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Old 01-10-2008, 04:14 PM
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Ignition Timing Affect Vacuum

Does advancing, or retarding ignition timing affect vacuum available at the manifold? Aside from changing engine RPM.

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Old 01-10-2008, 04:41 PM
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Yes to a point... The increase in rpm at idle that increased timing creates, allows you to close the throttles back down creating a little more vacuum at the original idle speed. The more charge dilution @ idle (exhaust gas egr effect of a big camshaft) the slower the fuel burns, needing more spark advance at idle.
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Old 01-10-2008, 05:43 PM
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^^ ditto what he said
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Old 01-10-2008, 06:51 PM
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it can affect cranking vacuum as well.

most high compression engines wont even crank with 20 degrees inital timming.
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Old 01-10-2008, 07:01 PM
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Set inititial advance to around ten to twelve degrees. Hook your distributor vacuum advance can directly to a full vacuum port, not a timed port. You won't have any trouble cranking and when the motor starts the vacuum advance will give you another seven to ten degrees of advance which is just what you want.
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Old 01-10-2008, 07:07 PM
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Quote:
Originally Posted by spinn
it can affect cranking vacuum as well.

most high compression engines wont even crank with 20 degrees inital timming.
There is a simple and inexpensive fix for that.
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Old 01-11-2008, 10:21 AM
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manually retard the start timming?
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Old 01-11-2008, 02:10 PM
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No, that would be neither cheap nor easy. Just set the initial timing to about ten to twelve advanced. Any engine will crank easily with that. connect the vacuum advance to full manifold vacuum. Don't use the so called "timed" port on the carb. That port is a smog crutch to make the motor run hotter at idle and reduce oxides of nitrogen. Also gives you a crappy idle and overheating in slow moving traffic. When the motor starts the vacuum advance will give you more advance at idle. Your motor will run smoother at idle and cooler in traffic. This will give you a faster idle which you can slow down with the curb idle screw but your advance will stay the same.
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