All mechanical advance is sometimes used for racing applications, but many engine builders recommend the use of vacuum advance to save the engine from detonation. If you have the same total advance on mechanical only as you have on vacuum and mechanical you will have detonation at full throttle pulling hills at too high a gear which the vacuum advance would prevent because the vacuum would drop and you would have less advance. When the load decreases, the vacuum will build back up as would the advance, giving better economy.
