Another timing question

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marion corvair
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Another timing question

Unread post by marion corvair » Mon Oct 02, 2017 3:10 pm

Another timing question, In specification 7 I see that total centrifugal advance in the distributor in a 95 hp. manual is 28 degrees, I
guess not including initial 6 degrees. this is at 4200 rpms and the vacuum advance is 24 degrees total I assume at say 2400 rpms.
Now if the vacumm port at the carborator is above the throttle plate in the carburetor, then as engine speed rises so does centrifugal
and vacumm advance, so at 4200 rpm you would have somewhere around 58 degrees total. That a lot of advance. What am I missing?
I appreciate all help. Marion Corvair

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bbodie52
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Re: Another timing question

Unread post by bbodie52 » Tue Oct 03, 2017 7:39 am

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Page 5 of the DELCO ROCHESTER - Models H, HV Carburetor Service Manual describes the carburetor vacuum advance port function as follows…
The Model H and HV carburetors have a vacuum advance port drilled just above the throttle valves to provide a timed distributor vacuum advance. As the throttle valve is opened, the spark port is exposed to manifold vacuum which acts on the distributor vacuum advance unit. This varies the advance unit so that the can be maintained in relation to engine load. This helps give maximum economy under varying road load conditions. Both carburetors have a spark port, however, the right unit is used to operate the distributor advance unit. The left carburetors spark port is blocked off by a plastic cap.
Modern automobiles utilize computer-controlled and sensor-based systems to regulate ignition timing advance and manage the fuel injection system. The ignition timing is regulated by a preprogrammed advance curve that is based on engine load, throttle settings, and sensor feedback to the computer as engine operating conditions very. In the 1960s, none of this type of sophisticated engine timing management was available. The technology used then was a combination of a simple centrifugal advance curve that was based on engine RPM and used advance weights that were restricted by spring tension to manage the centrifugal advance. The impact of the centrifugal advance mechanism was based solely on engine speed. The use of a vacuum advance mechanism essentially made the distributor a little smarter. Intake manifold vacuum is relatively high at engine idle. However, the vacuum port (spark port) does not have access to intake manifold vacuum at idle, because the spark port is connected to a drilled passage that accesses the carburetor's throat at a point that is physically above the throttle butterfly valve. When the butterfly valve is closed, intake manifold vacuum is below the throttle butterfly valve, but the spark port connection is physically above the closed throttle. As the throttle is opened partially, engine vacuum begins to impact the vacuum advance mechanism. The engine needs increasing timing advance at part throttle, but the engine speed is too slow for the centrifugal advance to have any impact. At partial throttle and cruising settings, the engine RPM is still relatively slow, but engine vacuum that is being applied to the spark port is relatively high at the almost closed, part throttle settings. So under these driving conditions the vacuum advance is giving the engine the timing advance settings that it needs. Increased throttle opening causes engine speed to increase, which begins to bring the centrifugal advance online. At the same time, engine manifold vacuum tends to be reduced as the throttle butterfly approaches wide-open throttle. At this point the high-speed circuits in the carburetor provide fuel flow to mix with the airflow based on the Venturi effect, with air flowing rapidly through the carburetors throat. At these throttle settings, intake manifold vacuum that would impact the spark port is relatively low, since the spark port is well below the Venturi portion of the carburetor throat, so rapid airflow through the carburetor throat is not generating a vacuum at the spark port. Essentially, the vacuum advance system provides needed timing advance at low RPM engine settings based on part throttle operation. As the carburetor transitions from part throttle to a more open throttle condition, centrifugal advance increases while vacuum advance decreases. This handshaking effect between centrifugal advance and vacuum advance is constantly varying, depending on engine speed and throttle position. Springs in the centrifugal advance and vacuum advance canister provide a dampening effect that allows the engineer to design timing curves based on the amount of vacuum applied to the vacuum advance canister and the engine RPM. Although not quite as sophisticated as a sensor-based computer, the end result is similar.

The two columns in the timing chart indicate when centrifugal advance begins to come online and its maximum settings that can be attained when RPM reaches a point that completely overrides the centrifugal advance springs. The vacuum advance column shows the maximum advance that can be attained at high vacuum, when high intake manifold vacuum completely overrides the internal spring in the vacuum advance canister. The timing advance of one is based on engine RPM, while the timing advance of the other is based on high engine vacuum. But high RPM and high engine vacuum tend to not occur at the same time. High RPM is generally associated with a more wide open throttle/reduced intake manifold vacuum condition, while low RPM, part throttle conditions are generally associated with higher engine vacuum being applied at the spark port. This creates a dual timing advance system within the distributor that is responsive to varying engine loads and engine speeds. The design engineers make use of spring tension, centrifugal advance weights, and a known quantity of vacuum advance at varying engine load settings to produce an overall timing advance curve that matches the needs of the engine.

The explanation below was obtained from a website that I located with a Google search. It may help to improve your understanding of the timing management systems found in older cars that utilize carburetors and mechanical distributors.
Everything You Wanted to Know About Vacuum Advance and Ignition Timing wrote:...As an engine revs up, we need to allow even more of a head start for the spark plug in order for complete combustion to occur. For this reason, a mechanical advance is built into most distributors. As the distributor spins faster and faster with engine RPM, centrifugal forces fling out weights inside the distributor housing, moving a cam mechanism and advancing the timing. This mechanical (also known as a centrifugal) advance is an extremely reliable and simplistic approach to controlling engine timing at given engine speeds. It can be adjusted by changing the stiffness of the springs on the distributor’s weights, and the amount of mechanical advance can be increased or decreased based on stop-bushings in the mechanism. We make mechanical advance sound pretty great– and in theory, it is – but there is a major problem with it as the only source of ignition timing compensation. Mechanical advance relies on one input, and one input alone: RPM. It cannot take into account engine load, fuel mixture or any of the many other variables that dictate ideal ignition timing. For that reason, it is best paired with another form of ignition advance: you guessed it, the vacuum canister.

If you were to put a timing light on a car going down the highway with the vacuum advance properly connected, you would be extremely surprised to see somewhere around 40-50 degrees of ignition timing. Ping city? Detonation central? Nope. Not on a flat stretch of highway. In that situation, what many might deem a radical amount of timing is actually quite beneficial to engine performance.

Timing that could potentially damage an engine at wide-open throttle (WOT) can actually help it achieve significant mile per gallon improvements on the highway. You see, lean fuel mixtures burn very slowly and, at cruise, the engine should be approaching a stoichiometric ratio of right around 14.7:1 (about the leanest it will ever operate). The added ignition timing from the vacuum advance allows the lean cruise mixture to achieve as complete a burn as possible during the power stroke and maximize engine efficiency.

But how does the vacuum advance know when to engage? Simple. As a car cruises down a flat stretch of highway, the throttle plates in the throttle body, or carburetor, are barely cracked open as it takes very little horsepower to move a vehicle down a flat stretch of road in high gear.

With the engine turning highway rpms of between 2000-3000 rpm and the throttle cracked ever so slightly, manifold vacuum shoots way up. This negative pressure exerts a pulling force on the diaphragm inside the vacuum advance can which has a mechanism linked to it to advance timing.

Lets say you encounter a hill or go to pass another car while cruising down the highway. As you apply more throttle, air rushes through the carb, into the intake manifold increasing pressure and pushing the diaphragm in the vacuum can right back out, retarding timing back to wherever it would normally be, given engine RPM and mechanical advance...
:link: http://www.superchevy.com/how-to/additi ... on-timing/
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DELCO ROCHESTER - Models H, HV Carburetor Service Manual.pdf
DELCO ROCHESTER - Models H, HV Carburetor Service Manual
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Brad Bodie
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Image 1966 Corvair Corsa Convertible

marion corvair
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Re: Another timing question

Unread post by marion corvair » Tue Oct 03, 2017 1:57 pm

THANKS FOR THE IMFORMATION BRAD.
MARION CORVAIR

Bob Helt
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Re: Another timing question

Unread post by Bob Helt » Fri Oct 06, 2017 8:06 am

You got a lot of information but the simple answer to your question is that engine vacuum will peak at low to medium rpms and decrease as the speed and the engine load increases. So it is unlikely that you can get max vacuum advance together with max centrifugal advance.
Bob Helt

marion corvair
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Re: Another timing question

Unread post by marion corvair » Fri Oct 06, 2017 3:14 pm

Hello Bob, I think that,if you have port vacuum, that is a port above the carburator butterfly plates. your vacuum will not advance
past its total advance, but it will hold that total until you let up on the gas. That said if you have manifold vacuum, that is vacuum
below the carburator plates then as you increase engine speed you will lose or decrease vacuum to you vacuum advance.
I enjoy everybody's input on this because it is of interest to me.
THANKS, MARION CORVAIR

Bob Helt
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Joined: Sat Sep 21, 2013 9:05 am

Re: Another timing question

Unread post by Bob Helt » Sat Oct 07, 2017 9:11 am

Hi Merrion,
Once the throttle plate opens, ported vacuum becomes the same vacuum as manifold vacuum, so the resulting vacuum for the spark advance becomes what you attributed to manifold vacuum.

Bob Helt

marion corvair
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Re: Another timing question

Unread post by marion corvair » Sun Oct 08, 2017 5:22 am

Hello Bob, I hope your day is good, I did not know that you had authored a book and I am sure you very qualified.
The only thing that I have to compare to that is my 70 years in auto mechanics. The statement you last made is true.
But may not be when applied to the operation of a vacuum advance
Thank you for your time. Calvin

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bbodie52
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Re: Another timing question

Unread post by bbodie52 » Sun Oct 08, 2017 7:45 am

The following explanation may help in understanding the continuously varying engine vacuum levels that affect the vacuum advance mechanism on the Corvair distributor. Intake manifold vacuum is high when the throttle butterfly is fully closed. If the vacuum advance mechanism was connected to a vacuum port that was physically connected to engine manifold vacuum at a point below the throttle butterfly, the vacuum advance would be fully activated at engine idle. However, the goal is to apply vacuum advance at partial throttle settings when engine RPM is low but increasing demand is being placed on the engine to accelerate the vehicle. At this point during operation the centrifugal advance would not yet be engaged because of low engine speed. But gradually increasing throttle settings place more demand on the engine to increase the vehicle speed. A partially opened throttle butterfly begins to expose the spark port to intake manifold vacuum, which activates the distributor vacuum advance to provide the engine with more timing advance. As speed continues to increase the vehicle operator typically applies more pressure to the accelerator pedal, which further opens the throttle butterfly valve. This is a transitional period when engine RPM is increasing (activating centrifugal advance), while at the same time the increasingly opened throttle butterfly valve is causing reduced intake manifold vacuum as the intake manifold is exposed to outside ambient air pressures. While intake manifold vacuum levels within the carburetors throat are decreasing, air velocity through the carburetor throat is increasing with engine RPM. At higher speeds the carburetor no longer depends on intake manifold vacuum to draw fuel from the carburetor float bowl. Instead, the carburetor transitions to dependence on the Venturi effect that produces an effective vacuum from airflow velocity through the Venturi, which now draws fuel from the float bowl to mix with the airflow at a proper ratio that is restricted by the main jet size. A proper fuel/air ratio is maintained to satisfy the engine at higher speeds. At this point the centrifugal advance in the distributor has gradually taken over to provide timing advance for the engine, while the vacuum advance has relinquished control of timing advance as engine vacuum at the spark port steadily decreases. This "handshaking" between the centrifugal advance and vacuum advance continuously regulates the overall distributor timing advance curve at a rate that was determined by engineers to be correct for all levels of engine operation.

Manifold vacuum

The rate of airflow through an internal combustion engine is an important factor determining the amount of power the engine generates. Most gasoline 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.
:tongue:
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 (engine speed -RPM- in a vehicle 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 produced. It is also a major source of engine drag (see engine 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, output power is limited by the ambient barometric pressure. Superchargers and turbochargers boost manifold pressure above atmospheric pressure.

Manifold vacuum vs. venturi vacuum
Manifold vacuum is caused by a different phenomenon than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect which, for fixed ambient conditions (air density and temperature), depends on the total mass flow through the carburetor. In engines that use carburetors, the venturi vacuum is approximately proportional to the total mass flow through the engine (and hence the total power output). As ambient pressure (altitude, weather) or temperature change, the carburetor may need to be adjusted to maintain this relationship.

Manifold pressure may also be "ported". Porting is selecting a location for the pressure tap within the throttle plate's range of motion. Depending on throttle position, a ported pressure tap may be either upstream or downstream of the throttle. As the throttle position changes, a "ported" pressure tap is selectively connected to either manifold pressure or ambient pressure. Antique (pre-OBD II) engines often used ported manifold pressure taps for ignition distributors and emission-control components.

Manifold vacuum in cars
Most automobiles use four-stroke Otto cycle engines with multiple cylinders attached to a single inlet manifold. During the induction stroke, the piston descends in the cylinder and the intake valve is open. As the piston descends it effectively increases the volume in the cylinder above it, setting up low pressure. Atmospheric pressure pushes air through the manifold and carburetor or fuel injection system, where it is mixed with fuel. Because multiple cylinders operate at different times in the engine cycle, there is almost constant pressure difference through the inlet manifold from carburetor to engine.
:doh:
To control the amount of fuel/air mix entering the engine, a simple butterfly valve (throttle plate) is generally fitted at the start of the intake manifold (just below the carburetor in carbureted engines). The butterfly valve is simply a circular disc fitted on a spindle, fitting inside the pipe work. It is connected to the accelerator pedal of the car, and is set to be fully open when the pedal is fully depressed and fully closed when the pedal is released. The butterfly valve often contains a small "idle cutout", a hole that allows small amounts of fuel/air mixture into the engine even when the valve is fully closed, or the carburetor has a separate air bypass with its own idle jet.

If the engine is operating under light or no load and low or closed throttle, there is high manifold vacuum. As the throttle is opened, the engine speed increases rapidly. The engine speed is limited only by the amount of fuel/air mixture that is available in the manifold. Under full throttle and light load, other effects (such as valve float, turbulence in the cylinders, or ignition timing) limit engine speed so that the manifold pressure can increase—but in practice, parasitic drag on the internal walls of the manifold, plus the restrictive nature of the venturi at the heart of the carburetor, means that a low pressure will always be set up as the engine's internal volume exceeds the amount of the air the manifold is capable of delivering.
:ballnchain:
If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or pulling the car up a hill) then engine speed is limited by the load and minimal vacuum will be created. Engine speed is low but the butterfly valve is fully open. Since the pistons are descending more slowly than under no load, the pressure differences are less marked and parasitic drag in the induction system is negligible. The engine pulls air into the cylinders at the full ambient pressure.

More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as engine braking, is used to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). This vacuum braking should not be confused with compression braking (aka a "Jake brake"), or with exhaust braking, which are often used on large diesel trucks. Such devices are necessary for engine braking with a diesel as they lack a throttle to restrict the air flow enough to create sufficient vacuum to brake a vehicle.

Example of varying vacuum that is dependent on throttle position...
Prior to the introduction of Federal Motor Vehicle Safety Standards in the USA by the National Traffic and Motor Vehicle Safety Act of 1966, it was common to use manifold vacuum to drive windscreen wipers with a pneumatic motor. This system was cheap & simple but resulted in the comical yet unsafe effect of wipers which operate at full speed while the engine idles, operate around half speed while cruising, and stop altogether when the driver depresses the pedal fully.
:cussing: :banghead:

:link: https://en.wikipedia.org/wiki/Manifold_vacuum
Brad Bodie
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Image 1966 Corvair Corsa Convertible

marion corvair
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Re: Another timing question

Unread post by marion corvair » Sat Oct 14, 2017 1:53 am

Good morning, to anybody that's up.
Brad , its 3:30 in the morning and my coffee must have me thinking more clearly. I was totally wrong
in My first post here on the forum. ( I guess I was stupid ) I was thinking that you could add vacuum advance
to cent. ( can't spell that word) Advance because, there is just so must advance in a Distributor. When
you get to 28 degree the plate will not go any farther. Maybe the coffee not working as well I think. You tell me.
Thanks again Marion corvair

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Re: Another timing question

Unread post by bbodie52 » Sat Oct 14, 2017 6:06 am

Actually, the vacuum advance timing and the centrifugal advance timing can possibly add to each other. The vacuum advance works by shifting location of the ignition points (or electronic sensor if you have a breakerless ignition system) by moving a portion of the breaker plate that the ignition points are attached to. Looking down at the distributor in the pictures below, the vacuum advance pulls the breaker plate and ignition points to the right (counterclockwise). This can add a nominal 22° to 24° of advance on most distributors if the vacuum advance lever moves to its maximum travel point.

The centrifugal advance mechanism works by rotating the position of the distributor cam to the left (clockwise). So the two mechanisms are independent of each other. The vacuum advance moves the ignition points to the right and the centrifugal advance moves the distributor cam to the left. The actual moment when the points open electrically and cause the ignition coil to discharge depends on the relationship and physical position of the ignition points rubbing block that comes into contact with the distributor cam lobe. So if the points are physically moved by the vacuum advance to the right and the cam lobe is physically moved to the left by the centrifugal advance mechanism, the total amount of timing advance can be cumulative as the two components shift in opposite directions.

The timing advance for an engine starts with the static timing that is established by rotating the distributor housing in relation to the crankshaft rotation position. This timing advance setting is established with the engine at idle speed in the vacuum advance disconnected, so that the centrifugal and vacuum advance mechanisms are not engaged. 12°-14° is common for the 110 hp engine, and 18° is common for the 140 hp engine. As the throttle opens and engine RPM increases, the vacuum advance mechanism begins to dial in more ignition timing advance (at part throttle). If the vacuum advance mechanism was stuck at full advance, the centrifugal advance mechanism would begin to add to that as engine RPM increased enough to cause the centrifugal advance weights to move and shift the location of the distributor cam. But in reality, the amount of vacuum applied to the vacuum advance begins to decrease as the throttle mechanism opens more to increase engine RPM. So as the centrifugal advance comes online, the vacuum advance begins to decrease. In other words the distributor cam is moving in one direction while the ignition points are shifting back to its original neutral position. The total amount of ignition timing advance at any moment may be a cumulative total of the initial (idle) timing setting, plus the vacuum advance, plus the centrifugal advance. But there is never a point in engine operation where the vacuum advance and the centrifugal advance would both be fully engaged and maxed out at the same time. The vacuum advance comes online first, but begins to decrease as the centrifugal advance begins to increase. The two timing mechanisms play against each other depending on the throttle butterfly position in the carburetor and the total engine RPM.
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Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible

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