The only real difference between ported/manifold vacuum is that manifold is adding timing at idle and during coasting. Some users report cooler running as a side benefit during low speed operation but I have not done enough testing to substanciate this. The milage difference would be negligable as both apply the same vacuum to the canister just off idle. It must be noted that on some factory carbs there are "weak" ported vacuum fittings. These have the internal opening in the carb high above the plates and do not work well as a distributor source. A correctly located ported vacuum source provides distributor vacuum just off idle or during any off idle driving situation and will mimic a vacuum source on the same engine. Therefore milage/off idle performance will be the same. Hope this helps some....Cliff
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https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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