Vacuum advance
August 18th, 2011, 03:52 AM
#1
Vacuum advance
I'm a fan of running vacuum advance on a street car however a group of Mopar Fagz I hang around on Thursday nights do not run vacuum advance on their street cars. Every one of them have it capped and tell me what their total mechanical advance is. A couple have some overheating problems. I have heard that the engine would run cooler with a bit more throttle response with more advance at cruise. but these guys know everything. As a matter of fact I watched them ruin 3 alternators because they kept grounding one of the field terminals and ended up buying a complete new engine compartment harness. lol... So, the question is, on a street car, is vacuum advance a good thing? And how can it be explained to these geniouses.
August 18th, 2011, 04:34 AM
#2
Registered User
Join Date: Nov 2010
Posts: 601
Vacuum advance is there for a reason, and wouldn't run a street car without it. If not familiar, the carb plate's mostly closed, cruising on the highway and mechanical advance full in, amounts to about 50 degrees for better milage. Not sure what's with some of these Mopar guys, as know some too, and many just dont seem to like vacuum advance for some reason. If a drag strip car, really doesn't matter with the throttle plates wide open. I'm keeping the vacuum advance on mine
August 18th, 2011, 06:47 AM
#4
Registered User
Join Date: Jun 2010
Location: Rogues Island, USA
Posts: 3,613
Vacuum advance is purely a gas mileage enhancement and is great for any street car and will help with driveability. It won't hurt performance in any way because at WOT the vacuum advance comes out of the equation with the distributor backing down to total advance (initial + mechanical). I've run several cars without vacuum advance on the street with no issues at all but it will help economy no doubt.
Your Mopar friends fall into the clueless category and could learn a thing or two from old school technology.
Your Mopar friends fall into the clueless category and could learn a thing or two from old school technology.
August 18th, 2011, 06:58 AM
#5
Connoisseur d'Junque
Join Date: Sep 2010
Location: The Hudson Valley
Posts: 21,183
The answer to this question is Yes, and it is well established over the past 80 years or so (since the demise of the last manual-advance cars), and not subject to debate.
The above explanation is a good start -
A street car is driven at a wide variety of throttle positions, but only partially open the vast majority of the time.
A race car (of any description) is far more likely to be driven wide-open the majority of the time.
With the throttle wide-open, the manifold vacuum is zero, with the throttle closed or partially closed, it is higher.
Therefore, in the simplest terms, in a race car running zero manifold vacuum the majority of the time, the advance diaphragm would be in its no-advance position anyway, so who needs it?
In more complicated terms, the point of ignition timing is to make the spark happen at the right point in time, so that the burning gasses can push the piston down the barrel for the longest time possible, and have the least chance of fighting the engine by pushing it down when it needs to go up. That's why it's called "ignition timing."
The spark from the spark plug ignites a flame which propagates as a flame front, spreading from the spark plug to the far edges of the combustion chamber. This flame front takes a specific, measurable amount of time to get from Point A to Point B. That time is in milliseconds, but since the pistons of an engine spinning at 6,000 RPM are going up and down 500 times a second (ie: in half a millisecond), the time is important.
The flame front does not travel at a set speed, but its speed varies with conditions, such as temperature (faster), pressure (faster), and mixture richness (faster).
So, at part throttle, the cylinder is moving down and trying to fill the cylinder, but it's pulling against the throttle butterfly and can't fill itself through the opening given in the time allotted (it pulls a vacuum).
The mixture in that cylinder is therefore at a lower pressure as it is compressed.
Since it is at a lower pressure, the flame front will travel more slowly when ignited.
Since the flame front will travel more slowly, it will take more time to get from the plug to the edge of the combustion chamber.
Since it needs more time, you need to start the spark earlier.
Starting the spark earlier is another way of saying "advancing the timing" (this is easier to grasp in German, where instead of "Advanced" and "Retarded" they say "Spät" and Früh" or "Early" and "Late").
One way of advancing the timing at part throttle (when manifold vacuum is high) is to use a vacuum advance diaphragm, which translates manifold vacuum (a good throttle position signal) into ignition advance.
So, at a given throttle opening, the piston pulls in less mixture than it wants to, the mixture is compressed at a low pressure, and the spark starts earlier because of the vacuum advance. You step on the gas, open the throttle, admit mixture at full atmospheric pressure, increasing the compressed pressure, and the vacuum diaphragm releases, retarding the spark (triggering it later), because the flame front now needs less time to get across the combustion chamber.
If the diaphragm hadn't retarded the timing, it would have been too far advanced, the flame front would have propagated before the piston was all the way up, the expanding gasses would have tried to push the piston back down the cylinder against the direction of rotation, this would have caused more work, reduced power, increased load on bearings, etc., and caused detonation, because excess heat and pressure would have caused unignited pockets of mixture to ignite on their own before the flame front could get to them (remember, the gas is trying to expand while the piston is moving UP and making the space SMALLER).
If, on the other hand, you have set the timing correctly, but without the vacuum advance, as your friends who resemble English cigarettes have done, then while you are driving at part throttle (darn near all the time in a street car), your timing is retarded (occurring too late) compared to where it should be - the flame front is still propagating as the piston is moving down the cylinder, and is "chasing" the piston down, providing minimal push, while still burning plenty of gas, and obviously making less power than it should.
Make sense?
- Eric
The above explanation is a good start -
A street car is driven at a wide variety of throttle positions, but only partially open the vast majority of the time.
A race car (of any description) is far more likely to be driven wide-open the majority of the time.
With the throttle wide-open, the manifold vacuum is zero, with the throttle closed or partially closed, it is higher.
Therefore, in the simplest terms, in a race car running zero manifold vacuum the majority of the time, the advance diaphragm would be in its no-advance position anyway, so who needs it?
In more complicated terms, the point of ignition timing is to make the spark happen at the right point in time, so that the burning gasses can push the piston down the barrel for the longest time possible, and have the least chance of fighting the engine by pushing it down when it needs to go up. That's why it's called "ignition timing."
The spark from the spark plug ignites a flame which propagates as a flame front, spreading from the spark plug to the far edges of the combustion chamber. This flame front takes a specific, measurable amount of time to get from Point A to Point B. That time is in milliseconds, but since the pistons of an engine spinning at 6,000 RPM are going up and down 500 times a second (ie: in half a millisecond), the time is important.
The flame front does not travel at a set speed, but its speed varies with conditions, such as temperature (faster), pressure (faster), and mixture richness (faster).
So, at part throttle, the cylinder is moving down and trying to fill the cylinder, but it's pulling against the throttle butterfly and can't fill itself through the opening given in the time allotted (it pulls a vacuum).
The mixture in that cylinder is therefore at a lower pressure as it is compressed.
Since it is at a lower pressure, the flame front will travel more slowly when ignited.
Since the flame front will travel more slowly, it will take more time to get from the plug to the edge of the combustion chamber.
Since it needs more time, you need to start the spark earlier.
Starting the spark earlier is another way of saying "advancing the timing" (this is easier to grasp in German, where instead of "Advanced" and "Retarded" they say "Spät" and Früh" or "Early" and "Late").
One way of advancing the timing at part throttle (when manifold vacuum is high) is to use a vacuum advance diaphragm, which translates manifold vacuum (a good throttle position signal) into ignition advance.
So, at a given throttle opening, the piston pulls in less mixture than it wants to, the mixture is compressed at a low pressure, and the spark starts earlier because of the vacuum advance. You step on the gas, open the throttle, admit mixture at full atmospheric pressure, increasing the compressed pressure, and the vacuum diaphragm releases, retarding the spark (triggering it later), because the flame front now needs less time to get across the combustion chamber.
If the diaphragm hadn't retarded the timing, it would have been too far advanced, the flame front would have propagated before the piston was all the way up, the expanding gasses would have tried to push the piston back down the cylinder against the direction of rotation, this would have caused more work, reduced power, increased load on bearings, etc., and caused detonation, because excess heat and pressure would have caused unignited pockets of mixture to ignite on their own before the flame front could get to them (remember, the gas is trying to expand while the piston is moving UP and making the space SMALLER).
If, on the other hand, you have set the timing correctly, but without the vacuum advance, as your friends who resemble English cigarettes have done, then while you are driving at part throttle (darn near all the time in a street car), your timing is retarded (occurring too late) compared to where it should be - the flame front is still propagating as the piston is moving down the cylinder, and is "chasing" the piston down, providing minimal push, while still burning plenty of gas, and obviously making less power than it should.
Make sense?
- Eric
August 18th, 2011, 07:32 AM
#6
Old(s) Fart
Join Date: Mar 2007
Location: Northern VA
Posts: 47,726
August 18th, 2011, 06:28 PM
#8
I got that resemble english cigarettes too! Too funny!!! Thanks! I tried to explain this but I don't have the technical knowledge as you people. Plus when I'm trying to explain to a few closed doors...These guys just say they have like 34 degrees total advance and that's that... blah blah blah and they are teaching this stuff to a guy who just bought his first hobby car. a nice 68 Road runner. and as I said before, they have no problem spending his money and filling him with wisdom. I will copy and paste this info in an email and see if the light comes on at all:-) Thank you all again.
August 18th, 2011, 06:44 PM
#9
Connoisseur d'Junque
Join Date: Sep 2010
Location: The Hudson Valley
Posts: 21,183
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