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Normally car manuals say to let the car break in gently, not accelerating or breaking fast for first 1,000 km or driving below 80km/h. Is that really necessary in today's world? Is there any proof anywhere that this makes the engine run better in the long run?

Is there also any validity in the opposite approach "Hard break-in"? It's linked below, but essentially it's just heating up your engine and then letting it run hard for some time. http://www.mototuneusa.com/break_in_secrets.htm

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    The manual for my car just said to avoid driving at constant speeds for a long time for the first 1000 miles. – Sam I Am Jun 22 '12 at 13:44
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    Not sure if this is the case but I remember reading in an older maintenance manual that engines had burrs and other minor flaws to the metal due to the manufacturing process that were worn down the first couple hundred miles which is why they needed to be "broken in". – rjzii Jun 22 '12 at 14:19
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    @RobZ you still want to avoid constant speeds so as to "sit all the gears in" properly and not just one. This question has been answered at mechanics.se – vikki Jun 22 '12 at 14:51
  • @vikki - That pretty much is in line with what I read when I was searching around on the question. Even then though, it really seems to boil down to the engine and manufacture involved. – rjzii Jun 22 '12 at 15:01
  • Do modern owners manuals still talk about a need for a breaking in period? Mine doesn't. – DJClayworth Jun 22 '12 at 16:02
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Maybe, it really depends upon the type of engine you are talking about. If you do some searching around, the usual reason cited for the break-in period, as summarized by Wikipedia is that modern engines are built with higher tolerances than previous years which eliminates the need for the break-in period. However, on smaller engines such as ones used for snowmobiles or or motorcycles tend to recommend that break-in procedures be followed to allow for proper seating of the piston rings and/or cleaning out manufacturing contaminates.

With regards to the larger engines used in automobiles, it seems that the piston rings are once again cited as the common reason for the break-in period. That said though, the actual need for a break-in period seems to be dependent upon the engine itself and generally there isn't a need for one.

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    What is it about low speeds/low engine speeds that allows for proper seating of piston rings/cleaning of contaminates? Seems to me the exact same reason could be given for constantly revving the engine for the first few weeks, so the explanation seems lacking. – Oddthinking Jun 23 '12 at 1:12
  • @Oddthinking - I haven't really found much in the way as an explanation as to why low speeds and revolutions are better than high ones or revving the engine. Most explanations are fairly general or that it is dependent upon the engine and manufacture if it even needs to be done or not. – rjzii Jun 23 '12 at 15:31
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Every mechanical engine, gasoline, diesel or other (such as LNG) is made up of a multitude of parts manufactured to accepted engineering "standards."

These standards spell out, down to the smallest detail, what engineering criteria the finished part will meet. Quite frequently however these individual parts come from different factories and various countries. But because each manufacturer subscribes to the same engineering standards, a wrist pin manufactured in Korea will mate to a piston manufactured in India for example.

So far, so good. There is, however, the rub called "Manufacturing tolerances." Which are accepted as a necessity in the above engineering standards. Thus, the commercially produced wrist pin made above may be allowed to have a circumference + or - variance of say .0005" to .0009", it will still function for its intended purpose and meet the manufacturers requirement for MTBF (Mean Time Between Failure). The cost to get that same part to be dead-on perfect, with absolutely minimal deviation from its design specification would drive the cost much higher than the intended market (you and me) could afford.

That's one reason why the engine in a Lamborghini, Aventador Roadster costs so much more than the powerhouse of a Chevrolet Corvette: Manufacturing tolerances (i.e. variations from the design) in the Lambo are almost non-existent.

All of which Which leads us to: Breaking in a new engine! Let's say you could only afford the Corvette and not the Lambo, but you want your prized beauty to last a long, long time... What do you do? You break the darn thing in properly of course! Here's the engineering reason: All those parts in your new engine manufactured to that above mentioned +or- tolerance need to mate to each other (think of it as dating before marriage). Every part, commercially machined has microscopic hi and low spots on its surface called "asperities." These asperities need to rub against each other under load in order to literally break off the high spots and bring the load bearing surfaces closer to the ultimate goal of 100% contact. The other element is called "wear hardening." Piston rings, cylinder walls and camshafts along with many crank shafts in most non-exotic, passenger vehicle engines are made from nodular cast iron. IF these cast iron parts are allowed to break in properly, they will develop an extremely hard, long lasting wear surface. It may only be .001" to .003" or so thick, but it will stand up to many years of use.

This all happens on a microscopic level of course. But I guarantee you that, as they came from the factory, the rings to cylinder wall contact and the cam lobe to lifter contact etc on your new Corvette were NOT optimal for (a) full performance and (b) engine longevity. Breaking it in carefully will, however accomplish this.

Recommended break in procedures vary by manufacturer, but essentially it boils down to a few basics: (1) NEVER rev a cold engine! (2) Vary your speeds for the first 500 miles and don't maintain a constant speed (like 60 MPH) for extended periods. (3) In the first 500 miles, accelerate evenly and allow your engine to coast frequently (i.e. coast from 60 to 50 foot off he gas). This creates a high vacuum in the combustion chamber and pulls more fresh oil to the rings ad cylinder walls. (4) Change your oil and filter after the first 100 miles and again at 500 miles. (5) Only use high quality oil, preferably synthetic. I've rebuilt many types of engines and some of them exceeded 150,000 miles on that rebuild. Break an engine in right, treat it good and it will last you a long time.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    Welcome to Skeptics! This is an answer based purely on a theoretical model. We expect answers to be based on empirical evidence rather than speculative predictions. Please edit it to add references to empirical data. – Oddthinking Oct 25 '13 at 1:26

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