I advocate breaking in any new or rebuilt engine hard. It's not so much necessary with a new OEM engine or a block that has been properly plateau honed, but every engine with cylinder walls and piston rings needs a bit of break-in. Yes, rings come pre-lapped and cylinders plateau honed, but a "perfect seal" between the compression rings and the cylinder walls doesn't exist until x number of revolutions have been completed by the engine. It takes much longer on a rebuilt engine with rough cylinder walls and raw-faced rings.
I won't go into the specifics of the procedure on a rebuilt or new race engine, but for a new car it's best to take it for some hard pulls with lots of engine braking as soon as you have the opportunity to drive off the lot without dealer supervision. Our Z had 23 miles on it on the lot - probably one or two test-drives - but not too late to do a proper break-in!
We declined a test-drive and took it straight to a nice strip of private road to do a few pulls through the gears. You should start out at about 1/2 throttle, up to 5000 rpm or so, then start getting more aggressive with each set of pulls. Always downshift and use engine braking to slow the car down during this phase. It helps keep the cylinder walls from getting glazed by keeping oil from sitting on the cylinder walls during combustion @ idle. The engine braking action actually forces the piston rings out and scrapes as much oil as possible off the cylinder walls. Never let oil temperature get too high during this phase - you must let the engine cool off if too much heat accumulates. It should only take a few sets of pulls to establish a good ring seal... it's most important not to let your engine idle or sit at the same load or rpm for a long time. For the first few heat cycles of the new engine, you should make sure to do some aggressive pulls.
The purpose of this is to basically file down the peaks of the cross-hatching on the cylinder walls (some decent info and illustrations here:
Stealth 316 - Engine Break-In) and create a proper surface finish on the rings. The depth of the cross-hatch "valleys" must be correct to allow proper oil retention without causing glazing (where oil ends up getting coked to the cylinder walls by the extreme heat of combustion).
Anyway, that's some basic info for you and how it applies to a new OEM engine. It's not critical to do this to a new OEM engine, but it definitely will ensure a better seal than if you hadn't done a similar break-in procedure. And contrary to popular belief, the only thing done to a car similar to the Z at the factory is a brief idle and "safety" run up/test to make sure all of the critical components of the car are in order. They do
not stick every engine on an engine dyno to measure its performance or to break it in. This is done for most sport bike engines
AFAIK, but not many car engines.