When I was starting this stuff the old timers claimed "Break it in like you'll drive it".

That's pretty much what I've always done.

My last new Challenger actually had a different tune in it for the first 500 or so miles, and started running noticeably better after 5000.

My new bikes the dealer always says 500 easy miles before pinning it.

A lot of the "break-in" logic is from the old days when fuel systems and ignitions were not very precise like today. Back then a properly broken-in engine would last 100K miles if you were lucky. People also did not drive like today. Even in the '80s a 100K car was considered worn out. Today it's considered low miles.

Today, you can beat the crap out of one and as long as you don't overheat it get 200K like nothing.

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Official Cookie and Mater Tormentor.

Note that I do not shy away from WOT on engines during break-in (and off-the-gas decal to create high vacuum), but I do limit RPM. Regarding the Mototune guy's argument, WOT at lower RPM should give you highest cyl pressures. I had not seen that argument before for higher cyl pressure, but it could be right.

Dennis is right about mixture and timing being FAR better controlled today than in the 60s-70s, so that removes some crazy variables.

Again, probably none of this makes too much difference, but...

Lou

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Home of Slant6-powered fun machines

exactly,
get the machining/clearances and assembly process right and regardless of the break in process it will be good
get screwy machining/clearances and poor assembly process and no amount of care during break in will fix it.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8