Thread: Vacum???

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This is probably a dumb question, but as I have no gauge to check this so I thought someone else might know.

Were is the highest point of vacume on a carborated engine? Just below the throttle plates, Or is vacume equal throughout the intake system. I have noticed power brake booster usually attached to port in carb under throtle plates. I wasn't sure if this made a difference or was just a point of convience for the auto manufactures.

And yeah I can't spell for S#@%.

Originally Posted by dlotraf33

This is probably a dumb question, but as I have no gauge to check this so I thought someone else might know.

Were is the highest point of vacume on a carborated engine? Just below the throttle plates, Or is vacume equal throughout the intake system. I have noticed power brake booster usually attached to port in carb under throtle plates. I wasn't sure if this made a difference or was just a point of convience for the auto manufactures.

And yeah I can't spell for S#@%.

I'm no expert, but I think the highest amount of vacuum is at the intake manifold. Most Fords, at least the ones I have had, have both the trans and power brakes vacuum source on the rear of the intake manifold itself. An Edelbrock intake that I have on my 302 also has the power brake/trans vacuum source on the back of the intake.

I just got a new Edelbrock carb, and it says to connect the power brakes at a fitting at the back of the carb so I assume this is full vacuum. I have neither power brakes nor an auto trans on this particular car so its not a situation for me.

With the throttle plates closed, the highest point of vacuum is in the manifold.

If you would put a vacuum gauge port in the manifold immediately below the throttle plate, and another just above the valve furthest away from the carb, you would see equal vac at both spots with the throttle closed. When you opened the throttle, let's say WOT for example, the gauge at the carb base would register the drop in vacuum before the one at the valve, but it would equalize almost instantly. So, you could say that anywhere in the system between those two points would be equal, for most purposes; there is not enough volume in the system to make much difference. Even in a very large volume, the vacuum will equalize very quickly. OK, you engineers, it's your turn.

There are two different Vacuum signals..Manifold vacuum is from the intake or carb base is for boosters, crankcase ventilation and modulators. Port Vacuum is on the carb and is only present when the throttle plates are open and should be used to activate the vacuum advance and secondary carb functions..Hope this helps.

Any vac assist brakes that I have worked on were always tied to manifold vac.

Please excuse the "partial hijack" of the thread, but here's some real good information on "manifold vs ported" vacuum. I know I've scratched my head on this topic more than once!

As has been stated above, when an engine is idling or at a steady cruise (when the load is very low) intake manifold vacuum is high because the throttle butterflies are nearly-closed; under acceleration, the throttle butterflies open wider, and intake manifold vacuum drops; it is essentially zero at wide-open throttle. The strongest vacuum signal will always be found in the manifold at idle. As stated – it is consistent anywhere in the manifold +/- scant percentages.

In my opinion, ported vacuum should be avoided at all times. Ported vacuum was introduced by GM as a component of their Air Injector Reactor (remember AIR?) system. It pumped fresh directly into the exhaust manifolds to encourage an afterburner effect to incinerate pollutants in the exhaust manifolds. For this system work at maximum efficiency it was necessary to retarded spark at idle. With retarded idle spark timing, the combustion burn in the cylinder begins late, and is not quite complete when the exhaust valve opens. This accomplished two things which (in theory) would reduce emissions: 1) the incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX). 2) The very significant increase in exhaust gas temperature helped ignite and consume the hydrocarbons in the exhaust flow as the fresh oxygen-rich air was introduced from the air pump.

As many of us can remember, these engines ran poorly. The problems centered on the enormous amount of wasted “heat energy” that was transferred through the exhaust port walls into the coolant, causing these engines to run very hot at idle. This then caused cylinder pressure to fall, overall engine temperatures went up, combustion efficiency suffered greatly, and fuel economy went down with it.

“Ported Vacuum” was simple to implement – GM just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where it was exposed to full manifold vacuum) to above the throttle plate, where it was only exposed to manifold vacuum after the throttle plate opened. This meant that the vacuum advance was inoperative at idle - retarding idle spark timing from its optimum value. These engines had low initial timing settings; they were usually set at 4 degrees before TDC and some even had initial timing settings as much as 2 degrees after TDC. The vacuum advances still worked at highway speeds, but not at idle, which gave GM a deserved reputation real dogs for several years.

Ported Vacuum was introduced as a component in early emissions control strategy. Anyone who tells you that ported vacuum is a good thing for performance and “driveability” is mistaken– it’s not.

(Information above is excerpted from “TIMING AND VACUUM ADVANCE 101” by John Hinckley)

i have AIR on my 78 chevy truck it runs and drive perfectly fine with emissions and a stock small block. low rev but still very torquey and managable

Glenn, thanks for the primer by John Hinckley. I'll come back here and copy and paste it when this subject comes up again at other forums.

Originally Posted by 78c10

i have AIR on my 78 chevy truck it runs and drive perfectly fine with emissions and a stock small block. low rev but still very torquey and managable

I'm very pleased for you that you are happy with what you have.