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vacuum advance

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Rick60

Rick60

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Some time ago I found out that the vacuum tube between carbs (HD8) and distributor on my engine was 'leaking' severely. So I guess I had no ignition advance due to vacuum.

Ok, fixed that the other day and had a closer look to the vacuum takeoff at the front carb. Noticed for the first time that the takeoff was 'upstream' the throttle disc.

Now my question is, how can there be vacuum or negative pressure? Engine draws air through the carbs into the cylinders. The amount of air the engine can breathe in is conditioned by the throttle disc amongst others. I always thought that the vacuum created between throttle disc and combustion chamber was used to advance the ignition, but that would mean the takeoff has to be 'downstream' the throttle disc.

Am I missing something or can anybody please enlighten me how this works?

Regards
 
Eric,
The vacuum advance is controlled by the basic venturi principle, known as Bernolli's Law. The simplest correct statement of Bernoulli's law is:

"When the speed of a parcel of air increases the pressure decreases."
This statement speaks about the change in speed of one parcel of air versus the change in pressure within that parcel.

The kinetic energy of a parcel of air of mass, m, and velocity, v, at point 1 changes to a different kinetic energy at point 2 when work, W, is done on the particle.

Work is force times distance, W = f d.

To create Bernolli's law divide each erm by the volume, V, of the parcel of air, assume the parcel is incompressible and so does not change from point 1 to 2, amazingly for subsonic flows this is a good approximation.

1/2 m/Vv12 + W/V = 1/2 m/Vv22

Mass per unit volume is density, r
Work per unit volume is force per unit area or pressure difference, DP = P2-P1

1/2 r v12 -P1 = 1/2 r v22 - P2

This relates the pressure and airspeed at one point to the pressure and airspeed at another.

Sooooo... that said, in the vacuum system (auto) the location of the Venturi port is calculated for negative pressure (vacuum) and is located to provide the most efficient change in negative pressure (vacuum). It does not matter if that port is before or after the butterfly plate in the carb. As long as the port is located to be subjected to the desired increased and decreased airflow to produce the venturi delta.

You may note that some are located in the manifold, some between the butterfly and the manifold, and some, as in your case, before the butterfly. To see the effect, just take the air cleaner off the carb and, with the engine running, put you hand near the intake of the carb (DO NOT place your hand over the carb or you will get a rather unpleasant injury to your hand), and you will feel a significant airflow. This airfow is what is used to produce the venturi effect as calculated by Bernolli's Law... Whew.... Ron
 
Wow
 
By the way, This is how we can generally explain how airplanes fly, your household vacuum sucks, airspeed indicator in your airplane 'indicates' (a whole 'nother story), gyro instruments, well, gyro, sails on sailboats, uh, sail, ad infinitum... We should all thank Mr. Bernolli.... Ron
 
[ QUOTE ]

Ok, fixed that the other day and had a closer look to the vacuum takeoff at the front carb. Noticed for the first time that the takeoff was 'upstream' the throttle disc.

Now my question is, how can there be vacuum or negative pressure? Engine draws air through the carbs into the cylinders. The amount of air the engine can breathe in is conditioned by the throttle disc amongst others. I always thought that the vacuum created between throttle disc and combustion chamber was used to advance the ignition, but that would mean the takeoff has to be 'downstream' the throttle disc.
Regards

[/ QUOTE ]
The "short" answer is that vacuum advance is usually taken off somewhere on the intake side of the throttle plate (manifold vacuum) or it is taken off a port located a bit outboard of the throttle plate (ported vacuum).

In your case, it is ported vacuum. This means that at slow idle, when the throttle plate is nearly closed, no vacuum is being applied to the advance. The reason being that engines do not hot idle so well with a full extra 20 or so degrees of advance applied. The vacuum advance is zero at idle, & progressively increases as the throttle plate is opened the first little bit to uncover the advance port. As the port is uncovered, the applied vacuum now corresponds to manifold vacuum in the normal manner.

Engines that use manifold vacuum advance instead of ported, have different distributor vacuum & centrifugal advance calibrations.
D
 
Dave, Well articulated, and, unlike mine, short and to the point. I tend to be a bit verbose... /ubbthreads/images/graemlins/cheers.gif Ron
 
Ron,
I fully understood & appreciated your explanation. Just thought I would shorten it a bit for those who might not understand the formulae. Not trying to degrade your explanation.
D
 
Most associate Bernoulli's priciple with air though it was actually fluid dynamics. But the real question is...What are the three sepatate numbers on the vacuum advances? Example 4-17-6. It is not a date of production. It deals with how the advance reacts when.

Alan T
 
Dave, No exception nor umbrage taken... Thanks for the thought, though... /ubbthreads/images/graemlins/cheers.gif Ron
 

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Alan, You are absolutely correct. However, in nature and in applying the Bernolli Law, air (generally speaking) can be considered a fluid with very low density. The numbers and formulae still apply. Wow, this could get very far off the topic of the location of a vacuum line..Fun tho.. /ubbthreads/images/graemlins/grin.gif Ron... /ubbthreads/images/graemlins/cheers.gif
 
Urban legend is that Bernoulli was watching sewage flow and thought this would be a good time to apply mathmatics .

Alan T
 
Alan.... GREAT COMMENT... /ubbthreads/images/graemlins/lol.gif Ron
 
[ QUOTE ]
But the real question is...What are the three sepatate numbers on the vacuum advances? Example 4-17-6. Alan T

[/ QUOTE ]

Marking of 4-17-6 on the vacuum can would indicate that vacuum advance:

Starts at 4 inches of Hg.
Reaches maximum advance at 17 inches of Hg.
Maximum advance is 6 degrees distributor or 12 degrees crankshaft
D
 
Thanks , Dave. I am asked that question about every ten years or so. I need to write it down somewhere. /ubbthreads/images/graemlins/crazyeyes.gif

Alan T
 
Gentlemen, Thanks for the conversation. I am bidding you farewell. /ubbthreads/images/graemlins/thumbsup.gif

All the best to you and yours... Ron (One '66 MGB MkI, one '57 MGA/1500, and one '58 100-6 BN4...) /ubbthreads/images/graemlins/patriot.gif
 
Gentlemen,

thanks for your ample explanations. It was fun as well as instructive to read.

Thanks to Ron, who threw a light on the academic side of this subject and whom I promise to retrace his formulas - time permitting.

Thanks to Dave R., who made it all clear to me. I knew Bernoulli's principle before, even though I couldn't have it explained as Ron, but the reason, why the vacuum takeoff is upstream the throttle disc escaped me.

My understanding is, that it will not come in effect when the butterfly is closed and therefore the engine is idling. Sounds logical to me.

Thanks again.
 
Yes Rick60 that is logical but you must remember that there is airflow around the worn/partially open throttle disk and via the Tickover screw and it's associated ducting.---Keoke
 
Yes, Keoke, you're right, but I assume on tickover the air is travelling relatively slowly near the vacuum takeoff tube and - since increasing air speed decreases pressure (and vice versa) [thanks Ron /ubbthreads/images/graemlins/savewave.gif] - it won't effect the timing too much. Nevertheless, I'll disconnect the vac tube when timing the engine. Thanks for your hint.
 
[ QUOTE ]
The "short" answer is that vacuum advance is usually taken off somewhere on the intake side of the throttle plate (manifold vacuum) or it is taken off a port located a bit outboard of the throttle plate (ported vacuum).

In your case, it is ported vacuum. This means that at slow idle, when the throttle plate is nearly closed, no vacuum is being applied to the advance. The reason being that engines do not hot idle so well with a full extra 20 or so degrees of advance applied. The vacuum advance is zero at idle, & progressively increases as the throttle plate is opened the first little bit to uncover the advance port. As the port is uncovered, the applied vacuum now corresponds to manifold vacuum in the normal manner.
D

[/ QUOTE ]

Dave, one thing I don't understand is, how can the "port vacuum" correspond to the "manifold vacuum" after the throttle disc has opened a certain amount?

I mean, this depression within the intake system is used to advance the timing when the engine is revving and the throttle is partly open. The engine is sucking air and the flow of the air is restricted by the disc. Thus creating a depression within the manifold. The combustion chambers are filled suboptimally, therefore the gas needs a bit longer to burn. To compensate for that the timing is advanced. That was my understanding up to now.

When applying ported vacuum I cannot see the same principle being applied. Could you (or somebody else) enlighten me on the principle of ported vacuum, please (except, what Bernoulli would have said about it /ubbthreads/images/graemlins/wink.gif)?

You see, manifold vacuum increases the higher the engine is revving and simultaneously the throttle is closed, ported vacuum increases the higher the engine revs, regardless of the throttle disc (once it's been opened) due to the air speed!?! /ubbthreads/images/graemlins/confused.gif

Thanks for your thoughts.
 
Hi Eric,
We don't really need to get air speed into the discussion. Perhaps my explanation was not complete or clear. If you have more questions, ask away. I don't always do explanations that everyone understands. We can eventually work it out.

The "port" is effectively on the engine manifold side of the throttle disc as soon as the disc is opened slightly. The port is located in a position that "exposes" it anytime the disc is opened even a slight amount. There is always vacuum pulling against the disc unless it is full open.

Or, once the disc is open a bit, the port IS effectively on the manifold side of the disc.

Ported vacuum only blocks the vacuum advance when the disc is closed at idle or on overrun. At all other times, there is vacuum advance which corresponds to manifold vacuum. A typical carb will have the vacuum port located somewhere between 1/32" & 1/16" outboard of the disc's idle position. Even a fast idle will start to bring the ported vacuum advance into play. This is one of the the reasons that ignition timing at idle is usually set with the vacuum line disconnected. BTW, this "extra" fast idle advance actually helps with the first few minutes on cold startups when the mixture is "thin" due to incomplete vaporization.

The further open the disc is, the lower the manifold vacuum, the lower the vacuum against the disc & the advance port, the less vacuum advance. Under part throttle cruising conditions the disc is only maybe 25% open, the manifold & port vacuum are both high & timing is advanced to compensate for the "thin" mixture. A typical vacuum advance might give 20 crankshaft degrees at 18" of Hg, reducing to zero degrees at 7" of Hg. Vacuum advance only improves fuel economy & part throttle driveability. As you know, at full throttle, it is not even in the picture.

There are several "emission control" schemes which operate differently. Some deliberately apply full vacuum advance at idle. Some apply vacuum retard at idle, some control vacuum advance/retard with temperture, some with computers. I am not addressing these "newer" schemes which often turned out to be ineffective "band-aids".
D
 
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