We all know the manufacturer's recommended procedure. 600 miles, no constant speed, go up and down through the gears, etc.
And then there are those that insist that method encourages improperly bedded rings with premature compression and blowby issues to appear later on down the road.
And those folks are the ones that usually tout the 'dyno break-in' or something similar.
1. Get the engine up to operating temp
2. Run it up through the gears up to 60 per and back down again.
3. Let it idle for a minute to stabilize temps
4. Shutdown and let cool completely
5. Repeat steps 1-4 twice more. Heat and cool cycling is key.
6. Start, let oil press. stabilize
7. Run hard up through gears to redline as fast as you can shift.
8. Steps 3-4.
Anything internal that had improper clearances or torquing will have reared its head at this point and rings and valves will be firmly bedded in their sleeves and seats. Engines broken in with this technique typically will make more power and less blowby/leakdown.
Lets Go Racing!!
I bought an '87 5.0 H.O. Mustang LX from Cal Worthington when they first came out. It was a demo model that had been flogged repeatedly on test drives by different drivers. To say it had a hard 'cycling' break-in would be an understatement. That car in stock form would do 150mph, and would pull away from buddies five-o's that were stock as well. I put 125,000 HARD miles on that car, a lot of it a 1/4 mile at a time, ran a full syn oil from break-in. Changed every 3k with a double filter change.
The conn rod bolts finally stretched from being banged against the limiter 10 zillion times, the only weak link in this motor. We tore that motor apart and found ZERO wear, everything a nice golden hue, no carbon.
Im not saying Im breaking in MY B this way......