CHT Voodoo

Advice about cylinder head temperatures seems to me to be an area rife with voodoo. On page 143 of the new edition of Fly the Engine, I quote from a recent Lycoming service publication (SSP-400, for Piper Mirage owners) that says this about CHT and cylinder life:

No matter what approved power setting is used, cylinder head
temperatures should not exceed 435 degrees F in level flight
cruise. For optimum service life, cruise cylinder head temperatures
should be maintained below 400 degrees. [emphasis added]

This bit of advice can be traced to statements made in the O-290 manual going back 50 years. I've never seen or heard of any substantiating evidence for it. It's hard to imagine Lycoming running engines for the thousands of test-cell hours of testing that would be needed to gather statistically meaningful data to back such a claim up. If they're relying on "data" from engines in the field, how good could that possibly be? (Think about it. How accurate is the average CHT system? How much CHT data could owners be giving Lycoming?) And why 435 degrees F (224 Celsius), in particular? What's magical about that number? And why, also, 400 F? The only thing magical about 400 is that it's a conveniently round number. It's the same as saying you should keep your cylinders below 859.7 degrees Rankine.

I strongly suspect this decades-old advice is based either on no data, or poor data. I intend to look into it further. Meanwhile, if anybody knows the real story on this, please write to me at fly.the.engine@gmail.com. It's time aviation got past the voodoo stage.