So how do you break in a modern gasoline powered internal combustion engine?
Firstly, what does the term "breaking in" actually mean? Breaking in a motor describes the processes and transformation from a freshly assembled engine to that of an engine whos rotating assemblies and mechanical parts are worn to optimal status, using various heat cycles and engine loadings.
There are two common methods, both of which I will talk about. I'll title these as the "babying" method and the "proper" method.
Babying Method
Most manufacturers recommend the owner to drive the car tamely, avoiding excessive engine speeds and high RPM's. Now you probably assumed by now I do not suggest this method, but there is a good reason why a manufacturer would choose this. It is simple. The typical user will understand these restrictions, and they do not dig into the specifics, so even your average low-IQ Joe can figure it out. It works, but isn't ideal, sooner rather than later, you will notice increasing oil consumption and even loss of compression.
So this procedure works. It is simple. Most users can undersdtand it, and follow the directoins with ease. Should you use this procedure? No.
Proper Method
This method in one way or another has been described to me by a few veteran engine builders. There was even a test performed using 600cc Honda Motorcycles to see if there is an actual power difference between the two break-in procedures, and there was, about 15% better compression at the end of the race day, less oil consumption, and dyno'd more HP/TQ. Although those findings are exxagerated on motorcyles due to tighter build tolerances and sustained high RPM's, the idea remains the same. Proper break-in allows for optimum ring seating and near perfect compression. You get more power, and the motor will hold compression longer!
This procedure is once again based off the principal of reducing heat and high engine loading.
If this is a freshly built motor, it is critical that you have a good basemap. The number one reason a built engine will have low compression is due to washing of the piston rings, when there is too much fuel to be burnt, it prevents the rings to seat 100% and results in perminant loss of compression. You will not need to worry about this if the car is new from the factory, as the factory ECU is perfectly matched to the engine.
** Any step with a double-astriks indicates a step performed only on a freshly assembled, 0 mile motor.
1. With the ECU fuse removed to prevent flooding and washing of the piston rings, crank the engine over for 30 seconds to move oil through the various passages in preparating for initial startup.**
2. Initial startup: Idle the motor for 25-30 minutes. If the cylinders are circle and the rings are gapped properly, most of the seating is accomplished in this step. A few reasons why such a long idle; a) Allows you to check for any leaks, fuel, oil, or coolant, that otherwise may have been missed. b) Allows any assembly lubricant and grease will be flushed through the system. c) Gives you the oppourtunity to check your air-fuel ratio at idle, make sure it is stoich between 14:1 and 16:1.**
3. Change the oil. If this is the first time you've brought home your new car, or you have just finished step 2, change your oil. Any assembly lubricants and greases will be flushed out of the system and new clean oil installed. Change the oil filter as well.
The rest is easy. You can check your compression frequently if you wish. This will tell you how well your break-in is going, and tell you when the engine is broken in and ready to rip.
4. Drive the car. Keep your engine speeds below 5,000-6,000rpm's. This will load the engine properly. Loading the engine at low engine speeds creates high cylinder pressure and low friction heat, the optimum situation for perfect ring seating. The best way to do this is to vary the engine speeds, and stay in higher gears, ie: 4th gear going up a hill. Remember, the key is to keep cylinder loading high while friction heat low, you want the rings to expand and seat correctly while lowering the risk of overexpansion and cylinder scorring/damage.
5. Oil change intervals. 50, 100, 250, 500, 1000miles. With oil filter. After 1000 miles, you can switch to synthetic engine oil. I use Castrol GTX for break-in. There are break-in specific oils, many dealerships sell their cars with these oils in them. Stay away from Synthetics, as they generally contain more detergents than regular "cheap" oils. While some detergents help the break-in process, there needs to be some metals in the oil to help the rings cut into the cylinder walls, so choosing an oil with a lower amount of detergents is ideal.
Alternative Methods...
I should note that almost every professional racing team breaks in their engines on an engine dyno in a series of loaded runs in high gears at low RPM's, followed by a sequence of full powerband pulls.
There are people who, even for their street car, break in the engine on the dyno. This is a good method as well, but unless you have access to a dyno, it is a bit more impractical. But if you are in a pinch for getting a motor together and ready for a race, a dyno break-in is the best way to achieve this, seeing as you will have 100% ring seal in less than 5 minutes on the dyno.