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techtalk:ref:engmech04 [2023/12/22 20:24] – hippysmacktechtalk:ref:engmech04 [2024/11/30 06:52] (current) – [Differential Pressure (vacuum and air pressure)] hippysmack
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 More molecules inside the engine than outside = inside air pressure. \\ More molecules inside the engine than outside = inside air pressure. \\
 Less molecules inside the engine than outside = inside is vacuum pressure. \\ Less molecules inside the engine than outside = inside is vacuum pressure. \\
-57-76 engines are subject to the most volume of vacuum in the crankcase, rocker box, oil tank and primary compartments. \\+57-76 engines are subject to the most volume of vacuum in the crankcase. \\
 77-85 engines are subject to the same volume of vacuum in the gearcase, rocker box, oil tank, gearcase and primary compartments. \\  77-85 engines are subject to the same volume of vacuum in the gearcase, rocker box, oil tank, gearcase and primary compartments. \\ 
  
 Oil scavenging: \\ Oil scavenging: \\
-Positive crankcase air pressure aids scavenging. It pushes the oil to the oil pump. \\ +Positive crankcase air pressure aids scavenging. \\ 
-Negative (vacuum) pressure makes the pump's job harder. High vacuum makes the downstroke push to scavenge less powerful. \\ +It pushes oil against the suction chamber of the pump creating more Net Positive Suction Head Available (NPSHA) to the pump which is a requirement of the pump design. \\ 
-The oil pump wants to receive more oil, high vacuum slows down the delivery of oil to the oil pump. \\+This push is needed in order to assure there will be oil at the suction inlet of the pump especially upon higher RPM. \\ 
 +With lower NPSHA, the oil may may not enter the pump's suction chamber fast enough which can lower the amount of oil the pump will pump. \\ 
 +Negative (vacuum) pressure on the incoming oil makes the pump's job harder. Vacuum makes the downstroke push to scavenge less powerful. \\ 
 +The oil pump wants to receive more oil on higher RPM. Higher vacuum is created in the suction chamber near the gears as the pump speeds up. \\ 
 +Without a positive "push" ushering the oil to the suction chamber, vacuum in the chamber can slow (or partially choke) oil coming into the chamber. \\ 
 +This slows down the delivery of oil to the oil pump (lowers NPSHA) and can result in wetsumping the motor. \\
  
 Ring seal and pumping loss: \\ Ring seal and pumping loss: \\
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 It puts internal pressure against the weakest structural points (gaskets and seals). \\ It puts internal pressure against the weakest structural points (gaskets and seals). \\
  
-Symptoms include: ((Jorgen of the XLFORUM http://xlforum.net/forums/showthread.php?t=75740&highlight=reed+valve&page=9)) \\+Symptoms include: ((Jorgen of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-motor-engine/sportster-motorcycle-engine-conversions/38153-xr750-883-series-race-engine-venting-etc/page9?t=75740&highlight=reed+valve&page=9)) \\
 Sweating oil from the cylinder base gaskets and rocker boxes. \\ Sweating oil from the cylinder base gaskets and rocker boxes. \\
 As well as the push rod tubes and lifter blocs on the other side. \\ As well as the push rod tubes and lifter blocs on the other side. \\

Trace: • REF: Engine Mechanicals