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1951 288 Vacuum Advance Source
#1
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kunzea
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Manifold or ported? Why?

Posted on: 9/1 12:31
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Re: 1951 288 Vacuum Advance Source
#2
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HH56
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I believe Packard mostly used the ported type. The vacuum source is a fitting on the carb where the vacuum comes from a port in the carb located above the throttle plate. When the throttle is closed during idling there is almost no vacuum going to the advance so the timing is not being affected and can be set with the vacuum still connected. On most if not all Packard instructions on setting timing there is no mention of needing to disconnect and plug the vacuum line to the advance.

With the manifold type, vacuum is taken from the intake manifold or the carb somewhere below the throttle plate. When the car is idling and throttle is closed the vacuum is substantial and advance is in full play and affecting whatever setting you made. On those cars the instructions will generally say to disconnect and plug the vacuum before making any adjustments.

Probably doesn't answer your question but there are advantages and disadvantages to both methods and others more into engines will better be able to explain them or there are several hot rod forums that dive into the differences and some reasons for choosing one or the other.

Posted on: 9/1 12:47
Howard
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Re: 1951 288 Vacuum Advance Source
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JWL
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I believe one reason for having the distributor vacuum chamber connected to full manifold vacuum is to reduce engine heat at idle. At idle the distributor is at full vacuum advance. It is retarded ignition timing that can cause more engine heat. Setting the engine timing with full vacuum to the distributor does involve a couple of extra steps over the ported version.

Remember, the distributor vacuum chamber is just a way to modulate the timing along with the mechanical advance.

Some Ford products only used a vacuum control for the timing and some other manufacturers - early on - used only mechanical control for the timing. Engines used for racing tend to use only distributors with only mechanical control as engine vacuum is low due to valve operation of the competition ground camshafts.

Others here probably have much more knowledge on this subject.

Posted on: 9/1 13:29
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Re: 1951 288 Vacuum Advance Source
#4
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DavidPackard
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Kunzea;

I’ve had two vehicles that featured 100% operator control of the spark advance. My impression is that too much advance at low speed can result in a rude engine . . . not at all smooth. The operating instructions for both of those engines was to operate them at, once started, full advance. The ported configuration entirely avoids that type of operation, so my bet is car manufacturers accepted a little bit of cooling system load for a smooth idle. If you want to plumb the vacuum chamber directly to the manifold (as many on this forum have) you will be moving in the direction that most manufacturers rejected . . . that may not be a showstopper. I always come back to the subject of octane rating of the fuel, that is, today versus what was available post war when those design decisions were being made . . . too much advance at low speed while running on 60-70 octane fuel might awaken the driver, and we can’t have that!

If your car has a Delco distributor, I have a short story about vacuum chambers. I was replacing the unit on my ’54 and decided to make a few measurements. In retrospect I shouldn’t have done that! What I found was a large difference in the total stroke of the two diaphragms, so much so one unit produced less than the specified amount and the other about twice the specified amount. I concluded that the upper stroke limit is established by a tang on the operating arm that attaches to the breaker plate, and the arms had different numbers stamped on them. To this day I don’t know the significance of those numbers, it could be a simple part number or an indicator of the calibration of the entire unit. For the unit that had too much stroke I ultimately fabricated a spring guide that acted as a high stop. The point of the story is if vacuum chamber is installed that, one – has too much stroke, and two – is plumbed directly to the manifold, then you might have a situation that has an objectionable characteristic. If that same chamber is connected as a ‘ported’ system the highest advance would likely occur during high RPM coast-down and go unnoticed.

While I was writing this response I was thinking of the few engines that I worked on that had a propensity to back-fire if the throttle was snapped open from low idle. IIMC the ‘fix’ was always to take some timing away, so you might find that a limiting condition, even though others have not. You’re only a few fittings and a length of tubing away from having the answer. I surely have been thinking of trying a ‘manifold’ system on my ’48. Don’t be surprised if a carburetor mixture and idle speed adjustment is required.

dp

Posted on: 9/1 16:36
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Re: 1951 288 Vacuum Advance Source
#5
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PackardDon
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Quote:
Some Ford products only used a vacuum control for the timing and some other manufacturers - early on - used only mechanical control for the timing. Engines used for racing tend to use only distributors with only mechanical control as engine vacuum is low due to valve operation of the competition ground camshafts.


I have a 1940 Packard Autolite distributor that is mechanical-only.

Posted on: 9/1 18:10
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