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Old 09-04-2008, 11:04 AM
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Jmark Jmark is offline
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This is a subject that always draws out both sides.

Personally, I always ran ported on my truck. When I really got into researching it, I decided to do some real world testing. I "T'd" in a vacuum gauge and went for a long drive. Up some hills, down some hills, mild acceleration and hard acceleration. I took good notes on speed and my best guess on pedal position and vacuum readings on hills and cruising, following the same route one time on ported and one time on manifold vacuum.

My bottom line was that at cruise, I had almost identical vacuum readings, and identical timing added, whether on ported or manifold. During idle, of course I had max advance on manifold and no added advance on ported. At WOT, there was no vacuum signal so neither added anything.

When going from an easy cruise to a harder acceleration, on manifold vacuum, the vac. signal falls off very quickly, which is good since the mix goes rich quickly once the metering rods OR power valve are now open and giving a rich mixture.

From all I could tell, the biggest advantage is the added timing at idle, which, depending on cam selection, can be a great bonus in idle quality and a boost in idle vacuum. For me, the biggest drawback is when emissions testing comes around. Idling with 18 degrees of combined timing, my HC% levels are through the roof. If I just pull over before going in and yank the hose, my HC% is always within spec.

So for now, i'm running manifold vacuum for a little better idle quality since my cam is a tad tall for my converter stall.

and now back to work!

Mark
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