User:Wdl1961/Rod/Stroke ratio

Rod/Stroke ratio
The rod/stroke ratio is the ratio of the length of the connecting rod to the length of the piston stroke. A longer rod will reduce the sidewise pressure of the piston on the cylinder wall and sress forces increasing engine life. It also increases cost and engine height.

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Rod/Stroke ratio
5454 The rod/stroke ratio is the ratio of the length of the connecting rod to the length of the crankshaft's (or piston's) stroke. An increase in the rod/stroke ratio (a longer rod, a shorter stroke or both) results in a lower piston speed. A longer rod (and consequently, higher rod/stroke ratio,) can potentially create more power, due to the fact that with a longer connecting rod, more force from the piston is delivered tangentially to the crankshaft's rotation, delivering more torque. A shorter rod/stroke ratio creates higher piston speeds, but this can be beneficial depending on other engine characteristics. Increased piston speeds can create tumble or swirl within the cylinder and reduce detonation. Increased piston speeds can also draw fuel-air mixture into the cylinder more quickly through a larger intake runner, promoting good cylinder filling.

Rod length and stroke length are independent variables. Rod length is expressed as center-to-center (c/c) length. An engine with a particular stroke can be fitted with rods of several c/c lengths by changing the piston pin location or block deck height. A rod that is longer in relation to stroke causes the piston to dwell a longer time at top dead center and causes the piston to move toward and away from TDC more slowly. Long rod engines with a particular stroke also build suction above the piston with less force, since the piston moves away from TDC more slowly. Consequently, long rod engines tend to produce a lower port air velocity, which also reduces low speed torque. Long rods place less thrust load on the cylinder walls, thus generate less parasitic drag and result in less frictional losses as engine revolutions rise. A "short rod" engine has the opposite characteristics. “The short rod exerts more force to the crank pin at any crank angle that counts i.e.--20° ATDC to 70° ATDC” (Jere Stahl ). Short rod engines tend develop more torque at lower engine speeds with torque and horsepower falling off quickly as engine RPM rises to high levels. Long rod engines generally produce more power due to reduced engine drag, especially as engine RPM increases. Regardless of rod length for a given stroke, the average piston speed (usually expressed in ft/s or m/s) remains the same. What changes as the rod length becomes shorter or longer in relation to the stroke, is the RATE of motion as the piston rises and falls in relation to the crankshaft. A long rod fitted to a given stroke generates less stress on the component parts due to the lower rate of acceleration away from and toward TDC. The average piston speed is the same; however, the peak piston speed is lower with long rods. A "square engine" is an engine with a bore diameter equal to its stroke length. An engine where the bore diameter is larger than the stroke length is commonly known as an oversquare engine; such engines have the ability to attain higher rotational speed since the pistons do not travel as far. Conversely, an engine with a bore diameter that is smaller than its stroke length is known as an undersquare engine; such engines cannot rotate as quickly, but are able to generate more torque at lower rotational speeds.