A Question Of Timing

[Joe M]

I've heard a lot of different things about the benefits of running vacuum advance on the street and about whether to use ported or manifold vacuum.  I found this essay on the Internet which seems to agree with what I know about running vacuum advance but I thought I would run it by the experts!  What do you guys think?

http://rockridgefarm.com/vettdoc/Timing_101.pdf

[jayb]

Interesting writeup.  I didn't realize that spark ported vacuum was an early emissions type fix.  If that is correct, then straight manifold vacuum is indeed the way to go, for stockish type engines anyway.  However, I would have to disagree with the author regarding running vacuum advance on all street engines.  The problem that you run across on an engine with a healthy cam is that the idle speed is not stable (rough idle), so the vacuum signal is not stable at idle speeds.  In some cases it turns out to be all over the place, varying by as much as +/- 4 inches with some cams.  The net result on a car equipped with a vacuum advance distributor is wildly varying timing at idle, which then results in even more variation in idle speeds and mixtures, etc. etc.  So the rough cam itself will cause idle speed variation, and then this is amplified by the timing variations caused by the vacuum advance distributor.  You just can't get a reliable idle speed in some cases.  So on any engine with even a medium performance cam, I always ditch the vacuum advance system. 

Maybe if you want to run a healthy cam and take advantage of the vacuum advance at cruise, you can use spark ported vacuum, since it doesn't come into play idle?  How ironic...

For what it's worth, even using spark ported vacuum I have had trouble with the idle quality of rough cams, and in the past have been able to improve the situation by ditching the vacuum advance system.

[Faron]

I will say this , WRONG , early cars ran Venturi vacuum, not manifold , many cars had NO Mechanical advance at all ( Fords Loadamatic ) it was simple and worked for low compression low horsepower engines , I guess all the Hi horsepower engines of the 60's got it wrong they used Mechanical only Dist , when a Manifold vacuum setup would of been the ticket , I love these articles , a properly set up Distributor using Mechanical , and Vacuum advance connected to a ported source is optimal , if for any reason it runs better with Manifold Vacuum , the Mechanical part has the wrong curve

[jayb]

That's a good point about the stock no vacuum advance distributors, Faron.  Apparently the author knows more than Ford 

[Joe M]

Thank you for the great information and for the comeback.  I'm always grateful to folks with more knowledge who are willing to teach me something!   I remember something called a Ported Vacuum Switch which screwed into the water neck.  It was my understanding that it would switch from ported vacuum to manifold vacuum if it sensed an overheat condition.  It would advance timing at idle and also increase idle RPM to bring the engine temp back down.  It was an early band-aid emission device.  I'm pretty sure the 428CJ had vacuum advance with a PVS.  I can certainly see how a pretty wild engine wouldn't do well with vacuum advance but on a milder street engine I would think it would help low speed street operation both in a cleaner idle and helping reduce engine temperature.  Educate me!  LOL!

[jayb]

Well, I think you've got it right.  On a mild street engine, vacuum advance makes a lot of sense.  On a more radical street engine, not so much.

[Joe M]

Thanks again for the response and the info.  You guys rock!