers approximately 4% higherconstant volume). Considering these challenges, it becomes evident For example, if an engine was tested on a cool than those corrected to SAE J1349. Unfortunate- that there is a need to be able to test engines January day where the barometric pressure was ly, SAE J607 conditions are not very realistic. The under observed operating conditions and correct relatively high, observed engine performance most commonly accepted standard is the SAE the results to a standard set of conditions. will be better than the same engine tested on J1349. This corrects to a more practical set of at- a hot, muggy August day when a storm was mospheric conditions and utilizes coeffi cients to SOME DEFINITIONS coming in. Also, engine tests performed in higher compensate for an 85% mechanical effi ciency. Observed Operating Conditions are measured altitudes have lower observed barometric pres- Please note temperature is converted Rankine near the entry of the carburetor or inlet air sure and engine performance is lower. degrees in both formulas. system of the engine. These conditions include CORRECTION FACTORS inlet air temperature, wet bulb temperature and actual barometric pressure. Several correction factors exist and this article while the engine is running. It typically uses a (1) SAE J1349, June 1990 Data corrected and horsepower ratings is the measured torque valuewill deal with two of them.FYI about Ford Racing catalog torque Observed Torque calibrated load cell. This load cell measures to 77° F and 29.31 in Hg 85% effi ciency. Ford Racing push rod crate engines are rated the work the engine is doing in real time. The (2) SAE J607, Data corrected to 60° F at STP-corrected operating conditions. This observed torque value is then used in calculat- and 29.92 in Hg. correction factor is actually SAE J607, and also ing the observed horsepower value. SAE J1349 referred to as “Racer Corrected.” Simply work (generated by the engine) is being done. temperature and wet bulb readings to calculate ambient conditions and corrects the observed represents how fast theThis formula utilizes the observed inlet airspeaking, the STP correction factor takes the Observed Horsepower This is calculated by the following formula: saturated, current and corrected vapor data to 60° F, a barometric pressure of 29.93" (observed torque * rpm)/5252. pressure. The corrected vapor pressure is sub- pressure. This correction factor is used by mostof mercury and no correction for vapor Observed Barometric Pressure is atmospheric tracted from the observed barometric pressure. dyno shops because it yields the highest pressure measured near the engine air inlet. It is subtracted because this pressure is due numbers to advertise. self-explanatory. barometric pressure. “SAE” corrected operating conditions. This atFord Racing Modular crate engines are rated isto water vapor in the air. This yields corrected Observed Inlet Air Temperature achieved by evaporating water into the ob- metric pressure of 29.31 inches of mercury. Once referred as just “SAE.” This correction factor iscorrection factor is SAE J1349, and typically is the temperatureThe conditions for correction are 77° F and baro- Wet Bulb Temperature served inlet air. This is accomplished by using a the corrected barometric pressure is calculated used by automotive manufacturers to rate wick with one end in a vessel containing water and the observed inlet air temperature is known, torque and power for new vehicles. This correc- and the other connected to a thermometer or those values are plugged into the following tion factor adjusts the ambient operating thermocouple. This reading is used in calculat- formula. The correction factor conditions and corrects the observed data to ing vapor pressure, humidity and, ultimately, formula is: 77° F, barometric pressure of 29.31" of mercury correction factor. C.F. = 1.18 #(29.31/Corrected Barometric Pres- and compensates for corrected vapor pressure. Corrected Torque is the measured torque times sure) * {(Observed Inlet Air Temp+460)/ (537)} Some of Ford Racing's Modular crate engines the correction factor. ^.5-.18 are direct vehicle replacement engines, and Corrected Horsepower is the measured horse- SAE J607 since these engines/vehicles are originally power times the correction factor. This formula utilizes the observed inlet air advertised with SAE-corrected numbers, they Corrected Barometric Pressure is the observed temperature and wet bulb readings to calculate also are advertised in the catalog with SAE barometric pressure minus the corrected saturated, current and corrected vapor pressure. numbers. This keeps the engines' torque and vapor pressure. The corrected vapor pressure is subtracted power ratings consistent with O.E. advertised Standard Barometric Pressure is stated in the from the observed barometric pressure. It is numbers. See Ford Racing Dyno Tech articles on defi nition of the correction factor. subtracted because the pressure is due to water page 59. vapor in the air. This yields corrected barometric For important information about the proper usage of performance parts, please see page 14. 60 Techline (800) FORD788 See pages 286-292 for important safety, emissions and warranty information.


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