Some Reasons Why Piston Speed is So Important
Things inside your engine are a lot more complex than horsepower and torque ratings. Let’s look at a few reasons piston speed is one of the most critical.
Define Piston Speed
Piston speed— compromised of mean speed, maximum speed, and piston acceleration—can be measured with each serving as a different yardstick on engine performance. When you speak about piston speed it is very much like the speed of any other moving object, measurements are stated in miles per hour or meters per second. For the sake of this article, I’ll refer to things in mph to keep things consistent with our units of measurement.
Mean Speed
Mean Piston Speed is calculated based on RPM and is, therefore, is subject to change, though it’s typically measured at the engine’s maximum horsepower rating. As much as enthusiast like to brag about how high their engines rev, it is actually mean piston speed that is the limiting factor in performance as opposed to a sky-high rev limiter. This is the yardstick by which engines are measured and can tell you a lot about the performance bracket they fit into. Let’s look at how the numbers stack up.
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Small Increases Equal Big Differences
Increases in mean piston speed directly correlate to the performance level of an engine. Typical internal combustion engines will run around 35mph at peak power with high-performance street cars operating close to 40mph. Just 5 miles per hour can be huge when it comes to digits with Top Fuel dragsters seeing around 67mph. Higher mean speeds translate to more air pumped, but it’s never that simple with cars, is it? While this is just part of the equation it’s good to have some numbers should you hear someone comparing engine specs in the future. Anything over 40mph in the mean piston speed department is getting serious.
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Acceleration and Maximum Piston Speed
Piston speed changes during the length of the stroke with peak velocity having a big impact on reliability. Forged alloys are chosen to combat the violent onslaught of force that each component will feel during the combustion cycle, but the weight is just as important as the material itself. This is because as your piston travels in one direction it must eventually slow down to travel back up (or down). By keeping materials light it puts less stress on things like the wrist pin in a similar way that a lightweight car puts less stress on brake pads. From micro to macro, piston speed teaches you that many engineering principles are true whether you’re discussing an engine or braking system.
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Real World Implications
Pistons are constantly accelerating and decelerating under extreme forces as they move throughout the combustion process. Knowing this, we can choose the proper pistons, connecting rods and wrist pins to ensure the engine lasts as long as possible with minimal wear. It also helps in understanding what you can rev an engine to safely, based on the forces involved. A good engine builder will know these things and calculate them, so be sure to ask if you are considering cracking open an engine for a full build. Mean piston speed and the forces involved during the compression and power strokes are vital to having a safe, reliable and fast platform that you can enjoy at any race track in the world. Thanks for reading.
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