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Re: [T3] '68 SQ w/ dual carbs questions

On 10 Dec 2002 at 22:59, Greg Merritt wrote:

>  With a vacuum advance system, when you open the throttles, 
> the ignition timing retards, no?  It "kicks in" (advances) under a 
> strong vacuum (i.e., throttle shut), right?

This is exactly where the confusion starts. Manifold vacuum acts the way you 
describe here, but very few distributors actually use manifold vacuum.

Most vacuum advances use vacuum from carefully drilled holes somewhere in the 
carb, usually near the throttle or in the venturi. The holes are positioned to 
give a particular vacuum response as a function of various conditions.

The fuel economy drillings are near the throttle valve so that there is high 
velocity (remember Bernoulli's Principle) there when the throttle opening is 
small, but none when the throttle is either closed or fully open.

Systems that use vacuum advance for the major source of advance want to respond 
to the load on the engine, so they put the drilling in the venturi where the 
vacuum is simply a function of the amount of air drawn into the engine.

Ports used for vacuum retard in some late cars are drilled near the throttle 
valve in such a place that they see full manifold vacuum when the valve is 
closed, but that plate crosses over them quickly as soon as the throttle is 

I think in our cars, the only place that uses true manifold vacuum is the 
vacuum modulator for the AT.

Jim Adney
Madison, WI 53711-3054

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