Re: Advice on breaking in process
Everybody has different "theories" on this. If you search for MDS V8 Break-in I asked and got some good responses. What engine do you have?
I hate to make a lengthy post, but someone sent this to me a while ago from a BMW forum. I used it because it was analogous to what the Jeep Owner's manual says: "... short bursts of full acceleration at low RPM's..." I followed it pretty religiously except I changed my oil at 600 miles and will do it again at 1500 miles. I have the Hemi, so I used auto stick to prevent the Eco mode from forcing the engine into 4 cylinder mode for the first 300 miles. I didn't like the idea of having the engine running on half cylinders during its break in.
Take this for what it is and use what you are comfortable. I didn't write it but I liked the logic so I went with it. Sorry for the long read. Good luck and enjoy your ride!
The primary goals of engine break-in are: 1) achieving a good seal between the piston rings and cylinder walls, and 2) allowing the engine to operate correctly throughout its RPM range. The major enemy during the break-in period is localized heat buildup, mainly in bearing surfaces (most notably the crankshaft bearings).
Initial state: When the engine is machined at the factory, many wearing surfaces (places where parts rub against each other – cylinder walls, bearings, etc) are purposely machined more roughly than they could be. The reason for this is that it allows the engine to complete the machining/polishing as it operates, thus allowing for the individual variations inherent in any manufacturing process. This wearing process, when complete, produces parts which will fit together with very tight tolerances. However, the process also involves a great deal of friction, which in turn means a great deal of heat. As metal parts heat, they expand slightly. If the expansion goes beyond a certain point, the parts will tend to bind with and/or score each other. This must be avoided.
[To put this in plain English, the parts which rub against each other are left a bit rough, and as the engine runs the parts will scrape against each other until they wear down a bit and have a proper fit. While they're still in the process of scraping, they can get very hot; if they get too hot, they will damage each other in a permanent way.]
Since this sort of heat buildup is very localized, it will not show up on the engine temperature gauge. Therefore, it is important to operate the engine in such a way that the heat buildup will not reach a dangerous level. More on this later.
Stress and Variation: Although the engine parts are metal and, as a rule, quite rigid, they are still subject to slight deformation when stress is applied. The largest stress in a piston engine is that produced by reciprocating parts. The forces involved increase with the square of the RPM. Any deformation will necessarily involve a change in some tolerances inside the engine. Thus, in order for the engine to operate properly over a range of RPMs, it is important that it be exercised over this range during the break-in process so that the wearing parts will experience the range of tolerances they will be subjected to during normal (post-break-in) operation. Further, for the wearing surfaces of reciprocating parts (most notably the piston ring/ cylinder wall interface) operation at a single RPM for an extended period of time will cause the machining process to progress significantly further within the confines of the part's range of travel without progressing at the point just outside that range, thus building up a small ridge of metal just above the point of maximum excursion.
[In order for your engine to run well from 1000 to redline, you need to operate it at all those rpms while it is breaking in. If you don't, the parts won't be used to working at the rpms you neglected, and they won't work as well at those speeds]
Piston Ring Sealing: The seal between the piston ring and the cylinder wall is crucial to getting good economy and performance from the engine. A bad seal will allow more blow-by, reducing the amount of power the engine can produce with each power stroke and thus reducing both its horsepower and fuel economy, as well as allowing combustion gasses to get into the crankcase and contaminate the oil AND allowing oil to get into the combustion chamber and be burned, producing the characteristic blue-smoke-from-the-tailpipe syndrome (note that oil can also get into the combustion chamber via the valve stem guides, but that's not something we can do much about during break-in). The key to getting a good piston ring seal is high combustion chamber pressures. High combustion chamber pressure is produced under hard acceleration; also, the lower the RPM the longer that pressure is maintained during each power stroke. SO - to get a good piston ring seal, hard acceleration at low RPMs will give the best results. Since hard acceleration also produces more heat and more stress (leading to friction and still MORE heat), it should only be used in brief bursts, followed by a couple of minutes of "normal" low-stress operation to allow the heated parts to cool down.
Localized Heat Buildup: As previously mentioned, wearing parts will produce inordinate amounts of heat as they polish each other. This produces local points of intense heat inside the engine, with temperatures far higher than
the engine as a whole (which shows up on the temperature gauge) or even of the surrounding parts. The most susceptable points in an engine for this kind of heat buildup are the crankshaft bearings, which must withstand enormous stress and pressure. If the bearings are allowed to get too hot, they will expand to the point of scoring each other or (*gulp*) binding, producing a spun bearing. During the initial stages of engine break-in, there is no satisfactory way of keeping these bearings cool during even mild engine operation except to turn the engine off after every 10-15 minutes of operation and allow the bearings to cool down.
The theory I have outlined about should now be sufficient to explain the "practice" section of the break-in instructions. For the first 100 miles, keep the rpms low and the trips short to minimize the stresses and heat buildup in the bearings, and use short full-throttle bursts to seal the piston rings. From 100-500 miles, gradually increase the RPMs to allow the wearing surfaces to correctly mate, and continue using full-throttle bursts to ensure ring sealing. Use cooling periods (the 1-minute rule) to minimize the heat buildup produced by the high RPM operation and the full throttle bursts. At 500 miles, change the oil to flush out all the metal particles produced by the wearing process.
For the first 100 miles, only take short trips of <15 minutes. Do not rev above about 3500 rpm. Use full throttle in short (2-3 second) bursts at low rpms (say 2500) - 5th gear on the freeway is ideal for this. Do not do more than one full-throttle burst in the same 2-minute period. Avoid driving for more than 2-3 minutes at the same rpm - if you are on the freeway, vary your speed and alternate between 5th and 4th gears.
From 100-500 miles, increase the peak RPM you reach by 200 rpm each time you drive the car (but don't go higher than redline). Do not rev to your new peak under heavy throttle; instead, let the engine drift up to the rpm under light load. For instance, pulling away from a stoplight, leave the engine in first and accelerate lightly until you reach the desired RPM, then shift. Continue the full-throttle-burst procedure. Do not rev the engine high under full throttle, and do not do either the peak-revving or the full-throttle procedure more often than once a minute. Avoid driving for more than 5 minutes at any one rpm - again, alternating between two adjacent gears and varying your speed will work.
You will notice that each time you reach a new peak rpm, the engine will be quite loud at that rpm, but after a few runs up it will quiet down. This is a sign that the break-in is proceeding well. You will want to have revved the engine to 6500(5500) rpm a few times by the time you reach 500 miles. At that point I recommend you change the oil, as most of the metal wear and contaminants from break-in are released in the first 500 miles.
From 500-3000 miles (the extended break-in) you can operate your engine fairly normally. Most of the work is done. You should still run the engine at higher RPMs on a regular basis (assuming you don't in the normal course of driving ;-) ) and you should avoid prolonged high-speed/high-stress operation, like racing or cruising at 110 mph. I personally change the oil after 1500 miles since it will be dirtier at that point than it would be after 3000 miles of post-break-in operation, but it isn't critical. Be sure to change it at 3000 miles, however. Although there is some difference of opinion on what KIND of oil to use during break-in, the general consensus is to use normal (non-synthetic) oil of the recommended weight.
From 3000 miles onward, your engine is considered broken in. It will probably continue to "loosen up" a bit over the next 3000-6000 miles, so look for a small increase in gas mileage. Other than that, your engine is now ready for a long and productive life. Enjoy!
2011 Grand Cherokee Overland 5.7L V8 4x4
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