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techtalk:ref:engmech04r [2020/10/20 04:39] hippysmack [Measuring Engine Vacuum] |
techtalk:ref:engmech04r [2020/10/20 22:35] hippysmack [Measuring Engine Vacuum] |
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| **Air pressure at sea level** (considered one atmosphere) is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. \\ | **Air pressure at sea level** (considered one atmosphere) is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. \\ | ||
| + | Numbers are generally rounded to 14.70 PSI. \\ | ||
| **Gauge Pressure** is the measure of pressure relative to the ambient atmospheric pressure (or measured against the zero of atmospheric pressure). It is the difference between absolute pressure and the atmospheric pressure. Therefore, a zero value on the gauge pressure scale means that the absolute pressure of the system is equal to the absolute pressure exerted by the surrounding atmosphere. A gauge is the instrument used in measuring pressure. A gauge always needs a reference point since the reading is made with a deflection in the gauge caused by a difference in pressure. Usually, a gauge is vented, meaning it uses the pressure of the air as reference. This is why it is called gauge pressure. ((https://theydiffer.com/difference-between-absolute-pressure-and-gauge-pressure/)) \\ | **Gauge Pressure** is the measure of pressure relative to the ambient atmospheric pressure (or measured against the zero of atmospheric pressure). It is the difference between absolute pressure and the atmospheric pressure. Therefore, a zero value on the gauge pressure scale means that the absolute pressure of the system is equal to the absolute pressure exerted by the surrounding atmosphere. A gauge is the instrument used in measuring pressure. A gauge always needs a reference point since the reading is made with a deflection in the gauge caused by a difference in pressure. Usually, a gauge is vented, meaning it uses the pressure of the air as reference. This is why it is called gauge pressure. ((https://theydiffer.com/difference-between-absolute-pressure-and-gauge-pressure/)) \\ | ||
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| Engine vacuum is simply air pressure lower than (and is based on comparison with) atmospheric pressure. \\ | Engine vacuum is simply air pressure lower than (and is based on comparison with) atmospheric pressure. \\ | ||
| - | It is usually measured as Gauge Pressure showing changes from the current atmospheric pressure. \\ | + | It is usually measured as Gauge Pressure. \\ |
| It varies with altitude just as atmospheric (barometric) pressure does. \\ | It varies with altitude just as atmospheric (barometric) pressure does. \\ | ||
| As altitude increases, vacuum decreases by about 1" Hg for every 1000 feet above sea level. \\ | As altitude increases, vacuum decreases by about 1" Hg for every 1000 feet above sea level. \\ | ||
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| {{:techtalk:ref:engmech:gauge_pressure_by_hippysmack.png?direct&400|}} ((drawing by Hippysmack, referenced from https://sciencestruck.com/absolute-vs-gauge-pressure)) \\ | {{:techtalk:ref:engmech:gauge_pressure_by_hippysmack.png?direct&400|}} ((drawing by Hippysmack, referenced from https://sciencestruck.com/absolute-vs-gauge-pressure)) \\ | ||
| + | \\ | ||
| + | Example of crankcase pressure testing results from different elevations. \\ | ||
| + | Results below are both at 1000 RPM at operating temperature and the test point in each motor was the timing hole plug. \\ | ||
| + | From testing in these Sportsterpedia articles; \\ | ||
| + | * [[techtalk:ref:engmech04g|Slack tube testing on a 1998 1250S model]], (elevation app 630 feet); | ||
| + | * 1250cc | ||
| + | * The crankcase pressure had a mean vacuum of 21" of water column. | ||
| + | * At 630 feet above sea level, ambient atmosphere is applying app. 29.24" Hg of force (397.52" WC or 14.36 PSIA). | ||
| + | * Vacuum at idle created by the pistons lowered ambient atmosphere inside by 21" WC. | ||
| + | * So the mean pressure against the gaskets and seals at 1000 RPM (breather valves closed) was 376.21" WC (27.67" Hg or 13.59 PSIA). | ||
| + | * The crankcase cycled from a mean pressure of 13.59 PSIA (idle) to near 14.39 PSIA (5000 RPM) - (range 0.8 PSIA). | ||
| + | * [[techtalk:ref:engmech07p|Testing with a Slack Tube (Manometer) by bustert]], (elevation app 12.8 feet); | ||
| + | * The crankcase pressure had a mean vacuum of 30" of water column. | ||
| + | * At 12.8 feet above sea level, ambient atmosphere is applying app. 29.91" Hg of force (406.63" WC or 14.69 PSIA) | ||
| + | * Vacuum at idle created by the pistons lowered ambient atmosphere inside by 30" WC. | ||
| + | * So the mean pressure against the gaskets and seals at 1000 RPM (breather valves closed) was 376.63" WC (27.70) Hg or 13.60 PSIA) | ||
| + | * The crankcase cycled from a mean pressure of 13.60 PSIA (idle) to near 14.63 PSIA (5000 RPM) - (range 1.03 PSIA). | ||
| ====== The Effect of Atmospheric Pressure on Vacuum Level ====== | ====== The Effect of Atmospheric Pressure on Vacuum Level ====== | ||
