Mommy, where does engine vacuum come from?

[DierwulfBL]

:lol:

[Martin]

yea vacuum is needed to boil fuel and get it to atomize.

when ya hook up a Vacuum guage to a manifold you can tell a ton about diagnosing an engine problem.

There are charts with like 20 different ways that a Vacuum guage can fluctuate. There's really specific ways engine vacuum has to react in various situations.

Theyre like 14 bucks and neat little tools.

[.RJ]

yea vacuum is needed to boil fuel and get it to atomize.

:?:

[CaptainHenreh]

Theyre like 14 bucks and neat little tools.

Yes! Everyone should know about McMaster and their 16 dollar glycerin-filled "boost" gauge. +/- 1% accurate from 0-30mm/hg and up to 100psi (not that you'd be running that much boost, but they make them as small as 15psi)

Boost vaccum gauge! For 15.43 plus shipping! Seeing stuff like that makes me want to go beat Autometer with a stick.

[Martin]

Boost vaccum gauge! For 15.43 plus shipping! Seeing stuff like that makes me want to go beat Autometer with a stick.

haha yea, McMasters is the shit


yea you can get get gas to boil by either raising the temperature of the fuel AND/OR lowering the pressure on it (put it in a vaccum)

In a carbed car during startup, the choke butterfly slams almost shut and makes a ton more vacuum( at the expense of airflow) and the fuel can get to boiling point even in the winter.

Manifold Vacuum. - in the intake manifold

Venturi Vacuum - between the throttle butterfly and the choke butterfly. High venturi vacuum boils gas IN the carburator, (and simultaneosly the vaccum sucks more gas from the jets for a Richer startup mix)

[Mike]

ry ry got pwned

[.RJ]

What in the hell is a carburator?

If the fuel boiled it would be gas, not atomized as a vapor.

[Martin]

What in the hell is a carburator?

If the fuel boiled it would be gas, not atomized as a vapor.

who would have thought this newfangled fuel injection would catch on

I copied this from <!-- w --><a class="postlink" href="http://www.factorypro.com">www.factorypro.com</a><!-- w -->

The stuff is actually useful when you're trying to diagnose old cars or even newer bikes like my 2000 Honda CBR


To begin with, only gasoline vapor will burn, so fuel must change its state from a liquid to a gas at some point or the
engine won't run.

To do this it must absorb enough heat to boil.

" Boil the gasoline, now I know this guy's a dufus"!

"For the sake of argument how the hell can I boil gasoline when it's below freezing and icicles hanging off my nose?"

"Them carburetors may be magic but they ain't that magic!"

The answer is reduced pressure. What we call "vacuum".

Water boils at a lower temperature on a mountain top than it does at sea level because there's less atmospheric pressure
bearing down on it, right?

So the vacuum in the carburetor's venturi and intake manifold do the same thing only better, causing the gasoline to boil
and fuel vaporization to occur when very little heat is present.


The boiling point is reduced so that the fine atomized droplets that are sprayed into the intake stream vaporize from the \
latent heat in the air, no matter how little that might be. Within reason.

We do know about carb icing, right?

Of course, until the engine has warmed itself up, only a small part of the available gasoline actually turns to vapor, which
is why we need the choke to cut down on the air.

[.RJ]

To be perfectly honest I think thats a bunch of crap. First off, a choke isnt to cut down air, its to increase vacuum at idle so that the damn car will run when cold and the engine needs more fuel.

The heat (much more important) and slight pressure differential will cause some of the fuel to vaporize, but the fuel doesnt instantly boil as soon as it flows into a the vacuum.

[Martin]

I say that some of the gas will become gaseous as it hits the vaccum. In a carburetor theres really nothing to atomize the fuel, it just kind of bleeds out of the jets into the venturi and splatters in the airflow and vacuum.
You ever seen an accelerator pump squirt gas like a waterpistol when you pump the gas? It doesn't really atomize fuel. Thats the vacuum's job. When people say that a carburetor atomizes fuel, it does, indirectly.

Now a Fuel injector will atomize the fuel.




What a choke does is create an artificial pocket of high vacuum in the carburetor.

1. the choke butterfly closes
2. now, instead of a very low vacuum in the venturi, there's a mega dose of vacuum in the venturi up 15-22 lbs that the manifold vacuum supplies.
3. Since the airflow can't get in, the vacuum will suck fuel instead of air from the little jets.

That's the way I've always understood it.



I copied this from howstuffworks

"Similarly, the boiling point of the fuel is greatly reduced by the vacuum in the intake manifold. Fuel vaporization is a result of several factors, including intake manifold vacuum, atmospheric temperature, fuel temperature, and intake manifold temperature. "

[BLINGMW]

what a nightmare! hock: I really have no idea how a carb works and now I'm REALLY glad I don't know. Boiling the fuel with vacuum. Honestly. :wink: