Differences
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techtalk:ref:engmech04r [2020/10/20 17:00] hippysmack [Measuring Engine Vacuum] |
techtalk:ref:engmech04r [2020/10/21 03:34] (current) hippysmack [Measuring Engine Crankcase 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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| Absolute pressure cannot be lower than absolute zero as that is its zero point. On the other hand, gauge pressure uses atmospheric pressure as its zero point. Even with varying atmospheric pressure, absolute pressure is always definite. Meanwhile, due to varying atmospheric pressure, the measurement of gauge pressure is not precise. Absolute pressure units are sometimes suffixed with the letter “a” whereas units for gauge pressure use “g” as a suffix. ((https://theydiffer.com/difference-between-absolute-pressure-and-gauge-pressure/)) \\ | Absolute pressure cannot be lower than absolute zero as that is its zero point. On the other hand, gauge pressure uses atmospheric pressure as its zero point. Even with varying atmospheric pressure, absolute pressure is always definite. Meanwhile, due to varying atmospheric pressure, the measurement of gauge pressure is not precise. Absolute pressure units are sometimes suffixed with the letter “a” whereas units for gauge pressure use “g” as a suffix. ((https://theydiffer.com/difference-between-absolute-pressure-and-gauge-pressure/)) \\ | ||
| - | ====== Measuring Engine Vacuum ====== | + | ====== Measuring Engine Crankcase Vacuum ====== |
| 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. \\ | ||
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| And so, operating at higher elevations sometimes requires higher rated vacuum pumps to attain the same vacuum you would at lower elevations. \\ | And so, operating at higher elevations sometimes requires higher rated vacuum pumps to attain the same vacuum you would at lower elevations. \\ | ||
| And likewise, operation at higher elevations affects the air/fuel mixture in the combustion chambers since air is less dense. \\ | And likewise, operation at higher elevations affects the air/fuel mixture in the combustion chambers since air is less dense. \\ | ||
| - | + | ||
| + | However, it doesn't appear that elevation has much to do with the amount of vacuum created in the crankcase. \\ | ||
| + | A 12" pressure drop will still be a 12" pressure drop no matter if it drops from 50 or 5000 feet above sea level. \\ | ||
| + | The important number is the pressure swing. The more vacuum that is created, the farther away from atmosphere it becomes. \\ | ||
| + | The positive makeup pressure on downstroke normally brings vacuum back to near atmosphere. \\ | ||
| + | But an increase in vacuum lowers scavenging ability for the oil pump. \\ | ||
| Gauge pressure is a measurement taken from the current atmospheric pressure level (depending on current elevation above sea level). \\ | Gauge pressure is a measurement taken from the current atmospheric pressure level (depending on current elevation above sea level). \\ | ||
| When taking a vacuum measurement, you are measuring how much the air is lower than the current atmospheric pressure at your location (elevation wise). | When taking a vacuum measurement, you are measuring how much the air is lower than the current atmospheric pressure at your location (elevation wise). | ||
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| - | Example of crankcase pressure testing results from different elevations. Results below are both at 1000 RPM at operating temperature: \\ | + | Example of crankcase pressure testing results from different elevations. \\ |
| - | From this testing article , [[techtalk:ref:engmech04g|Slack tube testing on a 1998 1250S model]], the crankcase pressure had a mean vacuum of 21" of water column (elevation app 630 feet). \\ | + | Results below are both at 1000 RPM at operating temperature and the test point in each motor was the timing hole plug. \\ |
| - | And from this testing article, [[techtalk:ref:engmech07p|Testing with a Slack Tube (Manometer) by bustert]], the crankcase prerssure had a mean vacuum of 30" of water column. \\ | + | From testing in these Sportsterpedia articles; \\ |
| - | (both test results from the timing plug hole) \\ | + | * [[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). \\ (both outside in and inside out of a parked engine) | ||
| + | * 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). \\ (both outside in and inside out of a parked engine) | ||
| + | * 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 ====== | ||
