Alright, admitedly, I'm not the World's #1 Mechanic, so there are some things on a car that I don't quite understand yet. The whole "vacuum" concept is one of them. Not only on vacuum advanced carb's, but I also mean on EFI. So just to clear all this up (because I know I'm not the only guy in here who doesn't understand it) all of you knowledgable people please post on what it is, what it does, and how to use it correctly (fuel mileage). Tom P, I'll be disappointed if I don't see SOMETHING from you in here. U 2 Ked

 

ok, well since i'm home instead of pimping hoes i'll give it a shot. first, i don't believe there are any ways to manipulate vacuum for gas mileage. second,vacuum occurs when there is a difference in pressure(high and low). at idle, the outside air has low pressure, and inside the engine there is high pressure. low+ high= lots if vacuum. low + low= not so much vacuum. basically picture a piston going down on it's intake stroke. when it goes down it is literally PULLING air in. when your engine is idling, the butterfly is letting very little air in, causing high vacuum. when you open the throttle, the engines got plenty of air and it creates little vacuum. there is no restrictions so there are no differences in pressure which = low or zero vacuum.


i hope you understand that....i find myself guilty of explaining things very poorly.

EDIT: i forgot vacuum advance. as far as distributors, they are mostly on carbed cars(no computer). it uses vacuum to sense engine load and advances or backs off timing accordingly. computer controlled cars don't have vacuum advance. the computer does the job.


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Dan
1990 3.1L RS
80 Series Flowmaster
It's fast(er than a 3 cyl metro)

[This message has been edited by CaliCamaroRS (edited May 12, 2001).]

 

Here's what I can offer.
I do not know a thing about the MECHANICS of engine vacuum.
BUT...................
Ever since 1963, a car in our family had a Vacuum Manifold Gauge (this car I am referring to was a 1963 Grand Prix with 421 "Super Duty" engine-a Beautiful automobile! I was 7, then).
What does it do.
It tells ya (in my own words) how heavy your foot is on the gas pedal.
And the correlating effect?
Good or bad gas milage, or good sounding engine noises.
NOW....................
In a carb car, you can tell, when ya got your foot, heavy into the carb.
You can tell, when the secondaries open (even on a 1 bbl, you can "tell").
But, when I got my first fuel injected car, (my 1975 Opel Wageon, with Bosch L-Jetronic Feul Injection system), I couldn't tell, when I had my foot into the "secondaries".
And I couldn't tell, when I was wasting gas or not.
I got this Opel during the second "gas crisis" 1978. I installed a Vacuum Manifold Gauge, to help me get better milage.
It taught me to drive, as if I had an egg, on my gas pedal.
Lighter touch on the pedal, less gas I use. Then I could eventually tell, touching the gas pedal, more power or better crusing power (along with better gas milage).
NOW, I have seen something else. And read, too.
The more vacuum an engine pulls (makes, whatever) the more "sealed" the engine is.
What I mean is how well the bottom end is (how worn are the piston rings or cylinder bores). And how well the top end is (if the gauge flutters, you have worn valve guides). If it goes into the bad gas zone, at idle, you have a problem (a vacuum hose leak, perhaps).
So I am constantly monitering the condition of the internals of the engine (my new 40K 3.4 block pulls about 20 inches of vacuum at idle, outta gear, in gear about 15-17 inches). So, I know, from this reading, I have a very well, broken in, seasoned and sealed engine. And while driving, I have conditioned my self, to touch, lightly, the gas pedal, to produce the power (to keep up with traffic, avoiding jack rabbit starts) and that correlates to better gas milage for my rides.
I hope that HELPS you understand, how helpful this guague can be.
I have an Edelbrock, digital manifold gauge, in the Firebird.
In my S-10 Blazer, I have the one I got back in 1979 (it's a Steward Warner gas milage gauge-to reflect the "Gas Crisis" days). In my 1967 RS/SS Camaro Convertible 4-speed, I will be using a Pontiac Vacuum Manifold Gauge (big honking chrome one, just like the one in our old 1963 Grand Prix, painted black, with a "Yenko" decal covering the word Pontiac). My Corvette will have one on the "A" pilar, in a pod (like one sees often these days). My wifes 1968 Camaro? She has the 1 barrel, 6 cylinder. I just beat it to shet to get the most power I can. I do not have one in her car, yet. May never. It gets good milage, already, with the 6-er. Excpet when I drive it!!!!
Bye!

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

 

Well the engines vacuum is generated by the intake of the air it draws in through the intake manifold. This vacuum changes with the different loads put on the engine. Thats why vacuum secondaries in carbs can be more effective than mechanicals. Mechanicals open the secondaries no matter the intake draw on the outside air.
the vacuum should be directly related to engine load. Yes it can be manipulated through many means... The favorite is blocking off certain hoses, you can put in restrictor valves/smaller diameter hoses/connectors.
As far as manipulating it for better milage, it should be possible if you have the time to test it out.

 

What I knew about vaccum was covered by Dan!

There's also another type of vacuum used by engines- ported vacuum. We've got a good example on our MPFI engines! Follow some of the vacuum lines going to your throttle body; you'll see that they go to holes right above the top edge of the butterfly. The throttle plate actually blocks the port off when the butterfly's upright (gas pedal isn't pressed). (CMA disclaimer- I was told this; I haven't verified it myself on my 2.8.) By doing this, this makes sure that no vacuum will be present in this line when the engine is idling. Engine idling = no vacuum. EGR valves that don't work off solenoids (like ours do) work like this.

Thanks for the info on diagnosing an engine with a vacuum gauge, Karl! I'll have to hook up my dad's vaccum gauge and go for a ride to see just how bad my motor is!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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