Actually, a manifold vacuum tap location right near the valves would be a
poor source of vacuum because it would be contaminated with shock waves of
presure bouncing off the valves as they close. Abruptly breaking the
momentum of the fuel air charge travelling through the manifold causes
presure to bounce back towards the carb. This is why your supposed to tap
the manifold close to the carb. If you were to put a vacuum guage on the
port next to the valve the needle would bounce in sinc with the valve
closing. The port next to the carb would show a steady reading although
changing depending on throttle conditions but is still a good source of
vacuum for accessories.
Chris Reichle
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When I said 'maximum available vacuum' the key was 'available'. I
believe that you'll find the most vacuum the closer you get to the
intake valves. 'Ported vacuum' which I believe is very near (but not
before) the throttle butterflys will vary according to throttle position
-- that ain't something the power brake booster wants to know about but,
to an earlier point, can be useful to the distributor.
Bob Allen, Kansas City, '69CGT, '75TR6
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