Break-in question

[Baldeagle]

It seems every car manual states that unvaried, steady rpm (constant highway speeds) is bad for the engine during break-in. Does anyone know why? Does it damage the engine? If yes how? I can understand how heavy loads and high rpm can cause “rough edges” to scratch metal to metal contact points during break-in. But low load operation, like with steady rpm on the highway seems like it should permit a gradual wearing of rough surfaces. I’d appreciate any response. Thanks.

[OrbitalEllipses]

You couldn't ask in this thread?

[Baldeagle]

I thought about it, but my question is not about break-in procedure. It is more technical. It is a “why does this happen question” and not a “what should we do question.” To be candid, that other thread was like most forum break-in discussions 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.” I had hoped for a response based less on opinion and more on fact. It seems there are several knowledgeable members in this forum who may know the answer.


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

[Zgrinch]

Originally Posted by Baldeagle

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.

No offense, but to post a new thread on a subject that already exists because you don't want your question to get lost is a problem. What if everyone did this?

The forum would be more of a zoo than it already is with the constant new thread posts. Maybe a better approach would have been to post it the proper thread and see if you got a response, if you didn't get a response, bump it up again, and at that point, if no one responded, then maybe what you pulled here may have at least been understood. I hope you can understand why keeping new threads to a minimum is important.

Anyway to your question, which I will admit I am not an engine expert, mechanic, etc...but it's not really that constant speeds are bad, rather loading the engine at various RPMs ensures piston rings and valvetrain components seal properly.

Hopefully one of the mods will merge this into the break in thread where it belongs.

[Baldeagle]

Okay, I apologize to the forum for my redundancy. It won’t happen again.

So based on your response, 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.

[GMU-BRZ]

Glad you posted this question, I was wondering too. It's gonna take me forever to get to 1000 if I'm avoiding highway cruising speed trips.

[mrtodd]

Originally Posted by Baldeagle

...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.

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.

[Subaruwrxfan]

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

[meWant]

Originally Posted by Subaruwrxfan

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

use the navi to find an alternative route that doesn't go onto a freeway

[Baldeagle]

Originally Posted by mrtodd

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.

Excellent response. Thank you.