advancing the ignition timing

[grumpyvette]

ok basics first,
in an ideal engine running the ideal fuel ,the ignition would ignite the compressed mix of fuel air trapped and compressed over the piston at just after top dead center, and the burning mix would near instantly produce about the max pressure the engine strength would safely contain and keep that mass of burning fuel air producing a high and constant pressure over the piston until the piston was about 120 degrees past tdc,\
obviously you need to have a consistent base line advance curve to work with,
on most Chevy v8 engines that run cams designed for street/strip use Ive generally found a advance that goes from about 8 degrees at idle speed (800-900rpm in most cases) and smoothly advances the ignition to about 36 degrees or about 28 degrees advance from where it started at to reach 36 degrees at about 3200rpm , is generally a good place to start, or about 82 rpm increase per degree of ignition advance , up to about 3200rpm, where increase turbulence and squish tends to speed the burn process, you can then play with the engine and determine what changes MIGHT be require

watch video
http://www.youtube.com/watch?v=UYGU7mTw ... r_embedded

watch this

BUT in the real world the fuel takes time to ignite, time to burn and build useful pressure and its all effectively burnt as far as increased pressure is concerned well before the pistons more than 45 degrees past tdc, so you need to locate the pressure curve so the piston on its power stroke can use that pressure effectively,
this requires the fuel/air mixture to be ignited well before the piston reaches tdc, so ignition timing is advanced, the advance , is required, to allow the flame front to cross the cylinder and get the full charge of fuel/air mix burning and producing pressure,the down side is that the pressure builds rapidly as the piston is approaching tdc so it produces pressure that tends to resist the ideal rotation direction of the engine, during the first few degrees of that burn time that are before top dead center in the pistons rotation, taking away from its efficiency, and it burns out before the pistons reached its max mechanical advantage near 90 degrees past tdc.
with in limits imposed by detonation due to fuel octane ratings and a few other factors the higher the compression ratio the more power can be produced, but keep in mind that as the engine speed increases the time available to fill and evacuate the cylinder decreases dramatically
setting the ignition advance is a compromise designed to maximize the usable pressure over the piston on the power stroke and minimize the pressure building before tdc in the cranks rotation that resists the intended rotational direction.
as the ignition advance curve, & total timing or rate of advance in relation to the rpms increases so does the cylinder temperatures, and pressures and the tendency to get into detonation , so matching the ignition advance to the compression ratio, and fuel octane to maximize the pressure over the piston, is a compromise or balance that needs to be struck, you'll need to keep out of the detonation range in temp. and pressure, but ideally maximize the pressure after tdc to increase the torque, produced with the useful pressure curve in the cylinder

it takes time for the flame front to cross the cylinder , at lower rpms,up to 50 thousands of a second, but theres only 7-8 power strokes a second, as rpms increase the compressed gas turbulence in the combustion chamber increases burn efficiency , but the time available is also becoming much shorter, but by about 3200rpm the constant increase in ignition advance lead timing is no longer required, as the burn speeds have continued to increase.

yes reading thru the linked info takes time but youll be amazed at the info they contain, if you take the effort

standard vacuum connections

manifold to power brake booster
manifold to distrib vacuum advance
pvc to carb or air cleaner assembly
viewtopic.php?f=70&t=1411&p=3281&hilit=+detonation#p3281


http://www.circletrack.com/ultimateraci ... index.html

http://www.circletrack.com/enginetech/c ... index.html

http://temp.corvetteforum.net/c3/joevet ... urve.shtml

viewtopic.php?f=52&t=727&p=3212&hilit=+detonation#p3212

viewtopic.php?f=70&t=875

http://www.chevyhiperformance.com/howto ... index.html

viewtopic.php?f=38&t=1099&p=2152&hilit=volumetric#p2152

http://www3.fs.cvut.cz/web/fileadmin/do ... vac-Bl.pdf

http://www.kb-silvolite.com/article.php ... ad&A_id=36

http://www.tpub.com/content/altfuels02/ ... 340011.htm

http://www.tpub.com/engine1/en1-105.htm

http://www.chevyhiperformance.com/techa ... index.html

http://auto.howstuffworks.com/ignition-system.htm

http://www.chevyhiperformance.com/howto ... index.html

http://en.wikipedia.org/wiki/Ignition_timing

viewtopic.php?f=70&t=1411

http://www.centuryperformance.com/ignit ... g-219.html

http://www.hotrod.com/techarticles/sett ... index.html

viewtopic.php?f=70&t=875

http://www.msdignition.com/uploadedFile ... op_ten.pdf

http://circletrack.automotive.com/10831 ... index.html

 

[grumpyvette]

As a starting point on ignition advance curves ,Ive generally found about 12 degrees at 750 rpm and a consistent smooth advance to about 36 total at about 3100rpm to work well on both small and big block street cars with compression levels consistent with pump gas octane levels

look at curve (D) (pg 3) in this link

http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/distributors/8361_instructions.pdf


without a decent timing light and a well marked damper, on which youve carefully verified TDC, your basically guessing at timing
viewtopic.php?f=70&t=875&p=6241&hilit=+timing+light#p6241

be aware that some damper designs do tend to fail over time!
beating a balancer onto a crank, pulling it with the wrong type of tool, or letting it get fuel or oil soaked can damage a damper, this can easily result in the outer damper ring with the TDC mark rotating to a random location

if your front crank seal leaks, over time it can dissolve the elastic, between the inner and outer damper hub weight, beating on a damper tends to hurt the flex ring seal alsos

 

[grumpyvette]

http://www.pontiacstreetperformance.com ... curve.html
people ask me why I post links, so often?
well I type with two fingers and if someone has already done a reasonably decent job on a subject theres little to be gained by repeating their effort so lets give them the credit and take advantage of the resource and devote our time to a more productive use


http://www.gofastforless.com/ignition/advance.htm

http://www.pontiacstreetperformance.com ... curve.html

http://www.hotrod.com/techarticles/sett ... index.html

http://www.centuryperformance.com/ignit ... g-219.html

http://mg-tri-jag.net/tech6.htm

http://temp.corvetteforum.net/c3/joevet ... urve.shtml

http://www.crankshaftcoalition.com/wiki ... istributor

http://www.corvettefever.com/techarticl ... index.html

 

[Doodad]

Thanks for all the great info Grump I'll be reading all night...

 

[grumpyvette]

obviously you need to have a consistent base line advance curve to work with,
on most Chevy v8 engines that run cams designed for street/strip use Ive generally found a advance that goes from about 8 degrees at idle speed (800-900rpm in most cases) and smoothly advances the ignition to about 36 degrees or about 28 degrees advance from where it started at to reach 36 degrees at about 3200rpm , is generally a good place to start, or about 82 rpm increase per degree of ignition advance , up to about 3200rpm, where increase turbulence and squish tends to speed the burn process, you can then play with the engine and determine what changes MIGHT be require

http://www.hotrod.com/techarticles/sett ... index.html

http://www.chevyhiperformance.com/howto ... index.html

http://www.pontiacstreetperformance.com ... curve.html

http://www.setyourtiming.com/Timing_Set ... mingCurves

viewtopic.php?f=70&t=1015&p=1864&hilit=damper+tabs+tape#p1864

viewtopic.php?f=70&t=875&p=1372&hilit=damper+tabs+tape#p1372

viewtopic.php?f=52&t=966&p=6800&hilit=+dead+center+dead+center#p6800

viewtopic.php?f=52&t=974&p=1699&hilit=degree+wheel#p1699

obviously you'll need to verify the timing tab on your engine and the damper DO INDICATE true TDC (TOP DEAD CENTER) and the damper should have either factory inscribed degree marks or you can install a timing tape

http://www.gofastforless.com/ignition/advance.htm

Distributor advance

It doesn't matter what kind of ignition you have, if the advance curve is not set properly it won't make any power. The ignition is advanced so you reach peak cylinder pressure right after TDC (top dead center). If the spark comes too soon the burning fuel will try to push the piston back down the cylinder before it reaches TDC, resulting in a loss of power and possible engine damage. If the spark is too late you will not reach full power and get poor gas mileage. There are two major factors that effect how much advance is required, engine speed and load. You increase advance with rpm and decrease advance with engine load. You don't need a distributor test stand to curve a distributor. All you need are some basic hand tools, a timing light, and a tach.

The first thing you should do is find a shop manual for your particular car. Read about the advance mechanism and make yourself familiar with the different components. Before doing too much you should check the condition of your advance mechanism. If your vacuum diaphragm is bad replace it. Lube all the components and make sure they are moving freely.

You probably won't have enough timing marks to test total advance. You can buy stickers for the harmonic balancer or you can make one. I made some on my computer. If you are really cheap you could just draw lines on a piece of tape. Trace them from the timing marks you already have to get the right spacing. The easiest way is to buy a timing light with an advance dial. They are kind of spendy, about $60, but a big time saver.

The mechanical, or centrifugal, advance adjusts the timing based on engine speed. The faster the motor spins the more it will advance the timing. The factory advance curve is very conservative. By using a more aggressive advance curve you can greatly improve your engines performance. To setup your mechanical advance you first need to disconnect your vacuum advance line. Next you should hook up a tach and timing light and see what your timing is set at. Simply watch the timing marks with the light. The reading at idle is your initial advance. Rev the motor up until the distributor stops advancing and note what speed it maxes out. This is your total advance.

Total advance is the most critical setting. Short of a dyno the best way to find what total advance you need is some track tuning. The MPH reading at the end of the track is your best indicator of engine output. Make a run or two to get a baseline then increase your total advance and make another run. If the MPH increases advance it some more and run again. Continue advancing the ignition until the MPH starts falling off then pull it back to the point where you had the highest trap speed. It may take quite a few passes until you find the optimal setting. Total advance is initial advance plus mechanical advance. There are two ways to change the total, adjust the initial advance by turning the distributor or adjust the mechanical advance mechanism. The easiest, and cheapest way is to just turn the distributor until you get the total you want then just leave the initial wherever it ends up. If you want more control over the initial then you need to adjust the advance mechanism. If you have a total of 36° and want to run 12° initial then you need 24° mechanical advance. Aftermarket distributors will have replaceable bushings to adjust mechanical advance. Stock Ford distributors have two slots. There is a pin that sits in one of the slots to limit travel. To change slots you simply remove the armature assembly turn it 180 degrees and reinstall it so the pin is in the other slot. The slots are numbered as to how many distributor degrees it will pull in, double it to find the amount of crank degrees. So if you want 24° mechanical advance you need to find an armature assembly with a slot marked 12L. My distributor is setup with the stop in the slot marked 13L, this means 26° mechanical advance. The other slot is marked 18L which is 36°. I wanted less than 20° so I made the big advance spring a stop. I wrapped the coils with copper wire then soldered it so it was solid. Now I can adjust the total advance by bending the adjustment tab through the hole in the breaker plate. This makes it easier to adjust than the factory setup, and most aftermarket distributors for that matter.

Initial advance isn't very critical. Usually you just set the total where it needs to be and leave the initial wherever it ends up. Once you found the best total advance setting you can play with the initial. Basically you want to run as much as you can before the motor cranks over hard. With the motor warm pull the timing way up and try to start the motor. If it cranks real slow then pull it back until it spins normally. Lets say that is 20° and you ran best with a total of 36°, that means you need 16° mechanical advance. Any time you adjust the initial you need to adjust the mechanical advance so the total stays the same, this is easy to do with my distributor mod above. Now if you're running tall gears and/or a heavy car you will probably encounter some sort of pinging if you stand on it with that much advance. If it happens at real low engine speeds then you need to pull the initial timing back. If it happens at moderate RPM levels then you can probably fix it with the advance springs.

The springs adjust the advance rate. Where the distributor is bolted down determines the initial advance, the amount of travel in the mechanical advance determines the total advance, and the springs determine what RPM total advanced is reached. Lighter springs allow the mechanical advance to move more easily so you will reach total advance at a lower RPM. Stiffer spring will delay the total advance. Most distributors use two springs, a small one and big one. The big one will usually have a bit of lash so the small one does all the work at low speeds. This allows the advance to come up quickly off idle. Once the lash on the big spring is used up the weights will be trying to pull both springs so the advance rate will slow down. The chart below shows a typical stock advance "curve". It is the dual springs that give it the curve. If there were only one spring the chart would just be a straight line.

In this example it idles around 500 rpm and has 6°initial advance. At this point only the small spring is holding back the timing. It is a light weight spring so the advance rises fairly quickly until it hits 18° at 1800 rpm. Once you hit 18° the lash on the big spring is taken up so the advance rate levels off a bit. If you increased the lash on the big spring the advance will go further than 18° degrees before hitting the big spring, less lash will hit the big spring sooner. To adjust what rpm you hit the big spring you would change the small spring. A lighter small spring will cause you to hit the big spring before 1800 rpm and a heavier small spring will delay it until after 1800 rpm. In this example you hit total advance (28°) at 4000 rpm. As mentioned above the total is limited by the mechanical advance mechanism. The rpm total advance is reached is determined by the big spring. A heavier big spring will delay total until after 4000 rpm and a lighter big spring will allow total to come in before 4000 rpm.

If you experience pinging just off idle you should lower your initial advance. If your motor pings around the point you reach total advance you have two options; lower the total advance or put in a heavier big spring to delay the total. If its pinging well above this point you will need to pull back the total. If your motor is fine at low and high speed but pings in the mid range then you need to either reduce the lash in the big spring to lower the mid advance point or install a heavier small spring to delay the mid advance point. Making these fine adjustments can be a pain in the neck because adjusting one element often changes the others. If you want to bypass this whole mess you could use my programmable digital ignition. It allows you to adjust any of these points independent of the others and from the comfort of the drivers seat.

The mechanical advance is adjusted for high load WOT conditions. Under light load, part throttle conditions the manifold pressure is lower so volumetric efficiency is lower so the cylinder pressure is lower so the fuel mixture burns more slowly. This means you need to light the mixture sooner so you reach peak cylinder pressure at the ideal time. This is the purpose of the vacuum advance. The lower the load is the more it will advance the timing. Vacuum advance will improve gas mileage and drivability of a street driven car. A lot of guys think a vacuum advance hurts performance, this is not true. The vacuum advance is entirely independent of the mechanical advance. They are two separate systems that perform two separate functions. The mechanical adjust timing based on RPM where the vacuum adjusts timing based on load. Under high load, WOT, performance conditions there is almost no manifold vacuum so the vacuum advance does nothing. The only time the vacuum advance adds timing is at part throttle, low load conditions when there is manifold vacuum. So unless you race at half throttle a vacuum advance will have no effect on performance. It will however improve part throttle drivability so unless your car is a 100% race car I would recommend running a vacuum advance.

You're probably thinking, "Sure there is no manifold vacuum at WOT but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work. Someone told me he noticed less dynamic braking with the vacuum advance hooked to manifold. I didn't notice it on my car but it makes sense. If the motor is running more efficiently with the added advance it will make a less effective brake. So which should you use? Try both and see which you like best.

Once you have the mechanical advance setup to give you the most power, and no pinging, at WOT then you should setup the vacuum advance. A stock vacuum advance will pull in 20° or more. If your car is pinging or running rough after hooking up your vacuum advance then you need to turn it down. Most vacuum canisters are adjusted by sticking an allen wrench in the vacuum tube. Turning the wrench counterclockwise will reduce the timing. Just turn it down a bit at a time until the problem goes away. I had to turn my vacuum advance down until it only pulled in 5°.

 

[grumpyvette]

http://www.corvette-restoration.com/res ... ing101.pdf

TIMING AND VACUUM ADVANCE 101
John Hinckley

In this day and age, when modern automotive power trains are computer-controlled and engines
don’t even HAVE distributors any more, the knowledge of what distributors did and how they
operated to control ignition timing has begun to fade; for those just entering the classic automotive
hobby, the function of the distributor and the notion of “timing” is even more mysterious. To keep
your classic Corvette running reliably and at maximum efficiency, some knowledge about the
principles of spark timing and how it’s controlled is essential. The objective of this article is to demystify
the principles of “spark timing”, and to explain why and how your distributor-equipped
Corvette’s spark timing is controlled and varied to suit changing driving conditions.
I won’t get into the gory details of combustion theory, but let’s understand a little about what
happens as the piston is traveling upward on the compression stroke toward the point where the
spark plug “lights the fire”. Before we light the fire, let’s talk a little about what we’re lighting – the
fuel-air mixture that’s been metered by the carburetor and atomized in the intake manifold as it
heads for each cylinder’s intake valve.


Fuel/Air Mixture and “Burn Rate”: At idle and steady cruising speed, the load on the engine is
low, and the air-fuel mixture is “lean” (more air/less fuel); when accelerating, the load on the
engine is higher, and it’s fed a “rich” air-fuel mixture (more fuel/less air). These are two very
different conditions, as a lean mixture burns relatively slowly, and a rich mixture burns faster.
Remember this distinction – it’s a key factor in ignition timing.
Back in the cylinder, with the piston rising and compressing the air-fuel mixture, the idea is to fire
the spark plug at just the right moment such that the mixture is ignited (starting the “burn”, as the
flame front proceeds across the cylinder) and the rapidly-expanding gases reach peak cylinder
pressure just after the piston reaches TDC (top dead center), exerting maximum force to push the
piston down on the power stroke for maximum efficiency.
Spark Timing: Referring back to the burn rate comparison, slower-burning lean mixtures need to
have the “fire lit” earlier in the compression stroke (because they take longer to reach peak
cylinder pressure) than faster-burning rich mixtures (which take less time to reach peak cylinder
pressure). With either mixture condition, the objective is to reach peak cylinder pressure at exactly
the same point after TDC, which says they have to be “lit” at different points during the piston’s
upward travel – this is what “spark timing” is all about – managing the point at which the spark
plug fires under different operating conditions. This point is expressed as “spark advance”, in
degrees of crankshaft rotation before the piston reaches top dead center; when someone says
their initial timing is set at 10 degrees, that means the distributor is set to fire the spark plugs when
the crankshaft is 10 degrees of rotation before the piston reaches top dead center, which is “10
degrees of advance”. This is the “initial” or “base” spark timing which is checked and set at idle
during a traditional tune-up (with the vacuum advance disconnected); it’s fixed at the point where
it’s set by clamping down the distributor hold-down bolt, and doesn’t change – it’s always there.

Early Spark Timing: In the days of the simple, low-compression, inefficient Model T, spark
advance was set manually to a fixed level with a lever on the steering column; about all the driver
did was to “retard” (delay) the spark timing while turning the crank to start the engine, then move
the lever to “advance” the spark timing once the engine was running. If the driver forgot to retard
the spark when cranking and left the lever in the advanced position, the engine could “kick back”
while the operator turned the crank to start the engine, which could result in a broken arm (an
unforgettable lesson in spark timing). Once running, the operator could vary the spark advance
with the lever for best performance (such as it was), or just leave it alone (which most operators
chose to do). With that big, low-compression, slow-running, low-powered engine, little damage
could be done by improperly setting the spark advance.
Fast-forward to the 1960’s and high-compression 350-horsepower Corvette engines howling at
6500 rpm; suddenly the spark timing equation is much more complex, and spark timing errors can
result in scattering expensive engine parts all over the street at one extreme, and poor
performance and fuel economy at the other extreme. An automatic device has to recognize the
entire spectrum of operating conditions and manage the complexities of spark timing in a manner
completely independent of, and transparent to, the driver, who has other things to keep him
occupied – like traffic, flashing blue lights in his mirrors, etc.


The Distributor And Advance: This wondrous device that handles all that work is the distributor,
which lives quietly in the dark, at the back of the engine, hidden forever under the Corvette’s
ignition shielding, demanding only an occasional set of points, a condenser, and a rotor to
continue doing its job. Let’s talk about the two different ways the distributor manages spark timing
while you’re watching traffic and grabbing gears – centrifugal advance and vacuum advance.
The centrifugal advance mechanism under the rotor in the distributor advances spark timing
based solely on engine rpm (it’s driven at half crankshaft speed); a pair of weights pivot on pins,
and are retained by little coil springs. The faster the shaft turns, the more the weights tend to pivot
outward (centrifugal force), and the rate at which they move outward is controlled by the tension of
the little springs; lighter springs let them move fully outward at relatively low shaft rpm, and
stronger springs require higher shaft rpm for full outward movement. The pointed “tail” of the
weights, at the pivot end, bear against a cam (called the “autocam”) attached to the top of the
distributor shaft, and as the weights move outward, the 8-sided cam that opens and closes the
contact points (which trigger the coil to fire the spark plugs when the points open) is “advanced” so
it opens the points earlier than when the weights are fully retracted (as they are at idle). In most
distributors, this mechanism provides up to 20-25 (crankshaft) degrees of spark advance when the
weights are fully extended; the maximum advance this system can provide is limited by a bushing
installed over a pin which moves in a slot in the lower plate of the autocam. The system is
designed so that the weights don’t begin to move until slightly above normal idle rpm, so the initial
timing can be set accurately without any influence from the centrifugal advance mechanism.

Centrifugal Advance Calibrations: There are many different calibrations of weight configurations
and spring tensions specified for production Corvette distributors, depending on the performance
level of the engine, manual or automatic transmission, etc. The points between the rpm at which
the weights begin to move and the rpm at which they’re fully extended, providing maximum
advance, is referred to as the “centrifugal advance curve”, which is tailored to each engine
combination. The key point to remember here is that the centrifugal advance mechanism
advances and retards spark timing in response only to engine rpm, and nothing else. Its function
is to advance spark timing as engine rpm increases; as upward piston speed increases with rpm,
3
effectively shortening the time for the compression stroke, the spark has to fire sooner, as the
air/fuel mixture still takes the same amount of time to burn as it does at lower rpm. In effect, the
centrifugal advance mechanism handles only the basic physics of lighting the fire sooner at higher
rpm so peak cylinder pressure is still reached at the same point just after TDC.
Now we have the basic physics handled, but we still need another system to manage spark
advance based on all the variations of driving conditions and engine load variations experienced
in normal operation; this is handled by the vacuum advance system.

Vacuum Advance: The vacuum advance system consists of a vacuum diaphragm mounted on
the distributor body; the diaphragm is spring-loaded in the zero-advance position, and has a rod
which connects to a hole in the breaker plate, which is the movable plate the points are mounted
on. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the
breaker plate, rotating it with respect to the 8-sided cam on the distributor shaft which opens and
closes the points. When viewed from the top, the distributor shaft (and the 8-sided cam for the
points) turns clockwise; when the vacuum advance rod pulls on the breaker plate, it rotates the
breaker plate (and the points) counter-clockwise, which “advances” the opening of the points
(which triggers the coil to fire the spark plugs). A typical vacuum advance unit, when fully
deployed, will add about 15 (crankshaft) degrees of spark advance over and above what the
distributor’s centrifugal advance system is providing at the moment, which depends on engine
rpm; they are two independent systems, but they work together to provide the correct amount of
spark advance

Controlling Vacuum Advance: Let’s look at how the vacuum advance system is controlled.
Referring back again to burn rates, remember that lean mixtures burn slower, and rich mixtures
burn faster. Engine load conditions (idle, steady cruise, acceleration) result in how lean or rich the
air/fuel mixture is (the carburetor handles this), and the best indicator of engine load is intake
manifold vacuum. At idle and steady cruise, engine load is low, and intake manifold vacuum is
high due to the nearly-closed carburetor throttle plates; under acceleration, the throttle plates open
wider, and intake manifold vacuum drops; it is essentially zero at wide-open throttle. As a result,
intake manifold vacuum is a “free” indicator of engine load, which correlates nicely with fuel

effectively shortening the time for the compression stroke, the spark has to fire sooner, as the
air/fuel mixture still takes the same amount of time to burn as it does at lower rpm. In effect, the
centrifugal advance mechanism handles only the basic physics of lighting the fire sooner at higher
rpm so peak cylinder pressure is still reached at the same point just after TDC.
Now we have the basic physics handled, but we still need another system to manage spark
advance based on all the variations of driving conditions and engine load variations experienced
in normal operation; this is handled by the vacuum advance system.
Vacuum Advance: The vacuum advance system consists of a vacuum diaphragm mounted on
the distributor body; the diaphragm is spring-loaded in the zero-advance position, and has a rod
which connects to a hole in the breaker plate, which is the movable plate the points are mounted
on. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the
breaker plate, rotating it with respect to the 8-sided cam on the distributor shaft which opens and
closes the points. When viewed from the top, the distributor shaft (and the 8-sided cam for the
points) turns clockwise; when the vacuum advance rod pulls on the breaker plate, it rotates the
breaker plate (and the points) counter-clockwise, which “advances” the opening of the points
(which triggers the coil to fire the spark plugs). A typical vacuum advance unit, when fully
deployed, will add about 15 (crankshaft) degrees of spark advance over and above what the
distributor’s centrifugal advance system is providing at the moment, which depends on engine
rpm; they are two independent systems, but they work together to provide the correct amount of
spark advance.

mixture being supplied – lean mixture at high vacuum, and rich mixture at low vacuum.
At idle, the engine needs additional spark advance in order to fire the lean (and exhaust-diluted)
idle fuel/air mixture earlier in the cycle in order to develop maximum cylinder pressure at the
proper point after TDC for efficiency, so the vacuum advance unit is activated by the high manifold
vacuum, and adds another 15 degrees of spark advance on top of the fixed initial timing setting.
For example, if your initial timing is set at 10 degrees, at idle it’s actually 25 degrees with the
vacuum advance connected (a properly-calibrated centrifugal advance mechanism will not have
started to move yet at idle rpm).
The same thing occurs under steady highway cruise conditions; the mixture is lean, takes longer
to burn, the load on the engine is low (it only takes about 40 horsepower to cruise at 50mph) and
the manifold vacuum is high, so the vacuum advance unit is again deployed, and adds 15 degrees
of spark advance over and above whatever the distributor centrifugal advance mechanism is
providing at that engine rpm. If you had a timing light connected so you could see it as you cruise
down the highway, you’d see about 45-50 degrees of spark advance; your fixed initial advance of
10 degrees, 20-25 degrees provided by the centrifugal advance mechanism, and the 15 degrees
added by the vacuum advance unit.


When you accelerate, the fuel/air mixture is immediately enriched (by the accelerator pump,
power valve, metering rod piston, etc.), and that rich mixture now burns faster, doesn’t need the
additional spark advance any more, and when the throttle plates open, the manifold vacuum
drops, and the vacuum advance unit diaphragm retracts to its zero position, “retarding” the spark
timing back to what is being provided at that moment by the fixed initial timing and the centrifugal
advance mechanism. The vacuum advance doesn’t come back into play until you back off the gas
and manifold vacuum increases again as you return to steady-state cruise, when the mixture
again becomes lean and needs more spark advance for fuel efficiency.
Vacuum Advance Calibration: There are also many different calibrations of vacuum advance
units; some begin to deploy at different vacuum levels than others, and some provide more
degrees of advance when fully deployed than others. The original calibration was selected based
on the intake manifold vacuum characteristics of that particular engine/transmission combination
and how it was expected to perform in daily use. Vacuum advance units were connected to full
manifold vacuum for decades; in the late 60’s and early 70’s, when emissions began to become
an issue, many were instead connected to “ported” or “timed” vacuum sources. We’ll discuss this
aberration a little later.

The Advance Combination: Now we have two different advance systems working independently,
but complementing each other, to manage spark timing – centrifugal, based on engine rpm, and
vacuum, based on engine load and operating conditions. The centrifugal advance system is
purely mechanical and is only rpm-sensitive; nothing changes it except engine rpm. Vacuum
advance, on the other hand, responds instantly to to engine load and rapidly-changing operating
conditions, providing the correct amount of spark advance at any point in time, to deal with both
lean and rich mixture conditions.
By today’s engine management terms, this was a relatively crude mechanical system, but it did a
good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with
absolutely zero negative effect on wide-open throttle performance, as the vacuum advance is
inoperative under that condition. In modern cars with computerized engine controllers, all those
sensor inputs to the computer change both spark timing and fuel/air mixture 50 to 100 times per
second, and we don’t even have a distributor any more – it’s all electronic.
“Ported” Vacuum: Now to the widely-misunderstood manifold-vs.-”ported” vacuum aberration.
After 30-plus years of controlling vacuum advance systems with full manifold vacuum, that “free”
indicator of engine load and fuel mixture, along came early emission control requirements (seven
years before catalytic converter technology was introduced), and all manner of crude band-aid
systems were introduced to try and reduce hydrocarbons and oxides of nitrogen in the exhaust
stream. One of these crude, but effective systems was GM’s Air Injection Reactor (A.I.R.) system,
which pumped fresh air into the exhaust ports to “afterburn” pollutants in the exhaust manifolds.
The key to making this system work at maximum efficiency was retarded spark at idle; with
retarded idle spark timing, the “burn” begins late, and is not complete when the exhaust valve
opens, which does two things which were important for emissions. The incomplete burn reduced
combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and
the significant increase in exhaust gas temperature ensured rapid light-off and combustion of the
hydrocarbons in the exhaust gas stream when the fresh oxygen-carrying air was introduced from
the air pump.



Side Effects: As a result, these engines ran poorly, and an enormous amount of wasted heat
energy was transferred through the exhaust port walls into the coolant, causing them to “run hot”
at idle; cylinder pressure fell off, engine temperatures went up, combustion efficiency went down
the drain, and fuel economy went down with it. “Ported Vacuum” was easy to implement – they
just moved the distributor vacuum port orifice in the carburetor from below the throttle plate (where
it was exposed to full manifold vacuum) to above the throttle plate, where it was only exposed to
manifold vacuum after the throttle plate opened. This meant that the vacuum advance was
inoperative at idle (retarding idle spark timing from its optimum value), and these applications also
had very low initial timing settings; they were usually set at 4 degrees before TDC or less, and
some even had initial timing settings as much as 2 degrees after TDC. The vacuum advance still
worked at highway cruise, but not at idle, which caused all manner of problems. “Ported Vacuum”
was strictly an early pre-converter crude emissions strategy, and nothing more. Don’t believe
anyone who tells you that ported vacuum is a good thing for performance and driveability – it’s
not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing
idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don’t
understand what vacuum advance is, how it works, and what it’s for. There are lots of long-time
experienced mechanics who don’t understand the principles and operation of vacuum advance
either, so they’re not alone.
Summary: Now that we’ve covered the whys and hows of spark timing and its control systems,
you can appreciate what’s going on underneath your ignition shielding and how it affects
performance and driveability. Checking the operation of the centrifugal and vacuum advance
systems during periodic maintenance and tune-ups can pay real dividends that you can feel in the
seat of your pants. Well, you say, “how do I do that?” Tune in next month, when we’ll show you
how to check out those systems, how to “map” your advance curves against their design
specifications and verify proper operation, and pass along some simple tips and techniques for
improving your Corvette’s performance by “tweaking” its advance systems for peak efficiency.

 

[grumpyvette]

when your breaking in a new engine or cam, one common problem indicator is the headers running excessively hot



due to the effective delay in the ignition process because the ignition is not advancing to compensate for the lower time frame between ignition and the power stroke as the rpms increase as the rpms increase a greater percentage of the fuel/air mix thats being burn exits the exhaust still burning, greatly increasing the exhaust temps in the heads and exhaust manifolds


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EXAMPLE

btw notice the front header tube seems to be a bit cooler and each header tube as you move to the rear looks a bit hotter, thats because the engine compartment air flow cools the headers less effectively as its heated as it moves from the radiator rear ward

the picture above is commonly the result of having Your ignition timing too retarded for the 2500rpm-3500rpm your supposed to be lapping a new cam in at for the first few minutes,or the ignition advance curve rpm is to slow with a light load. Under a light load combustion is a slower process. Some of the combustion is still taking place after the exhaust valve opens which will make the headers glow.
if your running a LEAN due to either jetting, tuning issues or a large vacuum leak....the overly lean fuel/air mix tends to raise the exhaust temps, obviously an IR temp gun can be very useful in spotting this condition early, but its even more useful because it can easily tell you if only one or two headers are running significantly hotter, usually indicating a vacuum leak or tuning issue rather than ignition timing where all the header tubes tend to run hot.
now obviously you should have verified the correct oil and coolant levels and verified your ignition timing and advance and firing order before starting it , or seeing the headers glow before letting the engine run very long