Extremely oversimplified explanation follows.
The metal surfaces within the engine are not "smooth" if you look at them with a microscope. The surfaces have peaks and valleys. In the case of the piston rings and cylinders, the peaks and valleys prevent the rings from sealing as well as they could. Result - oil gets past the rings one way, and combustion pressure gets past the rings the other way. In the case of other contact points such as between cams and lifters, the imperfect fit between the components results in local high and low pressure areas.
During the break-in period, the highest points of these "asperities" get worn off, and the surfaces get smoothed out and "run in" to each other. Sealing improves, the engine stops leaking combustion gases out (resulting in slightly improved power as the running-in period is completed) and it stops leaking oil past the rings.
The best way to break in an engine is NOT what it says in most owner's books. The book wants you to take it easy, but run the engine at different speeds. The different speeds bit is okay but the take-it-easy part is not quite ...
Here's what I do on a freshly built race motorcycle engine ... starting out with cheap crappy 10w30 in the crankcase ...
1. Start engine, run at idle for 10 seconds, shut down and let it sit for a few minutes.
2. Start engine, run at idle for 1 minute, stop it and let it cool down.
3. Start engine, run at various speeds 2000 - 5000 rpm (max revs are 14,000 on this engine) until temp gauge gets to normal operating range (not more), obviously check for leaks or other problems at this time, stop it and let it cool down completely.
4. Start it up and drive it around at light loads and moderate engine speeds for a few minutes, stop and let it cool down.
5. First real drive: Get into 3rd or 4th gear. Go FULL THROTTLE for a short burst to about 2/3 of redline. Let it coast back to the original speed (not using the brakes), then FULL THROTTLE to 2/3 of redline again. Do this about 10 times, then increase by 1000 rpm at a time, until after 5 or 6 laps of the racetrack, I'm taking it right to redline at full throttle ... but not running it wide open all the time, as soon as it gets to redline in 4th (out of 6 gears) I upshift to 6th and cruise for a few seconds (generally until the next corner on the racetrack).
The engine is now broken in. 20 minutes (other than the cooling-off periods) and it's done. Come in, change the oil, put in Mobil 1, and have at it.
After an initial running-in at light loads, you want SHORT periods of full load to seat the rings, and you want to do this at gradually increasing engine speeds to make sure that the whole operating range of the engine gets covered.
'96 Passat TDI mit UPsolute
Well you shouldn't floor it off the showroom floor, but you shouldn't baby it either. One of the main reasons to break in a motor is to make the rings seat properly, also to imprint the lifters to the cams, help seat the bearings, etc. The point of breaking in a motor is partially to wear it in. Kinda like you would want to break in a pair of sneakers before you went into a basketball tournament. You want them to mold around your foot, and crease at certain areas that are specific to your foot. Thats why you shouldn't run synthetic oils during the break in period, they're too slick and dont allow the engine to properly wear.
I have no clue why C5s come with Mobil 1, maybe thats why they have more oil consumption issues than any other LS1 based engine
The key is varying the rpms, not going screaming to redline every chance you get but still allowing the engine to see 4K+ rpms after the first few hundred miles, avoid driving at a constant rpm for a long time, etc. Gettin into a new car and driving 500 miles on the interstate is a very bad idea.
OK, so if you take it easy during the break-in period, the engine really isn't broken in fully. So what's wrong with that? After all, when you eventually drive it hard later, then it'll break in then, no?
Or is there a break-in "window" that must not be missed?
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