Race Engine Rod Stroke Ratio Calculator

Race Engine Rod Stroke Ratio Calculator


Rod stroke Ratio

At 18000 RPM Formula One engines with an RSR of 2.56 achieve peak piston speeds of over 7000 feet-per-minute (FPM).  A NASCAR Cup engine piston speed reaches 8800 FPM at 10000 RPM with an RSR of 1.91.The F1 numbers are due mainly to the engines’ short strokes.




It is pointless to address the length of the connecting rod in any engine, racing or otherwise, without looking into the ratio of the length of the rod to the length of the stroke of the crankshaft.  This rod stroke ratio (RSR) is calculated by dividing the rod length by the stroke value. What is more, the piston pin location in the piston also affects the result of the calculations. Moving the pin closer to the thrust side causes the piston to acquire shorter rod characteristics. Conversely, when the pin is moved toward the non-thrust side of the bore makes the rod act longer.

Using the rod/stroke calculator and piston pin location calculator along with a modern advanced personal computer based engine development software unlocks a plethora of ideas and results that can be used to improve performance, increase BMEP, torque, power endurance and reliability.

Rod Angle

The rod to stroke ratio affects the angle at which the connecting rod applies force to the piston. A shorter rod will create more pressure on the piston skirts but it will have a smoother acceleration and deceleration curve at TDC and BDC.

At piston speeds above 5000 FPM, pistons must be equipped with forced pin oiling slots to cool the pins, the rings, and the pistons. This load increases at the square of the speed, and at higher speeds the rings, pistons, and pins receive less lubrication from the spinning crankshaft. Reduced oiling can result in overheated piston tops and skirts as well as the rings and pins.

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