My friend has overhauled his engine recently and he sought my advice about the most efficient way of breaking in his new engine (Peugeot 405 1.8L Petrol).
It is usually said that you should keep RPM below 3000 in the first 3000 miles and avoid revving the engine to prevent damage. This is the traditional and the most prevalent belief about engine break-in.
However, there are some people who believe you should rev an engine up to redline during break-in period to let piston rings rub off the roughness of cylinder walls and seal better in the long run. They recommend revving a totally warmed up engine in short burst accelerations (like revving the engine in 2nd or 3rd gear and letting off the throttle immediately). They say if you don't rev your engine during break-in period, you will end up with lower compression (= less power and higher oil consumption) in the future.
These two schools of thought are completely opposing! One says rev it, the other says avoid it! I don't know how to guide my friend during his essential break-in period! Who is right and who is wrong?