The break in process for an engine or other mechanical part is not well understood by many performance enthusiasts. Some feel it is unnecessary with modern machining and production methods. Others advocate a severe break in process to get the maximum performance out of the engine. What is not well understood is that the break in process applies to all mechanical systems in the car.
The two schools of thought are usually summarized with over simplified axioms, like "the engineers who built it know best do it exactly like the owners manual says!" or "drive it like you stole it!".
It is important to realize that the original manufactures has a different motive and intent behind his break in recommendations than the auto enthusiast. The manufacturer is giving a series of recommendations aimed at the lowest common denominator customer with very conservative guidelines that make it nearly impossible for the new owner to damage the car. The Manufacturer is also attempting to minimize warranty issues and maximize reliability during that warranty period with little regard to absolute maximum performance.
The auto enthusiast is interested in an entirely different result and has a different intent in his break in strategy. In this case the objective is usually maximum over all lifetime for the engine (200k Miles), maximum efficiency and gas mileage, or maximum power output in exchange for a reasonable reduction in the engines ultimate life time. As in all similar situations there is no free lunch and you must trade one attribute for another when you choose your break in process.
The manufactures break in procedures usually will give very long engine life, near zero chance of injuring the engine mechanically, acceptable (rated) performance, acceptable oil consumption, and acceptable gasoline milage.
The high performance break in puts highest priority on maximum achievable power production with out physical damage to the engine. Very low oil consumption, and high compression due to good ring sealing, in exchange for some small but real risk of engine damage during break in, and the possibility of a slight reduction in the long term operational life time of the engine.
If you follow the factory recommendations "exactly" you will have an average car that does not need any warrantee claims during the factory warrantee period. The factory breakin recommendations are to protect "THEM" not you. They want to have the lowest possible claim rate under warrantee. They make those recommendations with the fact in mind that a small percentage of the automotive customer base are certifiable idiots and could break an iron ball with a rubber hammer.
Repeated tests and standard practice by high performance users (ie race teams, and even factory recommendations for certain cars) are for the intellegent use of high load during break in. You want to adhere to the max rpm limit with rare exceptions during the first couple hundred miles, and absolutely do not exceed it for extended periods of time during the first few miles. The morons that take a new car out and try to see how fast it will go and do a 140 mph pull in 5th gear for several minutes will kill any engine. Taking a quick blast up to red line to merge onto a highway after the car is warmed up will not harm any engine that does not have a manufacturers defect.
To get best possible ring seal you need to put the engine under high load (high cylinder pressures) for a brief period of time then let the engine breath a bit and cool down any hot spots.
Probably 90% of your engine and transmission break in occurs in the first 1- 2 hours of driving, but the Subaru engine will continue to loosen up and gain power for almost 10,000 miles.
Break in is controlled wear, you MUST create high loads to get the best possible ring seat, you can get probalby 90% of best possible by following factory recommendations. You can ruin an engine (make it an oil burner for life) by being overly gentle with it during the first couple thousand miles.
As the saying goes moderation in all things. The factory knows full well that almost no driver can avoid taking the car up near red line a few times during the first hours of the engines life! There recommendations are to keep the idiots from taking it to red line and holding it there for seconds or minutes at a stretch. That will also kill a new engine.
Seating the piston rings is an important part of the break in process. No matter how hard the manufacture tries the engines cylinders are never perfectly round when a new engine is assembled, and the piston rings likewise have microscopic high spots. Breakin is a process of controlled wear.
What is happening in side the engine on a microscopic level, is the surface of the cylinder wall looks like sandpaper. Lots of sharp peaks and deep grooves. The Grooves are put there intentionally during manufacture to hold oil during the early stage of break in. In the final hone process as they finish the cylinder wall surface, they try to produces what is called a "plateau finish".
That means they first, using a coarse stone, create a rough surface (microscopically) with deep scratches and sharp points. Then they make a specific number of passes with much finer stones to "knock off" the sharp points but still leave some of the deep "cross hatch" grooves.
The end result if done properly, is a moderately smooth surface with lots of flat topped peaks ( the plateau) interspersed with deep scratches that can hold an oil film. (the surface would feel quite smooth to the touch, but on a scale of millionths of an inch is quite rough)
Likewise the Piston rings have a microscopically rough finish as well.
In the first couple hours of engine time is the ideal time to push the rings hard against the plateau finish in the cylinder wall, so that they both "wear in to each other"properly.
It is essentially a period of controlled wear which allows the piston rings to "grind themselves down to a perfect match" for the cylinder wall. This process rapidly wears down a few millionths of an inch of the cylinder wall surface and quickly takes the "bite" out of the cylinder wall surface. If you look at an engine cylinder wall after it has run a while, you will see that the rings have polished (worn) the cylinder wall to a near mirror finish.
If this happens before the rings have ground themselves into proper shape for a perfect seal, it will never happen as the cylinder wall becomes too slick to change the shape of the piston rings much, and the deep scratchs which hold oil during this wearing in process and protect the engine from over heating small areas of the piston rings and cylinder wall disappear.
As you can see, the process is in fact a pretty brutal process on a microscopic level and that is why the periods of high loading should only be maintained for a couple seconds at a time. (about the time it takes you to blast up an on ramp)
After that few seconds of high load, the deep scratches in the cylinder wall fill up with microscopic dust from the wearing in process and hot spots form because they can no longer hold adequate oil film. After a few miles of driving, the oil will wash all this crap out of the cross hatch and then it can do its job again at the next high load blast up an on ramp.
If you do it too hard too fast, you will pack crap in the deep scratches so hard they cannot self clean. This is commonly called "glazing" the cylinder walls. You have ruined their surface for proper break in at this point. Long duration high loads early in the engines life, will also create small areas of the cylinder wall that get heated to very high temps and then rapidly cooled by the cooling system. This forms a hard spot in the wall that will never wear at the same rate as the rest of the cylinder. If that happens you have a high spot that will last the life of the engine. High loads that last too long can also over heat the rings and make them lose their spring tension which provides the initial seal to the cylinder wall. That and the high spot issue I mentioned above, along with a couple other things can make the engine burn oil for its entire operational life time.
If you only drive the car easily at low engine rpm you defeat the entire process of engine breakin. When an engine is manufactured and assembled they carefully create a surface on the cylinder wall that is smooth but not too smooth. It has a crosshatch pattern of deeper scratches in its surface which are intended to hold an oil film. Likewise the piston rings are smooth to the eye but are also rough at a microscopic level.
When the engine runs the piston rings are forced against the cylinder wall by the pressures in the the cylinder as high combustion pressure push outward on the rear of the ring. The spring pressure of the piston ring is not adequate to create a tight seal and this "pressure capture" is essential to achiving a good ring seal.
If you only lightly load the engine as you break it in, you never force the piston ring hard against the freshly prepared crosshatch pattern in the cylinder and as a result the piston ring simply slides over the top of the high spots on the cylinder wall and piston ring, and the two surfaces polish each other smooth with no significant wear taking place. This eventually destroys the cross hatch pattern so the cylinder loses its ability to hold an oil film on the surface.
If this is the case (lightly loaded engine during break in) you will never get a good ring seal with the piston wall because you have prevented the breakin wear required to lap the two surfaces into a close full surface contact.
The proper way to break in an engine is to apply brief periods of high load to force the rings hard against the fresh cylinder surface while it still has the cross hatch pattern. This high pressure creates high wear on the high spots that are holding the surface of the ring and piston wall apart and they quickly "wear in" (break in) as these microscopic high spots get polished out. The presence of the cross hatch pattern holds a thin layer or oil on the cylinder surface which keeps this planned wear from going too far and prevents excessive metal to metal contact as long as the high load only lasts a short period of time.
Then you drive the car easily for a short period to allow the areas that just got polished down to cool off, and to re-establish the thin oil film in the cross hatch.
If done properly you will wear the high spots off both the cylinder wall and the piston ring, and end up with a smooth gap free sealing surface which applys pressure to the entire surface of the piston ring. This gives you a good pressure seal and a smooth sliding fit that needs very little oil to run with low friction and wear.
You need to apply load to the engine in the first few hours of its life to allow this to happen. If you drive the car too easy or too hard you will either create a seal that has gaps that never wore down before the cross hatch was worn off (very light break in), or you will over heat the piston ring and cylinder wall and damage them due to over load (very hard break in such as continuous high rpm operation before break in is complete).
This wearing in process as the rings conform to the exact shape of the cylinder and the cylinders wearing in to a true round surface is the process of "seating the rings".
Engine braking during the break in creates high vacuum in the cylinder that helps draw up oil and hold it on the cylinder wall. If done in moderation it is good to do.
The reason to vary engine rpm is that the components in the engine are not completely rigid and do stretch under load, so the shape of the cylinder wall changes with engine load as does the length of the total stroke as the rods stretch at high rpm and all the bearing clearences are squeezed down at high rpm.
If you do not vary engine rpm the piston rings will never reach the very top of their travel in a gradual manner and create a smooth surface at the very maximum reach of their travel. It is not as big of a deal as it was years ago but the effect is real and does make a difference in engine performance.
This is a balancing act between sufficient engine load to fully seat the rings and achieve maximum compression, and minimum oil consumption due to excellent ring seal.
As shown below aircraft engine designers recommend a variation of a high load break in as they try to achieve maximum reliability and maximum take off power in their engines.
From Lycoming Aircraft recommendations for engine break in
"A new, remanufactured, or overhauled engine should receive the same start, warm-up, and preflight checks as any other engine. There are some aircraft owners and pilots who would prefer to use low power settings for cruise during the break-in period. This is not recommended. A good break-in requires that the piston rings expand sufficiently to seat with the cylinder walls during the engine break-in period. This seating of the ring with the cylinder wall will only occur when pressures inside the cylinder are great enough to cause expansion of the piston rings. Pressures in the cylinder only become great enough for a good break-in when power settings above 65% are used.
Full power for takeoff and climb during the break-in period is not harmful; it is beneficial, although engine temperatures should be monitored closely to insure that overheating does not occur. Cruise power settings above 65%, and preferably in the 70% to 75% of rated power range should be used to achieve a good engine break-in."
If your interested in paying for it you can always get this SAE paper:
"Effect of Break-In and Operating Conditions on Piston Ring and Cylinder Bore Wear in Spark-Ignition Engines"
It is important to understand that you are breaking in more than just the engine. You are also breaking in the gears, seating and seasoning the clutch if it is new, heat cycling the brakes if they are new. Every bearing surface in the car, and even components like suspension bushings take a set and settle into their long term running position in the first few hours/days/weeks of use. Some changes also occur due to heat cycles. Tires firm up and stabilize as they go through their first few heat cycles, and become much more resistant to abuse if properly treated during those first few heat cycles.
Carrol Smith in his Tune to win book comments on how many racers kill their rear ends before they even race them by not breaking in the gears. The problem is the gear failure will not occur until many miles after the fact.
I suspect this is one of the elements in the transmission failures some folks are having. They cause micro fractures on the gears during the early break in period by beating on the car before it is well broken in, and then Poof the gears shatter while they are going to the grocery store with the 2 year old in the back seat.
For the gears you also need to give them brief periods of high load, followed by several minutes of low load operation so the gears can cool down, to a uniform temp. If you don't the tight points on the gear teeth will be over heated which destroys the strength properties designed into them and leaving you open to later random failure.
Example Break-In Procedure
In the 60's Chrysler corporation advocated the following sequence to break in their factory drag engines (street hemi's):
- Warm the car up to full operating temp then let it cool completely.
- Run a 1/4 mile pass at 1/4 throttle, let it cool completely.
- Run a 1/4 mile pass at 1/2 throttle, let it cool completely.
- Run a 1/4 mile pass at 3/4 throttle, let it cool completely.
- Run a 1/4 mile pass at full throttle, let it cool completely.
Mark Donohue the famous race driver describes how he broke-in race engines after rebuilding them in his book "Unfair Advantage". He liked to take the newly rebuilt engine and let it run at a fast idle for an hour or so, with a garden hose running in the cooling system to keep it cool. Then he would put it back in his car and take it to the track. By the time he finished his tuning laps and time trials for qualification, the engine was ready to race.
Several major engine builders describe how they dyno break-in an engine and they also use the same pattern. 30 minutes or so of moderate fast idle to warm things up, followed by brief runs up through the engines rpm range under gradually increasing load, followed by cool down cycles.
Graham Bell author of the book "Four Stroke Performance Tuning" gives an example of a dyno run in process he uses on all his race and rally engines. To make it universal I converted the numbers to % of maximum torque.
rpm Torque min 3500 25% 10 4000 33% 30 4500 45% 30 5000 56% 30 5500 66% 30
Followed by cool down, adjust tappets, and retorque heads on cars that require those steps, then full power runs for tuning purposes are acceptable.
The break in process is actually a period of controlled wear. At a microscopic level there are little tight spots through out the engine. Every place one surface runs on another there are very small high spots that are taking the brunt of the load. During break in you are allowing these tight spots to wear down or be burnished down smooth with the surrounding surface. When you put a new engine under high load these tight spots get very hot. When they get hot they expand, and clearances get even tighter.
If it goes on too long the oil film breaks down and a small piece of metal gets ripped off the tight surfaces. You have now scored a cylinder wall or a bearing.
So if you get on it hard, just think about that situation and drive easily for a few minutes to allow things to cool back down to normal operating temperatures. Kind of like doing wind sprints, give the engine a chance to catch its breath.
The way to do big damage is to get on it and just keep whipping your horse until it drops.