Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Ported vs. Manifold vacuum
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Old 08-17-2012, 07:55 AM
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75gmck25 75gmck25 is offline
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For the original poster - based on the question in your 2nd post, I don't think you understand how ported vs. manifold vacuum works. Switching from one to the other should not require a change in static timing, and you should not need to adjust it to avoid pinging.

At idle
-- Ported - you will only have static timing (e.g. 10 degrees BTDC)
-- Manifold - you will have static timing, plug whatever you distributor vacuum advance adds (e.g. 12 degrees more, for a total of 22). In most cases you just have to adjust the idle speed to compensate for the additional advance at idle.

Light acceleration (medium/high vacuum) - both cases will usually have equal advance, since the port is uncovered and now it is providing vacuum to the vacuum advance. At this point your mechanical advance also kicks in since it is based on RPM. If you get pinging its probably because your vacuum advance (or mechanical advance) is adding too much.

Cruise (high vacuum) - both equal; you will have total advance that equals static, plus vacuum, plus mechanical based on RPM. The additional vacuum advance is what helps gas mileage at cruise, as long as it does not go so far that it causes pinging.

Full throttle (low vacuum) - both equal, static plus mechanical only, since vacuum is so low that there is no vacuum advance.

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