purple82

Unfortunately, that's not going to tell you anything. The mixture is regulated by the carb and there's no communication between it and the distributor so any variation in fuel economy won't be due to timing differences. It'll be due to warmer weather, more humidity, driving techniques and other things.

Rogue_Wulff

Actually, there will be variances in fuel economy. With the timing advancing as it should, the engine runs more efficiently, thereby using less fuel.
I do admit I like your idea about having someone impartial use a vacuum gauge. The downside is, very few cars are actually totally unmodded.
As for why engineers hold no place of respect to me, too many of them have been wrong, costing many lives, and major property damage. Just cause something works in theory, doesn't make it fact. Engineering stuff is just applying theory, and hoping for the best, all too often in this fast paced, money hungry economy.

djjjr42

Kentetsu and Rogue Wulff are absolutely correct on this! Why is it so hard to see that manifold vacuum is higher/ present only when the throttle plates are closed? The engine is spinning, attempting to pull in as much air & fuel as possible but is limited due to the restrictions imposed by the butterfly valve. Once you open the throttle, it is exposed to atmospheric pressure and the vacuum is filled! The vaccum returns to the manifold once the throttle closes again, allowing things like your brake booster to work (like Rogue said). That is why the brake booster system is pretty much foolproof, because braking generally occurs under deceleration conditions (throttle plate closed, manifold pressure higher). You want to slow down, so you back off the gas and hit the brakes! This is one of the only times where you will see manifold vacuum, the other being when you are descending a hill, to control speed (which corresponds to the whole idea of engine braking). If you continued to use the same amount of throttle going down the hill, your speed would increase, right? If you wanted your speed to remain constant, you would back off the gas (close the throttle) and utilize the braking effect of the engine to control your speed.

Forget about diagrams, gauges, degrees, etc. and use some common sense. Here's a simple experiment you can do right now to understand this principle. Close your mouth and pinch your nose closed, and try to breathe in as hard as you can. Try as you may to fill your lungs with air now, you can't! But you are creating a vacuum there by attempting to suck in air, right? Open up your mouth (throttle plate) and the vacuum goes away as the air rushes in to fill your lungs. Your throat is acting like the manifold here, your lungs being the engine.

AaronPhelps

Really no kidding ? I perfer to take advice from a tuner that i personally know that is #1 tuner in the world for Toyota Mr2 Turbos instead of from a book. I've read Hartman's book and its not the end all be all book. Information should always be taken with a grain of salt. I've also read quite a few other books and dabbled into tuning with an AEM my self.


My comment wasn't directed towards you but to anyone who is talking about education as far as school goes. I've met some incredibly stupid mechanical engineers and some incredily smart ones at the same time.


Why are you people still aruging? It is pretty common knowledge that venturi vaccum increases with load.

purple82

It'll never show up by driving the car around without making corresponding carb adjustments.

purple82

If that's all you think we do, you have no idea what an engineer does.

REVHED

Complete rubbish. There is high manifold vacuum whenever you are cruising or on light throttle not just idle. This is what the vacuum advance is there for. It is not designed to increase with load.

You use the exact same principal when tuning the ignition timing load map on an aftermarket ECU. You start with lots of advance at high vacuum and slowly ramp it down until you reach atmospheric.

REVHED

This is unbelievable. Once the throttle plate passes the port is under manifold vacuum! The diagram even shows that automatic cars have the vacuum source UNDER the throttle plate. How the hell would they work at all! Vacuum advance is not designed to increase with load. It is for cruise and light throttle situations. If you could only read and understand this sentence from the article you posted you would realise how wrong you are.

That means it has to work off manifold vacuum because there is not much air flowing through the carb. Why do you think we have idle and transition circuits? It's because there's not enough airflow to pull fuel through the venturi at idle and when cruising on light throttle.

djjjr42

Hmm, nice intellectual counterpoint there, "complete rubbish". No supporting facts to back it up either.

Here's a basic run down on how carburetors work. Note how many times it says that manifold vacuum decreases with the throttle opening. Also note things like "Bernoulli's principle" and that it emphasizes how important the venturis are. By the way that port in your fsm diagram for cars with AT is not below the valve, thats why Kentetsu noted that it was a misleading depiction with the throttle valve being open.

Basics


A carburetor basically consists of an open pipe, a "throat" or "barrel" through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

[edit] Off-idle circuit

As the throttle is opened up slightly from the fully closed position, the throttle plate uncovers additional fuel delivery holes behind the throttle plate where there is a low pressure area created by the throttle plate blocking air flow; these allow more fuel to flow as well as compensating for the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to metering fuel flow through the regular open throttle circuit.

[edit] Main open-throttle circuit

As the throttle is progressively opened, the manifold vacuum is lessened since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the velocity increases, pressure falls). The venturi raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the venturi. Sometimes one or more additional booster venturis are placed coaxially within the primary venturi to increase the effect.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

[edit] Power valve

For open throttle operation a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, engine vacuum decreases and the spring opens the valve to let more fuel into the main circuit. On two-stroke engines, the operation of the power valve is the reverse of normal - it is normally "on" and at a set rpm it is turned "off". It is activated at high rpm to extend the engine's rev range, capitalizing on a two-stroke's tendency to rev higher momentarily when the mixture is lean.

Frostycrowd

iTrader: (9)

Wow some great topics have come up in this thread

Frostycrowd

iTrader: (9)

There is no doubt that vac advance is a good thing to have, granted I don't think it would be a huge deal if you did not have it hooked up for a few weeks.

djjjr42

And here is manifold vacuum explained for you...note that it says it should not be confused with venturi vacuum, and again how the manifold vacuum decreases upon throttle opening. To paraphrase you, this is as clear as day, and I hope it puts an end to all this nonsense. If after reading this you still can't get it then there is no point in debating this any further. You can lead a horse to the trough but you can't make him eat.

"Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

Manifold vacuum is an effect of choked flow through a throttle in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine. In some engines, the manifold vacuum is also used as an auxiliary power source to drive engine accessories. Manifold vacuum should not be confused with venturi vacuum, which is an effect that is exploited in carburetors to achieve a fixed mix ratio between fuel and air.

The rate of airflow through an internal combustion engine determines the amount of power the engine generates, and most engines are controlled by limiting that flow with a throttle that restricts intake airflow. Manifold vacuum is present in all naturally-aspirated engines that use throttles (including carbureted and fuel injected gasoline engines using the otto cycle or the two-stroke cycle. Diesel engines do not have throttle plates.).

The mass flow through the engine is determined by the rotation rate of the engine, multiplied by the displacement of the engine, and the density of the intake stream in the intake manifold. In most applications the rotation rate is set by the application (road speed in a car or machinery speed in other applications). The displacement is dependent on the engine geometry, which is generally not adjustable while the engine is in use (although a handful of models do have this feature, see variable displacement). Restricting the input flow reduces the density (and hence pressure) in the intake manifold, reducing the amount of power that is produced. It is also a major source of engine drag (see compression braking), as the engine must pump material from the low-pressure intake manifold into the exhaust manifold (at ambient atmospheric pressure).

When the throttle is opened (in a car, the accelerator pedal is depressed), ambient air is free to fill the intake manifold, increasing the pressure (filling the vacuum). A carburetor or fuel injection system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a "naturally-aspirated" engine, total engine output is thus determined by the ambient barometric pressure. Superchargers and turbochargers can "boost" manifold pressure to above atmospheric pressure, usually by a maximum of 0.7 bar.

Some modern engines using the Atkinson cycle rely on variable valve timing to regulate mass flow through the engine, and hence have no throttle and no manifold vacuum at all. In those engines, mass flow is regulated by phasing the intake valve to change the effective displacement of the engine.


[edit] Manifold vacuum vs. venturi vacuum

Manifold vacuum is caused by a different effect than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect and depends on the total mass flow through the carburetor. In engines that use carburetors, venturi vacuum is proportional to the total mass flow through the engine (and hence the total power output)."

purple82

Everything you've presented says the same thing that REVHED is saying. I'm totally confused by what you're trying to argue.

purple82

Folks, don't listen to me, don't listen to anyone, google vacuum advance and read 5 or 10 of the thousands of articles that pop up and learn for yourselves.

Sgt.Stinkfist

iTrader: (6)

all ill say is that according to the diagram revhed posted, the vac. signal for the distributors advance comes from in front of the throttle, where the vac. signal will be almost non-existant at idle and will get stronger as rpm increases (higher rpm=> higher air velocity=> stronger vacuum=> more advance) What so hard to understand? its just like a 4 bbl holly carb w/ vacuum secondaries, the secondaries only open when there is a strong enough vacuum, a vacuum thats only present at higher RPM or load. not to mention that advancing timing is only really beneficial at higher rpms.

djjjr42

No, I haven't. You should read it again and see how we do not agree on many important points. Manifold vacuum has nothing to do with how the vacuum advance system works because the port for it is above the throttle plate. Manifold pressure exists behind the plate and only decreases as the plate is opened with the ultimate goal being zero manifold vacuum (same as atmospheric pressure) at WOT**.During that same motion, venturi vacuum increases at a rate proportional to the output of the engine, exactly opposite of manifold vacuum. He is further complicating things here by using the terms "engine load" and "rpm" as if they are synonymous. They are not.

**Though that goal will most likely never be attained due to inefficencies in moving air through the manifold, etc. and a slight vacuum may still exist, it will be far less than the vaccum than is being pulled at the narrow base of the venturi.

REVHED

Comprehension isn't one of your strong points is it? Everything you said in the previous posts is what I've been saying all along. Your problem is you don't understand when vacuum advance is required. It's for situations when there is high manifold vacuum ie, cruise and light throttle.

Load and rpm and not synonymous but they are closely related.

Hopefully this will help you understand.

REVHED

Here is the complete page from the factory manual along with test procedure. It tells you to test for vacuum at 1000-1200rpm or just off idle in other words. This is with the car stationary ie, NO LOAD. This means it has to be connected to manifold vacuum as there wouldn't be enough airflow to pull a vacuum at the venturi. Furthermore, the testing procedure for an automatic tells you to test for vacuum at idle. The diagram even shows the vacuum port under the throttle plate. Once again, MANIFOLD VACUUM.

If you or anyone else still can't understand this then I give up

Kentetsu

Again, an excellent argument Revhed. I can totally understand your logic, and how you came to your conclusions. However, your theories are still incorrect.

In your picture of the throttle plates, you claim that the opening is being exposed to manifold vacuum. Well, at that point there would be far more venturi vacuum than manifold vacuum. And it is the venturi vacuum that influences the vacuum advance. The faster air flows past that point, the greater the vacuum and the greater the advance (to a point).

djjjr52: keep up the good work man...

Kentetsu

Revhed; the answer is in your own post!

"Decrease engine speed rapidly from 4,000 rpms and make sure that there is no vacuum".

If this system was based on manifold vacuum, then there would sure as hell be vacuum there under those conditions! But on the other hand, if it were based on venturi vacuum then there would not be any vacuum under those conditions because the throttle plates are closed and there is no air flow through the carb to create the vacuum.

REVHED

No there wouldn't because it's just above the throttle plate when fully closed. That is why the manual tells you to test for vacuum just off idle.

REVHED

You really are thick. Just off idle there is far more manifold vacuum then venturi vacuum. What do you think carbs have idle and transition circuits for? It's because there's not enough venturi vacuum to pull fuel through the main circuit. The main circuit only comes into play under load. Try hooking a gauge up and seeing how much vacuum you're pulling at cruise. The answer is more than idle in many cases. Funny how you completely chose to ignore the test procedure in the FSM and the fact that automatic transmission cars have the vacuum port below the throttle.

djjjr42

Well genius, please explain how and why this system works with your theory for cars with an automatic transmissio. Since you are saying the port for them is below the throttle plate where it will always be exposed to manifold vacuum when the plate is closed, what you're really saying that it works the exact opposite way for an automatic!? If you note the title of the thread it, asks why Mazda retarded timing at idle, the answer being for emissions purposes. So at idle an automatic would clearly be advancing it with all that manifold pressure bgehind the plate if your theory was correct.

purple82

Just take a couple of minutes and read some of the articles on google.

purple82

If you read the first post, it asks how manifold vacuum works, that's what we're explaining.