[dkirk]

Chris – looking forward to your results.

Just a couple of comments on previous posts – column buckling on the Porsche connecting rod would be best studied by constructing a finite element model – the best way to examine the relatively complex shape. I don’t have the ability or training or software to do this. Volunteers?

Rod to stroke ratio – this is normally looked at as rod to crank throw ratio, commonly abbreviated as L/R ratio…L being con rod center-to-center distance and R being crank throw radius, which is stroke/2. L and R form the legs to what is known as the slider crank triangle. The equation describing this triangle allows calculation of piston position as a function of the crank throw angle, and can be differentiated a couple of times to get piston velocity and acceleration. Thus, the L/R ratio is the valued relationship. For the 3.2 L Porsche engine, the L/R ratio is 3.41. As a comparison, small block Chevy engines use 2 different rod lengths and a bunch of different strokes for various performance builds. These L/R ratios range from 2.87 (short rod – long stroke) to 3.80 (long rod – short stroke) so the 911 engine is right in the middle. Chris gave an example of the “ideal” rod to stroke ratio of 1.75, which converted to L/R ratio yields 3.50.

The lower the L/R ratio (short rod to stroke), the higher the piston acceleration at TDC (and lower piston acceleration at BDC) as compared to a higher L/R ratio (longer rod to stroke). Also, the lower L/R ratio means greater rod angularity at the mid stroke positions contributing to higher piston skirt side loads and frictional losses. At the other extreme, a high L/R ratio can make for an engine that’s not very compact and is typically seen on older engine designs that were undersquare (small bore – long stroke). The 3.50 L/R is a good compromise and the Porsche engine, being horizontally-opposed, is right on design using a 3.41 L/R as this keeps the package width at a minimum.

For our 3.2 L example, here are the numbers at the 8000 rpm red-line:

Piston Accel @ TDC – 3441.9 g’s (g being gravitational force)
Piston Accel @ BDC – 1882.4 g’s
Average Piston Vel – 19.8 m/sec
Max Piston Vel – 32.5 m/sec

None of these values are alarming for a high-performance automotive engine and indicate that there is built-in reserve that was designed into this beautiful machine.