Differences
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techtalk:ref:engmech07b [2020/10/02 07:26] hippysmack created |
techtalk:ref:engmech07b [2020/10/02 14:16] (current) hippysmack |
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| [[:techtalk:menu|{{ :techtalk:gototechnicalmenu.jpg|}}]] | [[:techtalk:menu|{{ :techtalk:gototechnicalmenu.jpg|}}]] | ||
| - | ====== REF: Engine Mechanicals - Sub-07A ====== | + | ====== REF: Engine Mechanicals - Sub-07B ====== |
| ====== Affects of the 45° Rod / Piston Arrangement ====== | ====== Affects of the 45° Rod / Piston Arrangement ====== | ||
| + | Piston / rod positions are relative to positive or negative pressure in the crankcase. | ||
| + | Forget about valve opening for a second. | ||
| + | |||
| + | - With the piston rising up, | ||
| + | * The top of the piston is compressing or expelling air pressure in the cylinder but bottom of the piston is pulling in air (vacuum) into the crankcase. | ||
| + | - With the piston descending down, | ||
| + | * The top of the piston is pulling air (vacuum) in the cylinder but the bottom of the piston is both expelling air vacuum and turning it into air pressure. | ||
| + | |||
| + | These two conditions create pulsating air in the crankcase from vacuum pressure (negative) to air pressure (positive). \\ | ||
| + | How crankcase pressure moves inside the engine does or doesn't contribute to wetsumping. | ||
| + | |||
| + | Intake vacuum is relative to piston / rod positions. \\ | ||
| + | An example with on carbed models with VOES / MAP: (although this is the same for all Sportsters) \\ | ||
| + | Vacuum is created in the cylinders which pulls vacuum from the carb. \\ | ||
| + | This vacuum is tapped into and used to operate ignition timing through the VOES / Map. \\ | ||
| + | So a vacuum gauge plugged into the vacuum line at the carb does reflect the position of the two pistons. \\ | ||
| + | I.E. the gauge moves when the pistons are on the way down and goes toward the resting position when the pistons are coming up. \\ | ||
| + | (of course, you'd then have to take in accountancy if the intake or exhaust valve was open at the time) \\ | ||
| + | So a gauge there would not reflect an absolute vacuum. \\ | ||
| + | It would reflect more of an average vacuum based on running conditions with all the change variables working on the air pressure simultaneously. | ||
| + | |||
| + | Some time and tune older V-8s with a vacuum gauge instead of a timing light. \\ For each piston going up, there is a piston going down. \\ | ||
| + | And intake vacuum stays (more) constant in the middle of the push / pull. \\ A vacuum gauge shows a more steady dynamic condition. \\ | ||
| + | Likewise are the forces in the crankcase (crankcase pressure). \\ | ||
| + | |||
| + | {{:techtalk:ref:engmech:sbc_350_by_hippysmack.jpg?direct&300|}} ((photo by Hippysmack)) | ||
| + | |||
| + | The rods \ piston action in the Sportster engine are close together (45° apart) and there is 360° around the flywheels. \\ | ||
| + | If the rods were 180° apart instead, one piston would be going up at the same time the other is going down. \\ | ||
| + | Just as in a V-8, crankcase pressure would be more equalized between them. \\ | ||
| + | |||
| + | Since Sportster piston movement is not equalized, we get the potato, potato sound we all love but the equilibrium in the crankcase is off by design. \\ | ||
| + | Take the PCV off the valve cover on the V-8 while running and it just smokes a little. Take the breather valve off the Sportster and oil pukes out. \\ | ||
| + | |||
| + | Likewise, a vacuum gauge on the carb vacuum line will be erratic and not a very useful test. \\ | ||
| + | Hook up a vacuum gauge to it and the gauge bounces from vacuum to no vacuum pretty wildly at idle. \\ | ||
| + | Crankcase pressure is doing the same thing. But with the addition of the breather valve, it is a more controlled chaos in the bottom end. \\ | ||
