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Thierry:
This subject was covered earlier, but once again, some observations:
1. Either way is no problem. My 1983 911SC was delivered to me at the factory with the concave side up; i.e., you could see the concavity under the bolt. This is the "stock" setting.
2. When I got serious about track and autocross events, the "trick of the day" was to change the washers to convex side up; i.e., just turn over the washers.
3. Warren is right, with the convex side up, there is more compression to the rubber, and therefore, the engine will twist less before it is restrained by the engine mounts/body.
4. Is this worth anything other than just minor, perhaps unmeasurable, vibration? There is no real answer. Only your common sense can dictate which way is best for you.
Therefore, if you wish to try to extract the maximum performace from your total package, do all the little things which might add up, includiung this change. Otherwise, if you are more concerned with the total package of a well-engineered auto, leave them whichever way you found them, or whichever way your mechanic suggests.
The difference is very very minor....Andras
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