Rather than bending moment would it be better to look at column buckling? The non-uniform cross section top to bottom complicates it. Re. ignition, twin plug, the perfect fuel would allow ignition timing at say 1 deg ATDC with max cylinder psig at 10-20 ATDC. No combustion pressure before TDC so no "negative work". Maybe about 40% H2 in air, has about five times the flame speed of gasoline in air I think. Anyone want to try it:).

They probably do, but not the rod bolts.

Porsche engines has another thing working against them, rod to stroke ratio. The shorter the rod, the higher the piston speed and piston acceleration.

Dave,

We should have our model finished by the weekend - have just been quite busy the last couple of weeks.

The 'ideal' rod ratio is normally considered to be 1.75:1.

You sure about that?

Here are the numbers.

BBC - 1,625,600 mm's traveled at 8,000 RPM
3.2 - 1,190,400 mm's traveled at 8,000 RPM

Can't look at just rod length, it has to be compared to stroke.

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.

What does the speed equate to into compressive and tensile forces?

Speed does not equate to compressive or tensile forces.

Force = mass x acceleration.

The applied stress is the result of the applied force divided by the cross sectional area of the component of interest.

I imagine piston speed is a concern when investigating the shear forces at the cylinder/oil film and reducing friction.