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OK heres the deal. When you hook up your distributor to manifold vacuum ( ported ) you give it full idle vacuum which boosts your timing at idle. That allows you to set it at 10 degrees static or so for easy starting with a stock starter. Once it is started you develop vacuum and your timing jumps up to 25-35 degrees or so . That gives you a smooth idle with anything over a stock cam. It also gives you excellent throttle response off idle. When you gas it your vacuum level drops since your throttle plates are opening up and with it the amount of vacuum advance you are gaining. So your vacuum advance goes down while your mechanical advance begins to take hold . That allows you to avoid detonation under full throttle while still giving you higher than normal advance under cruise conditions. I run a very mild cam and high compression for a street motor. With 92 octane I can run almost 45 degrees of total timing under cruise conditions. That drops to 32 or so under WOT so I avoid detonation .
For a motor you want to run good. Use 12 or so degrees timing with the distributor disconnected . Hook the vacuum line to the distributor up to manifold ( vacuum at idle ) vacuum.
If your goal is to maintain low NO2 levels at idle then by all means hook it up to venturi ( timed ) vacuum. Thats all it is good for.
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No truck :-(
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