Educate me on Vacuum Advance
10-07-2001, 09:11 PM
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Educate me on Vacuum Advance
I got my car back with the carb rebuilt, and the vacuum advance is set up different now. Before, it was hooked to a port that had vacuum at idle. Now it is hooked to a port that has no vacuum at idle. I think this is called "off idle vacuum"? Tell me how this works, and is one way better than the other?
10-07-2001, 10:43 PM
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St. Jude Donor '03-'10
Re: Educate me on Vacuum Advance (Eric 70)
Your car can't take as much timing advance at idle as it can higher in the RPM range.
If you hook up the vacuum advance so it kicks in at idle you won't be able to advance it as far later in the curve, so you don't help yourself out much.
You'll also likely start pinging really early unless you have your timing retarded without advance.
Hooking it up to the off idle vacuum port brings in the advance later allowing you to get correct timing.
(And to think...6 months ago I couldn't have pointed out the distributor under the hood. ;))
If you hook up the vacuum advance so it kicks in at idle you won't be able to advance it as far later in the curve, so you don't help yourself out much.
You'll also likely start pinging really early unless you have your timing retarded without advance.
Hooking it up to the off idle vacuum port brings in the advance later allowing you to get correct timing.
(And to think...6 months ago I couldn't have pointed out the distributor under the hood. ;))
10-07-2001, 10:59 PM
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Re: Educate me on Vacuum Advance (Chris Fowler '80)
Is the port that has no vacuum at idle that way because of a valve or is the physical location what determines whether it has a vaccum at idle or not? I haven't checked mine but it wouldn't suprise me if it was incorrect
10-07-2001, 11:53 PM
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Re: Educate me on Vacuum Advance (rickmhsd3)
Is the port that has no vacuum at idle that way because of a valve or is the physical location what determines whether it has a vaccum at idle or not? I haven't checked mine but it wouldn't suprise me if it was incorrect
-Pedro
10-08-2001, 03:28 AM
#6
Melting Slicks
Re: Educate me on Vacuum Advance (Eric 70)
As the other members stated, you want to use the "timed port". It will work when you hook it up to a full time vac port, but it will cause fluxating idle problems.
The purpose of the vacuum advance is to increase gas mileage by advancing the timing in a "high" vacuum / part-throttle driving condition. (like cruising down the highway) As soon as the engine enconters a load however, (going up a hill or trying to accelerate) the engine loses it's vacuum & the distributer retards the timing to avoid pinging. As soon as you take your foot out of it, the vacuum builds up & advances the timing again.
The purpose of the vacuum advance is to increase gas mileage by advancing the timing in a "high" vacuum / part-throttle driving condition. (like cruising down the highway) As soon as the engine enconters a load however, (going up a hill or trying to accelerate) the engine loses it's vacuum & the distributer retards the timing to avoid pinging. As soon as you take your foot out of it, the vacuum builds up & advances the timing again.
