The factory timing setting is not always the best one . One reason for this is because many cars had to meet emissions requirements, and in some cases, the timing settings were a compromise between maximum performance and meeting these requirements, and determining the optimal setting is called "performance tuning.
Also, cam chain wear can change the amount of engine timing that is optimal for a particular engine and so can compression which will be higher than stock if an engine is bored out but everything else remains the same.
Using factory timing settings is usually safe, however, that does not mean that it is the optimal setting for optimal performance.
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