Originally Posted by Baldeagle (Post 165111)

I thought about it, but my question is not about break-in procedure. It is more techncial. To be candid, that other thread was general and digressed to opinions about “drive it hard verses follow the manual,” “break-in oil verses synthetic,” “how long should you keep the break-in oil in the engine,” “drive hard during the first 50 miles to set the rings or not.” All those topics have merit, but I didn’t want my question to get lost or overlooked. I had also hoped to get a more technical response. It seems there are several knowledgeable members in this forum.


And by the way, it seems you have no idea why steady speed is bad either. Do you? That makes two of us.

Originally Posted by Baldeagle (Post 165115)

...it seems that constant speeds imply no-load, which is bad for setting rings. Does this imply no break-in occurs at low load? If yes, that is an interesting thought.

Originally Posted by Subaruwrxfan (Post 167661)

The manual doesn't say not to drive on the freeway, it just says don't keep the engine at a low rpm. I'm pretty sure if I vary my speeds between 55-85, along with merging on and off, making passes, going up and down hills etc it will vary the engine speed enough to be fine. What do people do that buy a car and have to drive it home? What if the freeway is the only way to get home? lol

Originally Posted by mrtodd (Post 167655)

You sir, asked a good question. During all my engine building and time spent on my previous forums/talking to other builders this logic seems to be very sound. Consistent RPM implies that engine load (which in turn is consistent with cylinder pressures) is relatively low - especially in a flat freeway type of scenario.

The fact that some manufacturer manuals state not to drive on the freeway during the break-in period implies that the rings aren't fully seated out of the factory. Running your car like this will smooth out the 'peaks' on the cylinder walls without grinding much off the contact surface of the piston rings... In the end, your piston rings will only be partially broken-in (ie not filed down to fit the cylinder walls perfectly) when compared to a person who has driven through the neighborhood with varying load conditions.

You seem to already understand that the gas pressure in the cylinder is what drives the compression rings to be forced against the cylinder walls, but here is a simple analogy anyway. Keep in mind that this analogy isn't perfect, but for some this may help to understand the basic concept of what's going on the cylinder walls in a brand new engine. Let's say you have a piece of wood and only one piece of sandpaper. If you sand lightly and gently, eventually the sand paper will become worn out, and you won't be able to remove anymore material off the wood. A decent amount of wood will have been removed, though, meaning the sandpaper did it's job for the most part. If do you the same procedure but sand with substantially more pressure, you will remove even more material off the wood before the sandpaper wears out. Now, the difference in material removed this situation would probably be minimal, which is perfect, because that's exactly what you want during engine break-in. You only have so much time before the rough 'peaks' of the freshly honed cylinder wall wear out and there will be no more friction (relatively). If you really FORCE those piston rings into the cylinder walls (again, relatively) then more material will be removed from the rings themselves so that they will fit the entire 360 degree surface of the cylinder even better, creating a better seal. The difference between poorly broken in rings and properly seated rings would be in the ballpark of less than one thousandth of an inch at different spots of the contact surface between ring and cylinder wall. The physical difference may be tiny; however, the ring seal is substantially better.



--So, to answer your question, yes the reason behind no freeway driving is to ensure proper ring seat by avoiding a constant low-load environment.