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Old 01-12-2009, 02:47 PM
red65mustang red65mustang is offline
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step #1: Read the dist install instructions!!!
(many dist are designed/engineered for and specify timed or manifold vac adv)

step#2: look up the calibration curve and min-max Hg versus degrees operating range of your specific vac model unit...
(there are dozens and dozens of different calibrations-curves models for different car/motor/gears combo's and it can add way to much (24*'s?) to be able to use manifold or not enough (6*'s?) for the base....

268 cam should be making 15Hg in park with 14-16* base which is plenty for fuel atomization at idle....
14* base plus 20* (normal) cent puts you right on the Vortec WOT mark...

best hwy mpg timing total=highest Hg at cruise=anywhere from 42 to 54* depend on the car...
rig your vac guage with a long hose into the car and test drive adding more vac advance timing till the guage Hg peaks holding the rpms steady at hwy speed
(if 46*=18Hg at 70mph and 48*-50*=18Hg at 70mph, set it to 46)
now test drive on a step grade with just heavy throttle to be sure there is no mid-throttle ping from to much added to soon....
it's not max degrees=best, it's max Hg=least pedal=best

my $.02
a 268 cam motor can better like using either port depending on the car....
there are definite good and bad points for using either!!!
as long as the dist instructions say you can use either, try both....
timed vacuum comes in at roughly 1200rpms, as soon as you step on the gas a fraction of a inch and open the butterflies, so does your car like vac adv added to the cent or does it like vac adv taken away from the cent with manifold from a stop sign

any bigger cam than 268 needs some manifold timing added, to raise the idle Hg and not have to use a 1000rpm idle to raise the Hg
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