If you read the old factory-recommended break-in recommendations for NEW engines, the issue of "no full power" and no speeds above 60mph ARE operative as the various moving parts learn to get along with each other AND the green block cures with the heat cycles.
After the initial, relatively-easy miles, then progressively higher road speeds are allowed. Plus some "passing gear" applications while cruising to help things along. Until by the end of the 500 miles, all of the break-in procedures are no longer needed.
One caution, through all of this, is "steady-state cruising". Reason is that a variety of rpm levels are needed, which relates to manifold vacuum levels. Just as moderate accelerations are recommended to "break-in" the thrust surfaces of the pistons and bearings, ALSO somewhat equal levels of "coast" times to increase the manifold vacuum levels to pull oil up into the ring and piston skirt areas, so the rings and such do not run dry, by comparison. This way, both orientations of power and coast sides of things get equal attention. Even in such cruise conditions, the speed variations can be as little as 5mph up and down, which normal traffic patterns might do anyway. Even using the cruise control can have varied loads with the same speed (in hilly areas). But running at the same speed on level terrain is not desired.
IF you live in a somewhat hilly terrain, the power and coast functions tend to come naturally, as you apply a bit of throttle to go up hills and then remove throttle to coast down the other side, then regain cruising throttle on the flat spaces between the hills. All within posted speed limits.
Consider, too, that each new car engine has already gotten 30 minutes of run time at the engine plant before it was shipped to the car assy plant. Which can be one reason that when the formal break-in period is not followed, really bad things tendee not to happen, in most cases. Which also usually did not result in failure as the salespeople told new owners to "Drive it normally, like you usually do."
Contrast this with what I discovered in a "Popular Mechanics" comment about breaking-in a REBUILT engine. The "break-in" period is mainly concerned with the piston rings rather than "everything", it stated. Of course, new bearings and such, with (usually, back then) re-using the old pistons. Much could also be related to the rings being used. I recall some softer rings being termed "quick-seater" rings, for example, by the manufacturer. But as they would wear-in quicker, their total durability could be less than the harder chrome rings, I suspect. A LOT depended on the slickness of the honed cylinder wall surface, too.
One local mechanic (with a decent rebuild record) said that when the engine was first started and the cam issues taken care of, the first thing he would do is go out "and get the engine hot". Which meant some heavier throttle applications in the mix. Then after about 30 minutes, let things cool down, change the oil, and hand the keys to the customer. Done. This was with a "rings and bearings" type of "overhaul". If the pistons had enough wear to "make noise", then a machine shop was needed.
As common as a honed cylinder wall surface might seem generic, it is an ever-progressing "science" as "plateau honing" is the latest advancement. Using a computerized honing machine and different abrasive types and grits to make it happen and unlock horsepower in race engines. Not to forget the thinner rings helping power, too, due to decreased drag on the cyl walls. Google "plateau honing rottler" to find some YT videos on this. This technique can allegedly make 100+ more horsepower in a Winston Cup engine, according to Lake Speed, Jr.
Personally, the "power and coast" orientation makes sense to me. Gradually applying more power as the miles/run times increase. With a CURED block, the machining will "stay where it is" and not "move" as can happen with a green, uncured block. Which is why Grumpy Jenkins preferred "cured" blocks to build his race motors with. "Move" is probably measured in .001" rather than anything larger, I suspect. When building a top output high-load race engine, such things can be important for max power production. On a street engine, not quite so much.
Still, great machining on a cured block, with high quality parts and "better-than-they-was-back-then" lubricants, the rebuilt motor should run longer than the original motor did, I suspect.
Enjoy!
CBODY67
