How do manufactures determine crankpin spec?

[Fordiesel69]

Not everyone will be familiar with this engine line, but lets use the Kohler K series single cylinder small engine. This question will apply to car/truck engines as well.

On the 12HP to 18HP models, the crankpin diameter & width is identical and rod is the same.

What I have noticed is that with proper maint, the high HP models have more wear on the crank in comparison to the smaller models.

So why would a manufacture think it is OK to increase HP ratings without increasing the surface area on the crankpin to support those greater loads?

Secondly, is there a calcuation the manufacture can use to determine how to size it?

 

[440Magnum]

The same can be seen in car engines. Back in my glory days of car tinkering, I frequently saw faster wear on Mopar 440 rod big-end bearings than on the 383. Main bearing wear was about the same, because the 440 got bigger mains, but used the same-size rod big-ends as the smaller 383 (and 361, and 400, and 413, and 426...). See the smallblock Chevy family... repeat conclusion.

I don't know how the determination is made *exactly*, but I can guarantee that a lot of it is more practical than optimal. Sharing a manufacturing line for common parts, sharing a single rod bearing size (and stepped undersizes) for all replacements, etc. are all huge money considerations when millions of copies are being produced over the years. From a purely engineering side, you'd have to consider the peak and averages forces on the crank pin, the structure of the crank (including the material), the bearing surface area, normal service life expectancy, and assorted safety margins. There's a very wide range in there where most consumers will never know or care whether their particular engine has an optimally sized crank or one that's overkill for the application.

 

[Kuato]

440's got it right; it's money, and common manufacturing.

Sea-Doo (tm) had an issue with engine failures several years back., they increased the piston size and hp without changing anything else. (At the time my brother was a mechanic working on them, should have heard him complaining!)