On my freshly rebuilt carbed 2.7l- do I need the vacuum advance on the distributor? Is there a performance increase or decrease from using it? If I don't have to use it, what is the best way to get rid of it? Just unplug the vacuum tube?

Vacuum advance purpose is the bring the advance on earlier do the acceleration is smoother. Without it you will still get full advance but the initial acceleration may more jerky or jumping from none to full advance.

So if I choose not to use the vacuum advance- can I just leave the vacuum hose off or is there anything else I need to do to not use the vacuum advance?

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100 years later..
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The latest ecu controls the spark and fuel delivery without the mechanical linkages.
Same effect though,and with finer resolution,for the improvement of efficiency and mileage.
So yeah,a spark advance from the starting setting is desirable,as the engine will not start at the high rpm advance setting.
The vacuum advance is relative to throttle opening and manifold pressure.
All of this is basic engine tech,has been for a century,the improvement of the infernal combustion engine.
Without the correct operation of the vacuum and centrifugal advances,the performance is poor.

/

Several ways to go about it. The easiest is to just leave it unplugged.

The best-functioning way is to get a replacement distributor that is set up for your particular application. The original distributor is set up with a curve that assumes vacuum retard (and possibly vacuum advance) plus a low-compression engine with a mild cam running fuel injection. Your engine violates pretty much all of those assumptions.

Plus the original distributor is 40+ years old, and will have some wear. A new distributor shouldn't have any wear.

I bet the guys at the Type4 Store would be happy to sell you a distributor. Others likely would as well.

--DD

i think that you need a dist. that operates on a vaccum advance, yes you need it. if it is a dist that operates on mechanical advance, no you dont need it. you need an advance/retard function,how that is accomplished is determined by the dist components. my indian motorcycle advance is determined by a lever on the handlebar and adjusted for riding conditions by the lever(twist grip) later triumps norton have centrifical weights to advance the timing by rpm. current cars the ecu does it. if you dont want to run vacuum advance, run a dist with centrifical advance. time it fully advanced.

Is yours a mechanical advance, or a mechanical retard?

Vacuum advance - will advance timing at idle and under light throttle. This will increase throttle response at initial throttle opening, but cuts timing back at low rpm acceleration. This is an advantage if your engine is at risk for detonation under these conditions. It will also advance timing at cruise which will improve fuel economy and let the engine run a little cooler.

Vacuum retard - well, it does the opposite. It retards timing at idle and under light throttle. The only applications I've seen for this is to reduce tailpipe emissions; primarily NOx by reducing combustion gas temperatures.

Either are refinements of the basic mechanical advance curve built into your distributor. Personally I have had the best luck eliminating the vacuum and focusing on refining the mechanical curve to fit my particular engine parameters and operating conditions. Vacuum advance can be an advantage if performance at cruise is important, but tuning the advance curve from the vacuum canister is challenging.

useful information here http://www.sparkingplugs.com/9.html

and here - https://dorkiphus.net/porsche/showthread.php?t=11540

Barry Hershon removed my vacuum advance and set the advanced timing mechanically. Along with rebuilding the unit - costly but I got a brand new dizzy that works great on my 3.0L with Webers.

http://flat6recurve.com/

Shoot, I was assuming a four-banger. If this is for a Six, that may change things somewhat...

--DD