How much HP can magnum rods take happily

As I understand it, the rod bolts fail more due to high RPM operation and the resulting inertial loads, rather than from things like boosted power. 6000-6500 RPM is juuuust getting to the point where the bolts get stressed, IMHO.

Locomotion is right... it's the strength of the materials that is the basic improvement of the H-beam over the stock rod; yield strength of 4340 steel is almost 50% higher than 5140. It is also the H shape versus the stock rods I shape. The H is fundamentally stronger for straight bending forces like exerted on a rod. So you can't go just by appearances.