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and another thing...
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Posted by Monster (more from Monster) on Wed, 14 Feb 2001 19:10:52 Share Post by Email
In Reply to: One more thing..., Jeff Cunningham, Wed, 14 Feb 2001 17:02:40
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Ari, you're talking about two different timing advance mechanisms. One is vacuum, and one is engine speed.

You can have high vacuum AND high rpm... like at the end of a heavy accel run when you lift the throttle. High vacuum doesn't necessarily mean the car is idling or the engine is turning at low speed. You can have high vacuum at any tach reading.

The mechanism I believe you're thinking about is the centrifugal advance. In the olden days they actually used a mechanical lever which moved something like a governer -- the faster it spun (higher engine speed or distributor shaft RPMs) the higher the timing advance. The centrifugal advance on older engines was a system of little weighted levers and springs inside the distributor -- I remember the days when performance tune-ups included new springs to tailor that mechanical timing advance.

As you have pointed out, the faster the engine is going, the sooner the plug should fire. At idle speeds, you don't want so much advance... you start the air/fuel charge burning later. That's what the mechanical or centrifugal advance does -- changes timing based on RPM. It's independent of the vacuum diaphram control.

In modern engines, timing is electronically controlled, and there are various ways of implementing the centrifugal advance, including non-mechanical ones. I'm not exactly sure how Saab does it (might be mechanical), but I'll bet Kevin K knows.

- = M = -

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