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Vacuum Advance revisited....

Guest Anonymous

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Guest Anonymous

Recently in the last couple of threads we (that's the "royal we") have recomended connecting the advance to the manifold or below the throttle. I remember my old setup was always above the throttle plate. It's my understanding (I could be mistaken)that ALL dizzies have some kind of mechanical advance that provides a baseline adaptation for rpm related advance change. The dizzies with vacuum advance or retard are designed to improve driveability and emissions by adapting the timing slightly to correspond with throttle position.

So I think hooking up the dizzy to the manifold or below the throttle plate will give "full time" advance (or retard).

Perhaps some of the real experts could chime in on this?

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Guest Anonymous

timing has a direct relation to rpm UNDER FULL POWER.

This is done mechanicly. Under full power, there's hardly

any vacuum in the manifold, because the throttle valve is full


But under partial load, there is less mixture entering the

cylinder then compared to at full throttle (at same RPM),

therefor less actual compression and a slower burning.

Realizing that, you can ignite earlier under partial load then

under full load (at same RPM), hence vacuum advance.

At least, that's how I think it is. Does it make sense?

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Guest Anonymous

At no load, the're a high vacuum in the manifold, advancing

the ignition greatly. Since very little mixture enters the

cylinders (your throttle valve is in the way...), it is little

compressed and burnes slowly. Therefore you can ignite

earlier. Opening the throttle (quick movement of the right foot)

makes the vacuum in the manifold disapear, gives better

cylinder filling, quicker burning mixture, so needs less

advance, which it gets because the advance thingy doesn't

get the vacuum anymore

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Guest Anonymous


I understood that the vacuum for the vacuum advance should be taken from slightly above the closed throttle plate (there is nipple for that on the base of the 32/36, I don't know about others). The result would be no vacuum advance at idle (for a nice smooth "tick over"), vacuum advance at partial throttle (like for cruising down the highway at the legal speed limit), and no vacuum advance under full throttle (like when you are passing the Yugo over Guanella Pass).

Okay...Fire Away.


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Guest Anonymous

From: mikeS,

Subject: I'll take a crack at it...

The fundamental idea behind ignition timing is that combustion occurs at a more or less fixed rate. When you apply spark to mixture, the rate of

combustion is determined largely by the nature of the fuel (which is fixed once you fill the tank), the air-to-fuel ratio, the pressure in the

combustion chamber, the temperature of the combustion chamber, etc.

Ignition timing is set such that the expanding combustion gasses put pressure on the piston at the proper time. If the explosion occurs too late

in the piston cycle ( "too retarded", when the piston is on the way down), then the motor will fail to transfer the maximum possible amount of

combustion energy into mechanical energy (energy of motion of the piston, crank, etc.). In this situation much of the combustion energy will be

transferred to the block, coolant and environment as thermal energy. (Really, most energy of combustion is turned into thermal energy

regardless, because internal combustion engines are very inefficient. Retarded timing causes more energy than normal to be lost as thermal

energy, which is why your car will run hot if the ignition timing is late.)

On the other hand, if the spark occurs too early, the "overly advanced" timing condition, then the combustion peak occurs as the piston is still

very early on its way up to top dead center. In this case the piston is forcing the expanding gasses into a very tight space, and much of the

energy of combustion is wasted by deforming the piston, connecting rod, cylinder walls, and cylinder head (that's the nasty rattle-ping-knock you

hear when your ignition is too advanced). Once the piston reaches TDC and starts to come back down, whatever combustion process remains

will apply pressure to the piston to produce useful mechanical energy. Also, because we're talking about a multi-piston motor, the mechanical

energy produced at one cylinder might be fighting the combustion at another cylinder, because the combustion is occurring too early in the

piston's up-stroke.

Ideally, the spark should be applied while the piston is on the way up, but not so early that the major combustion occurs before the piston is in a

postion that allows mechanical transfer of the combustion energy.

So, what does this mean for advance curves, and all that? Well the next big idea is that your motor runs at a variety of RPM, but as mentioned

above, combustion occurs at a more or less fixed rate. That means that it takes about the same amount of time for a cylinder to fire, whether

the motor is going 1000rpm or 6000rpm. (Actually that the piston compresses the exploding mixture will affect the combustion rate, but I don't

think that's an effect we need to worry about here, and I'm not an expert by any means). Therefore, at high RPM we will need to light the flame a

bit earlier in the piston's up cycle, so that it's in the proper position to turn the gas energy into mechanical energy on the way back down. That is

why distributors increase the ignition advance with RPM.

Motor speed isn't the whole story, though. The load that you're placing on the motor, the amount that your throttle is open, the temperature of the

motor and the outside air, all of them affect what would be the "ideal" ignition advance setting. The book "Bosch Fuel Injection and Engine

Management" has some very nice 3-d graphs of advance settings, as functions of various paramters. Modern engine management systems

take many variables into account to set the timing with every spark.

Our old-fashioned distributor systems can only give a crude approximation to the ideal advance function. So how about all these whacky

gizmos that affect timing: centrifugal weight mechanisms, vacuum advance, vacuum retard, etc. Let's take them one at a time.

The basic "high-performance" centrifugal-only distributor varies the advance with engine speed only. There's no other input to change the

advance, so you get one advance value for each RPM value, regardless of how hard you are stepping on the gas. These simple systems are

set up to maximize full-throttle performance. That is, the advance curve is set assuming that most of the driving will be "sporting" type driving,

with the throttles fully open as often as possible, and with the revs high as often as possible.

The vacuum advance mechanisms attempt to improve the timing setting to accomodate varying load conditions. When you're on a level

highway cruising part-throttle ("light load" ) at 60mph, your motor can handle more advance than it would be able to take with the throttles

wide-open. With this additional advance comes better fuel economy and lower motor temperatures. However if you set up your 008 centrifugal

distributor to give you the right advance under these light-load conditions, then you would hear a preignition knock whenever you put your foot

into it. That's why the stock bread-and-butter 2002 has a vacuum advance distributor. At light load the manifold vacuum is high, and the

diaphragm mechanism bumps up the advance a few degrees to maximize part-throttle power and fuel economy. Put your foot into it and the

vacuum signal goes away, so the timing backs off a bit while you're under load.

But what about vacuum retard distributors, and combination advance/retard setups? Well the idea is the same regardless. With a vacuum

retard setup, you apply a strong vacuum signal whenever you want to back off the timing. I can't think of a purely-retarding setup right now, but

I'm familiar with the two-line advance/retard setup that my '74 had originally. They used this system because of the EGR pollution controls

installed on the engine. When you take your foot off the gas at high speed, the carbs go to an over-rich condition. To reduce the pollution

caused by this situation they open up the EGR valve whenever the throttle closes at high speed, to give the exhaust a chance to burn a second

time. When the EGR valve opens the mixture leans out considerably, because you're feeding half-burnt gasses back into the manifold,

bypassing the carburettor. Lean mixtures require a relatively retarded (less advanced?) timing setting to burn well, so the engineers added a

complicated system of magnetic vacuum valves, multiple vacuum lines, etc. Under idle and part-throttle cruising, the vacuum advance kicks in

to augment the centrifugal advance mechanism. When you put your foot into it the EGR is closed, and the vacuum advance has no effect--all

advance is provided by the weight/spring gizmo. When you take your foot off the gas at high speed, the vacuum retard signal from the carb

throttle plate counteracts the vacuum advance signal provided by the manifold, and the EGR valve opens to re-burn the exhaust.

The bottom line is that if you don't care much about fuel economy and pollution, and you will be driving your car aggressively, a

mechanical-advance unit is fine. However if you run the common vacuum advance distributor with the line connected to the manifold, you will

enjoy better part-throttle fuel economy and lower motor temperatures under around-town and cruising conditions. If you care about pollution,

install a modern EFI system, because the hodge-podge EGR advance/retard oxygen-pump-equipped systems really didn't accomplish much

besides satisfy federal regs at the time.

Incidentally, the mechanical advance curves in all stock BMW distributors are about the same. The TII and TI curves were a little less

aggressive than the vacuum models because the high-compression sport motors work best when full advance is held off until slightly higher

RPM (like 3500 instead of 2800, or something like that). The TI/TII units offer less total advance than the vacuum units. You can run the

advance/retard unit just fine with only the advance line hooked up if you have disconnected all of your EGR stuff.

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Guest Anonymous

...otherwise, you would have very little vacuum under light throttle conditions, which is when you need the most advance.

You wouldn't want full manifold vacuum, because that is highest when the throttle is fully closed. Just under the throttle plate, vacuum is low at full throttle, relatively low at idle and relatively high at mid-RPM cruising with the throttle _almost_ closed.

Very little throttle is required for light load, mid-RPM (3-4K) cruising... perhaps a few percent opening. However, because of the speed the motor is turning over, there will be quite a bit of vacuum just under the throttle plate. At idle, the throttle plate is slightly less open, but the motor is just ticking over, so vacuum is commensurately less (and below the threshhold for the vacuum advance mechanism).

Chris B.

'73 ex-Malaga

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