Horrible mileage

[theastronaut]

Originally Posted by HEI451

On a non-emissions engine, ported vacuum advance is a second acceleration advance curve, and it just does not work

Non-emissions engines do not need, nor like two separate advance curves for acceleration

Ported vac advance does not act as an "acceleration curve" since the ported vac source sees zero vacuum at WOT, so there's no added vacuum advance. Vac advance drops back to zero, and the centrifugal advance is the only advance there is.

Originally Posted by HEI451

"Manifold vacuum pulls all the time, so setting the timing is somewhat backwards from ported vacuum." Well, this is so far from the truth, it isn't funny at all. Full manifold vacuum is strong at idle, and at low load engine operation, and virtually goes away when large throttle loads are used.

I did forget to say that manifold vacuum goes away at WOT, this is generally understood though. My point was that with manifold vacuum you are starting at idle with added vac advance, and it lessens as more throttle is applied. That is backwards from ported vac advance- zero at idle, added in at low loads (the only time it's needed), and drops back to zero at WOT.

Originally Posted by HEI451

Sorry, I don't buy the argument that there is no positive truth about vacuum source differences, I have fixed so many mis-applied setups in the 40 plus years I have been in factory engine development, racing and design to know quite differently to know different.

It worked for VW for millions of cars, and is simpler to time since both advance systems are completely separate, and the total advance under any condition doesn't have to be dependent on the vac advance. If you lose manifold vacuum, you lose your initial and part throttle timing. If you lose ported vacuum, your initial advance stays the same so your idle isn't affected. Either system can be "misapplied" if the distributor's initial and centrifugal advance isn't optimized for the vacuum source you're using.