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Old 01-04-2005, 10:05 AM
454C10 454C10 is offline
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You be the judge.

Try it both ways and see which works best.

I'm betting you will find ported is much better.


I still think it is simple, when you give it the gas of idle, you need advance. And the only way to get that with a chevy is to use a ported source.

A ported source will also advance some when going down the highway because the throttle is openned. That combined with some part of the mechanical advanvce makes a nice amount of timing for good MPG.

Let us know what happens.

red65mustang,

I guess you have a ford by your web name.
Yes Fords need to use manifold vacuum source. On a ford, the vacuum advance retards during high vacuum (idle). The diaphagm pulls on a spring and retards the timing, but once the vacuum drops, the spring in the vacuum advance system is what actually advances the timing. So when you give it throttle and the manifold vacuum drops, the timing advances.

Chevys work opposite, the spring retards and the vacuum advances. That is why a ported source is used on a chevy. But in both ford and chevy, the timing advances when the throttle is depressed.

Last edited by 454C10; 01-04-2005 at 10:03 AM.
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