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techtalk:ref:engmech07e [2020/10/02 22:05] hippysmack [Further Study of Internal Engine Pressure] |
techtalk:ref:engmech07e [2023/12/26 21:28] (current) hippysmack |
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| If anything they freed up the crankcase by allowing oil and air to expel to different holes. \\ | If anything they freed up the crankcase by allowing oil and air to expel to different holes. \\ | ||
| - | All HD cases already pull a vacuum especially highest at idle. That vacuum just can't get too high. How high is the question. ((Hippysmack of the XLFORUM http://xlforum.net/forums/showthread.php?t=2077771&page=6)) \\ | + | All HD cases already pull a vacuum especially highest at idle. That vacuum just can't get too high. How high is the question. ((Hippysmack of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/200736-79-85-breather-vent-mod-adding-a-secondary-vent-hose-at-6-00-position/page6?t=2077771&page=6)) \\ |
| At present there is little information known regarding how much vacuum Ironhead Sportster engines create. \\ | At present there is little information known regarding how much vacuum Ironhead Sportster engines create. \\ | ||
| The only way to know for sure on any specific engine is to test it manometer and see where it's at. \\ | The only way to know for sure on any specific engine is to test it manometer and see where it's at. \\ | ||
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| * Better oil scavenging and keeping down pumping loss at high RPM. \\ 76< engines breath and scavenge from the same hole. \\ They may be more sensitive to wetsumping than later models because of this. The air can't jump ahead of the oil to the rotary breather. \\ Oil is thicker and doesn't compress. What does that do to air pressure behind it? \\ 77> engines breath and scavenge from different holes. \\ Makes for a more free flowing internal system. | * Better oil scavenging and keeping down pumping loss at high RPM. \\ 76< engines breath and scavenge from the same hole. \\ They may be more sensitive to wetsumping than later models because of this. The air can't jump ahead of the oil to the rotary breather. \\ Oil is thicker and doesn't compress. What does that do to air pressure behind it? \\ 77> engines breath and scavenge from different holes. \\ Makes for a more free flowing internal system. | ||
| - | We know downstroke pushes oil toward the oil pump. \\ | + | We know downstroke pushes oil toward the oil pump. Excess vacuum slows that process, can back up oil and cause wetsumping. \\ |
| - | Excess vacuum slows that process, can back up oil and cause wetsumping. \\ | + | |
| But, if the underside of the pistons are slight negative, it helps the rings seat without as much vibration and turbulence. \\ | But, if the underside of the pistons are slight negative, it helps the rings seat without as much vibration and turbulence. \\ | ||
| - | As a rule, rings sit on their own springiness when pistons go up and are forced against the wall by the combustion pressure that is allowed to pass behind them. ((thefrenchowl of the XLFORUM http://xlforum.net/forums/showthread.php?t=2077771&page=6)) \\ | + | As a rule, rings sit on their own springiness when pistons go up and are forced against the wall by the combustion pressure that is allowed to pass behind them. ((thefrenchowl of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/200736-79-85-breather-vent-mod-adding-a-secondary-vent-hose-at-6-00-position/page6?t=2077771&page=6)) \\ |
| Hence, a very small or inexistent amount of blowby should occur on combustion stroke. \\ | Hence, a very small or inexistent amount of blowby should occur on combustion stroke. \\ | ||
| A small amount of blow by is only possible on the way up on exhaust stroke if vacuum is too high underneath pistons. \\ | A small amount of blow by is only possible on the way up on exhaust stroke if vacuum is too high underneath pistons. \\ | ||
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| In contrast, the 77-up breather closes when the last piston is at bottom dead center (BDC). \\ | In contrast, the 77-up breather closes when the last piston is at bottom dead center (BDC). \\ | ||
| - | Is that extra vacuum taxed to the piston? | + | Is that extra vacuum taxed to the piston? \\ |
| The better the gaskets and seals, the more vacuum you can run. \\ | The better the gaskets and seals, the more vacuum you can run. \\ | ||
