How should you run in engine?

Answer 1

There are two main schools of thought here, and they are almost in direct opposition to one another. One is the "Baby It" school: Drive easy for a few thousand miles, then drive normally. The other is the "Hard & Fast" school, which says to basically drive in a spirited manner right 'out of the box'. This subject is controversial and there are passionateadherents on both sides.

Probably the most visited site for the Hard & Fast school is:

http:/wwwzperiodzmototuneusazperiodzcom/break_in_secretszperiodzhtm

The main point of the Hard & Fast school is that there is a limited time window for a new/rebuilt engine during which the rings have to seat well. This time window is early in a new engine's life, while the cylinder bores are still abrasive which causes the rings to seat well against them. The rings need some pressure behind them to press them against the cylinder walls and seat well. This pressure comes from pressing on the gas pedal to build cylinder combustion pressure. Driving 'easy' can result in the cylinder walls getting 'smoothed' before the rings fully seat, with the result that the rings may never seat properly. My machinist recommends the Hard & Fast break-in.

Car manufacturers uniformly recommend the Baby It type of break in. This may be the safer approach the safer approach from the manufacturers point of view, to reduce possible mechanical abuse. My conjecture is that part of the Baby It appeal may be a leftover from years ago when machine tolerances in mass production engines were perhaps not as well held as they are currently.

People have had good results with the Baby-It method. But from what I've been able to learn, Hard & Fast seems to be the choice of most people who understand engines.