Increasing the stroke, if the displacement must remain unchanged, decreases boring. And then also the space available to the valves. This means that the ducts, in turn, have a smaller diameter. This translates into more theoretical average speed of the air is sucked into the engine at constant rpm. The best one for the engine breathing in this case is achieved by a system lower than what would occur if the valves were larger.It is correct to speak of speed “media” and “theoretical” because in fact the motion of gas in the ducts of reciprocating engines is the button. The speed varies continuously and one to which, for convenience, reference is made is an average value obtained by a simple calculation.
Since the diameter of the duct is not constant throughout its length, again for reasons of convenience in the calculation we refer to the restricted section (at the inner part of the site or immediately upstream of it) or, less frequently, the transition from valve seat and when it is at maximum lift. The theoretical average air speed (downstream of the throttle body, strictly speaking we should talk about air-fuel mixture) is proportional to the average piston speed, which varies only for a given race in the function of the rotation.
The curve indicates the trend of the volumetric efficiency as a function of engine speed rises gradually, reaching the maximum value (at a certain velocity in the pipe), after which begins to decline.By increasing the speed of the gas column increases its inertia, and this is advantageous, from a certain point onward, however, continuing to increase the speed, the pressure drop becomes higher and higher, hitting the engine breathing in size gradually growing.
In fact the situation is more complex, and there are other factors at play (obviously stands out as the “game” of the waves), these considerations are only schematic and rough, but certainly indicative. This also explains the fact that the optimum value for the theoretical average speed of the intake air, is not strictly fixed, but varies depending on the circumstances, while remaining within a relatively narrow field.Then there were also other considerations, to do beyond that order fluid dynamics, speaking of the relationship stroke / bore.At this point, some numbers. As an indication, in modern high performance engine power
Valve seats
maximum is obtained with an average speed of air in the order of 80 to 110 meters per second (ie, the maximum torque occurs with lower speeds, but in several cases the difference is not much). To get an idea of ??what happens inside of the intake, just think that 100 m / s corresponds to 360 km / h. Therefore, the columns of gas bearing positive and negative accelerations impressive, which occur very rapidly (at 12000 rpm each valve opens and closes 100 times per second!).
The very short stroke engines, where the bore is large, can deliver higher speeds, with the same average speed of the piston, and thus with similar mechanical stress than those with a C / D higher ( or with more travel, subject to capacity).They also allow the installation of larger valves, and then obtaining the best volumetric efficiency at a very high speed. This results in a higher specific power. In fact, race relations / bore very low that characterize the motor sport (not to mention those racing) are adopted for this reason. Therefore the valves are mounted always larger than is physically possible to install, or nearly so. However, if you endow a short-stroke engine valves as small as those taken on a long stroke of equal displacement, and architecture and construction characteristics quite similar, it would be possible to get a pair on the whole similar, and as much as the maximum performance of the curve!
Finally, we put in other words the question: Is it true that for most mid-range torque, regardless of the displacement, it is necessary to adopt a report stroke / bore taller? In this case, the answer is no!
