Re: 1951 288 Vacuum Advance Source

Posted by DavidPackard On 2022/9/1 16:36:39

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.


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