Quote:
Originally Posted by scott70
At what rpm do you like to see the timing start to advance?
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Centrifugal advance?
At least a hundred RPM higher than curb idle. If there's a throttle-kicker for the A/C, a hundred RPM higher than that.
Point being, I want the centrifugal advance to NOT start when the engine is idling in neutral under real-world conditions--A/C on, or off, engine hot, etc.
Getting some centrifugal advance at FAST idle is fine--even desirable, when the engine is cold, choke on, fast-idle cam engaged.
Vacuum advance is another story. Some engines benefit from vacuum advance at idle, some don't. And the reasons aren't all the same--I've seen engines have wicked off-idle throttle response because opening the throttle to move away from the stoplight suddenly adds ten or twenty degrees of vacuum advance from the ported vacuum connection, and the tires chirp. Other engines have unstable idle rpm because the manifold vacuum is constantly changing the timing...which affects the manifold vacuum...which changes the timing. And either way, ported or manifold vacuum to the vacuum advance, changes the throttle position for a given idle speed, which changes the relationship between the throttle plates and the transition slots in the idle air/fuel passages.
Tuning the vacuum advance is just as important as tuning the centrifugal advance, on any vehicle that's mainly driven at part-throttle. But you need to tune the amount of advance, the "starting" vacuum, the "all in" vacuum; and whether ported or manifold vacuum works better in your application.