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Old 12-01-2011, 02:48 AM   #1 (permalink)
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Vacuum advance - ported vs non-ported

I was tinkering on my recently acquired $400 Winter beater today. I set up the dwell and then went on to the timing. The spec is 7 deg advance at 750 rpm with the vacuum advance disconnected. Disconnecting it did not make any difference anyway, as it is ported in the carb. Just for fun, I hooked the vacuum advance directly to the manifold and not only did the timing jump to about 25 degrees (as would be expected), but the idle increased to about 1200 rpm as well.

Now I am thinking. Would the engine not be idling more efficiently at full vacuum advance, with the throttle position being identical yet the idle speed has increased? Would it do well to hook the vacuum advance up this way permanently and adjust the idle accordingly?

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Old 12-01-2011, 05:44 AM   #2 (permalink)
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I'd keep the plumbing stock and just advance the timing overall.
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Old 12-01-2011, 08:54 AM   #3 (permalink)
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Reverse it and you car is undriveable.

regards Mech
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Old 12-01-2011, 09:01 AM   #4 (permalink)
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leave it stock port, it was designed that way . If you go to main vacuum you could be pulling to much timing .
I used to play around with advance curves but the vacuum advance I leave alone .
Things that can work are heavier weights, lighter springs on mechanical advance (note lighter springs need to be checked they return correctly at lower speeds/rpm)
another trick is to reduce the amount of advance allowed by the centrifugal advance and bring up the initial /static timing the amount you reduced but you can't go to high or slow cranking will happen . there no set rule for this stuff so many variables you need to adjust .
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Old 12-01-2011, 02:40 PM   #5 (permalink)
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When I purchased my Jeep the PO had gotten the vaccume advance reversed and put it on manifold vs ported.

It took me a while to figure out what was going on while I was trying to tune the engine. It would start out at 25-30deg advanced then zeroed out when I hit the throttle. I couldn't get over 50mph until I fixed it.
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Old 12-01-2011, 05:45 PM   #6 (permalink)
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Thanks for the input, guys. I won't even try it.
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Old 12-01-2011, 07:33 PM   #7 (permalink)
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Quote:
Originally Posted by mechman600 View Post
Now I am thinking. Would the engine not be idling more efficiently at full vacuum advance, with the throttle position being identical yet the idle speed has increased? Would it do well to hook the vacuum advance up this way permanently and adjust the idle accordingly?
If the engine is turning more RPM at the same throttle opening it will be flowing more air and fuel so it is not more efficient.
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Old 12-01-2011, 09:58 PM   #8 (permalink)
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When you set the idle screw on the carb the throttle plate should cover half of the transfer slot or less. That is most efficient. If manifold vacuum gives you to high of an idle, switch to ported. You can also get an adjustable vacuum advance canister.

For max. mpg, I would run as much vacuum advance as you can without detonation once you set initial and centrifugal advance.

Initial timing should be set to give you reasonably easy starting but enough for increased cylinder pressure, which equals torque. You will have to set up centrifugal advance for total timing and how fast the timing comes in for max. power without detonation.
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Old 12-05-2011, 04:08 PM   #9 (permalink)
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Actually I wondered about this in my big block TransAm days. I always wondered why if I advanced timing th rpm went up, I would turn the idle down and then more timing advance, idle down. But I was going for horsepower out of a 425 cubic inch Oldsmobile big block not concerned with mpg

I would love to test this with a scan gauge hooked up. Maybe get rid of vacuum advance and just have the weights. Some Sport bikes have fixed timing. Always wondered how that worked.
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Old 12-05-2011, 04:15 PM   #10 (permalink)
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you want vacuum advance at part throttles and you want the most advance w/o going over a max total timing for CR and fuel used.
Plus not raising static to high for hard cranking .

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