Join Date: Jan 2006
Location: Colorado Springs, CO.
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Generally, vacume advance is needed (in addition to mechanical advance) when the engine will be operated under part throttle conditions. This is because the air/fuel will be less dense under part throttle and burn slower, requiring more advance to ignite at the right time.
If you're only running under full throttle, mechanical advance is all that's needed. No vacume under WOT, so vacume advance doesn't do anything. Mechanical advance still helps make starting easier and it'll run better around the pits, etc.
If you're only running under full throttle and it'll always be above ~3000 (like with a 4000 stall converter), you can lock out the advance and run a fixed amount of timing. Often a manual timing retard is installed to make starting easier.
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