Quote:
Originally Posted by nikitopo
Manual instructions are not only for the brake-in of the engine, but for the brake-in of ALL the parts of the car.
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^^This^^
Will you destroy anything by driving hard off the lot? No.
But as other have mentioned, it is not just the engine you are breaking in. Literally every part of the drive train benefits from a break in - as well as the brakes. It is not like the old days of flat tappet cams where you could destroy the engine, but almost all friction interfaces benefit from heat cycling. Things like differential gears won't fail, but might be a bit noisier if abused before heat cycling.
Do professional racers break in engines? No. But they also tear down their engines regularly, and they also pay far more attention to clearances than a factory assembled engine.
Lack of a break in probably won't contribute to component failure within warranty in a modern car, but many manufacturers actually enforce break in with their ECUs so it must be measurable over a large enough population.