Understanding Vacuum Advance

Shamu70

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Hi guys. When I bought the Newport years ago it had already had a stroked RB put in. And a little while later I’d noticed that was driving it with the distributor dash pot unplugged. Previous owner had jammed a bit into a vacuum line going to ported side of Edel. carb.
Car has always been a BLAST to drive.
Well after reading horror stories about washing down your cylinder walls with excess vapors, I hooked the dash pot back in. Yeah, it idles a little smoother, but boy is it tame above 50 mph.
I don’t have any specs on cam, compression; guy gave me nothing.
Why would vacuum advance calm things down? My limited knowledge say that increased timing gives more performance? I don’t understand.
 

CBODY67

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When the engine is running along at part-throttle, the air fuel mixture getting to the cylinders is not at "full strength", so it takes more ignition advance to get it to fire-off at or near TDC, for best power and fuel economy. When at WOT, the mixture is richer and easier to fire-off, other than the higher cyl pressure making it harder for the spark to happen, at the same time.

Vacuum advance is almost always hooked to ported vacuum on the carb base. Typically, there is one port which is full manifold vac and one which has very little vac at lower rpms, or until the throttle plates uncover more of the ported vac hole in the throttle body. So, making sure the vac advance line is hooked to the correct vac port on the carb is very important.

Basically, the engine idles with 0 degrees total distributor advance, being set at a fixed idle spark setting as spec'd by the factory. Once the throttle opens to accelerate from a stop, normal throttle or uo to about 3/4 throttle, the vac advance slot is exposed to intake manifold vacuum and the vac advance happens. It usually takes about 9" Hg to acrivate the advance, with max vac advance at about 14" Hg, with the throttle openned. Check the FSM for exact specs.

The behavior you describe with the vac advance hooked up sounds a bit unusual. IF the distributor has been modified with a much quicker advance curve, then the additional vac advance should be making the engine clatter under part-throttle acceleration. Usually, 2500rpm no-load vac+centrifugal+base timing can usually result in about 50degrees total advance BTDC, but it does not clatter because the engine is a partially loaded. More total timing and it should clatter if it is not already. Yet 38 degrees total at WOT is uaually considered optimum for a B/RB engine at WOT. BTAIM

Check to make sure the vac advance is hooked to the ported vac port on the carb. Even if there might be a slight vac at hot base idle, it should not be enough to advance the vac advance unit. Do also ensure that the vac advance unit holds vac.

Back in the 1950s and up until the middle 1970s, it was common for some speed shops to delete the vac advance and put the distributor into full mechanical centrifugal advance mode. If the engine rpm was at a certain level, there was always a certain known mechanical advance happening. Some road racers wanted that consistency, but they were always at or near WOT anyway, so the vac advance would have been of little consequence in that application. Many medium-duty gasoline engine trucks also had centrifugal advance only distributors, from the factory. With the aero load on their cargo boxes, they were usually operating at a manifold vac level where the vac advance would not have been operative. Those engines always sounded a bit doggy when you goosed the throttle on them, to me, compared to a normal car or 1/2-tone pickup truck motor. Yet their total WOT horsepower ratings were not that different, other than related to the lower compression ratio.

On our '66 Newport 383 2bbl, I noticed that the gas mileage had dropped to about 12mpg average. Base timing was where it should be, plus the carb was still working well, too. Then I thought to check the vac advance can. It was leaking, so no vac advance, which explained why the gas mileage had dropped, although it was not running bad.

So, you probably need to investigate what you have and that it is hooked up like it needs to be. Usually, seems like the ported vac port on Edelbrock/Carter AFB/AVS carbs is on the front, passenger side of the carb base plate. Full manifold vac is on the other side of the front of the carb base plate. Then check to see if the vac can is operational and working. Then determine what the hot base idle base timing is. Then how much centrifugal mechanical advance is in the distributor. I suspect that a hot base idle speed of 650-700rpm would be appropriate, with the idle mixture adjusted to "lean best idle" adjustments at that rpm level in "P" or "N". Please advuse if your findings.

Sorry for the length, but not a "sound byte" type of question.

Enjoy!
CBODY67
 

furious70

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Need to see what it's doing with a vac gauge and dial back timing light.
 

Shamu70

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Thank you very much for your replys. I had also read that some would add lighter springs or heavier weights, or vice versa, to the distributor. This “mechanical” advance feels like what it has. And man, it’s a beast! Yet quite civil to drive around.
 

1970FuryConv

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Thank you very much for your replys. I had also read that some would add lighter springs or heavier weights, or vice versa, to the distributor. This “mechanical” advance feels like what it has. And man, it’s a beast! Yet quite civil to drive around.
If you have a vacuum pump, I would put it on the vacuum canister nipple and check for a vacuum leak in the diaphragm of the vacuum advance/dash pot. A vacuum leak would hurt performance.
 
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