Read the manual. It clearly specifies what to do/not do. But is worthless to us who wish to know more… There are two major things going on during break in. Tight tolerances between brand new parts in the engine/trans/driveline and also engine piston rings seating/sealing. You have experience with the smoke show from an engine that didn’t seal.
Off the lot, the most critical thing is thermal cycles as one mentioned. We need the engine/trans to achieve full operational temps and allow all metals to expand , start seating/wearing in and also to allow cylinder bores/piston rings to start seating/sealing better. This initial period of 60 miles per the manual is to be below 50/55mph in order to allow all mechanical bits (bearings, gears, driveline, etc.) to wear a bit as the tolerances of these areas are the tightest they will ever be. Warm engines, with warm oil is a must at this phase as hotter oil is thinner and is the ideal viscosity for such tight clearances. Can't shove thick syrup where thin water needs to be... The 5.7L Hemi has 0.0009”-0.002” acceptable oil clearances for the main crankshaft bearings. A single hair from your head is 0.001"-0.003" depending on who you ask... you get the point. Oil has to be pumped into these tight area to provide adequate lubrication and to cool the area, cold oil cannot do the job 100%, it has to be thinner, hence 5W-20 oil. 20cst viscosity to get the party started when cold, 5cst viscosity to properly lubricate and cool under extreme stress/high performance/better MPG's . On a newer motor, those clearance all "should" be towards the tight side. This time is where all of the loose metal bits, shavings not removed during assembly, etc... get washed out and returned to the oil pans/sumps.
The next thing is driving at varying throttle at speeds up to 50/55 mph in order to start the engine sealing process. The pistons need varying degrees of cylinder pressure to allow the rings to be forced outwards to start seating and also to rotate themselves around. Due to the cross hatching honing in the cylinder walls, this causes the rings to rotate. By varying the throttle we are applying pressure to force outwards and releasing pressure to encourage rotation. During the assembly process the rings are supposed to be clocked at varying angles to prevent all ring gaps from lining up and any possible time, even during the ring rotation while in operation. The thermal cycles are important at this point to help the cylinder wall/ring materials soften slightly and to wear better. The combustion process will create high temps that slightly change the hardness of these ferrous metals slightly. Full throttle accelerations are extremely important at this phase due to cylinder pressures/ring seating. More throttle = more cylinder pressure = higher ring loads = better sealing. Understand this is short duration burst, low to mid RPM's, nothing over 3-4K RPM, not held down for a long period. Long periods of lugging a motor at low RPM's, for long periods, or at high rpm is a no no as the thermal cycle is too long and pressure is not released soon enough, too much throttle for too long, too high of RPM due to mechanical clearances as stated above. This isn't rocket science. Just get it floored safely from time to time, off idle to mid RPM and get out of it. They need this to seal faster. Driving around with no throttle is worse than full throttle everywhere (assuming no high rpm's). No long idle periods. Idling at this point for long periods is very bad. Shut it off. The thermal cycles are better than idling a tight, non-sealed motor.
On my 2.0L turbo track toy after rebuild, that has a larger bearing tolerances and make more power than a 5.7L Hemi, I start/warm it up, check for leaks, run about 1500 RPM (no idle) to start thermal cycles. Once oil pressures, coolant temps stabilize with a hot motor, road test. I do 3-5 basic pulls to 55MPH (obeying traffic laws, cough , cough...). Stop shut off, check fluids, start again, warm up. The road test to full throttle/power multiple times at 4K, 5K, 6K RPM. Once this is done, I cool down, shut off check fluids, and restart and warm up. Road test to full 7.5KRPM/full power multiple times. I do all of this in about 2-3 hours assuming no issues. Cylinder sealing/pressures are always good until I melt a hole in a piston or break one. I change my oil after this process. I break this toy in on pure synthetic, the same oil that it runs. All of this work goes into a $2K engine refresh on a toy car that isn't daily driven. Any serious drag racer will tell you "break it in the way it will be driven", they usually go wide open out of the gate. Most of their break in takes place at the track on the engines first run. I'm a nerd and take more time.
Bottom line is it needs to be driven lightly at start due to tolerances, then gradual speed increases due to this. Then some full throttle burst to help seal it up faster. The harder and faster the sealing the better for the rings as the higher cylinder pressures wear a larger flat area on the ring edges. Think knife edge vs flat edge, which would seal better? As a note, I change my hemi oil at 1500 miles or less due to these facts. At this time I switch to pure synthetic Pennzoil. Pennzoil is what comes from the factory as it is the only MS-6395 certified oil readily available in the USA (to my knowledge as of 3 yrs ago). Mopar oil is Pennzoil oil at a premium price and different container. Oil brands is like a religion to some, so I avoid these discussions... but understand there is only 1 oil brand that is sold on shelves in the USA that meets these specs (or paid money to be tested). This is always an avenue that dealerships go down to void people's warranties. I personally use AMSOIL for everything (including my lawn mower) but not the jeep due to the warranty issues.
The factory does almost no testing. There is a basic "knock test" where the engine is started, ODB port plugged into by a testing PC, sensors feedback verified, basic throttle testing, etc... but no/very light load. The guy behind the wheel gets a green light "good" or a red light "bad" at this point. This testing is semi automated, servers/computers being the go/no go gauge, the operator pushing the buttons/throttle/wipers, etc... This should be considered a validation of systems/subsystems not an engine break in. Some are randomly sampled for extended testing. The interns driving them off the line to parking areas does more real world engine break in 5 minutes than they do in the factory. Despite popular belief, every vehicle does not undergo a dyno test. Owners are getting a virgin engine essentially. There is no such thing as a Jeep 5.7 Hemi that is broken in at the factory. Break in takes time, miles, fuel, and thermal cycles, none of which Jeep is doing. These are SUV's, not corvettes. Every Corvette gets a dyno, but this is a sports car for Chevy and due to LS/LT powertrain complexity at higher HP numbers, seems justifiable in my opinion.
As a note, a brand new vehicle typically gets abused hard during test drives/dealer sales persons driving them. I would never buy a new vehicle with 100+ miles on it do to this fact. I would always order my own custom and wait vs rolling the dice on an abused lot vehicle. No one reads the manual, very few even know the engine needs broken in, few actually care to do it, even less actually do it remotely close to correct. The first 500-1000 miles is pretty critical in my opinion. Then the next comment you read will say "F it, just hammer down and it lasted 200K miles" or "F Pennzoil", I digress... My 2000 Dakota 4.7L is at 212K miles for reference, all original drivetrain, never been opened for any engine work except maintenance items per the manual. It still fries the tires well and has zero issues except clearcoat peeling. I would expect a 5.7 to go at least 250K miles easily, without issue, if properly maintained.
My 5.7 GCL arrived in August, April order.
