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Vacuum Retard Education


Guest Anonymous

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Guest Anonymous

When is the advance retarding occuring, at idle? Also if somebody wants to go into more depth explaining advance curves as it relates to engine configuration, that would be really cool (ti vs. tii vs. vac advance vs. vac retard). Also I would be most grateful, if anyone could recommend any sources that I could learn more about the subject.

Thanks,

-Jaycen

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Guest Anonymous

I think my current dist only has vacuum retard. It only has one line. I'll verify that though. Currently the line is plugged. Thanks for the info, I'm thinking about playing another distributor and needed a better understanding of advance curves before I did.

thanks,

-Jaycen

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Guest Anonymous

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.

Mike

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Guest Anonymous

at least on my car. When I first got the car, I was surprised to find that the single vacuum line was a retard, not an advance. The retard mechanism seems to only have an effect on the timing at idle, so I presume it is provided just for emissions control purposes.

I have checked the timing curve on my motor with an adjustable timing light and found that I get about 37 total degrees of advance, but very early on... peaks about 2200 RPM. Haynes sez that for a non-tii that centrifugal mechanism should provide 16 degrees of advance and the vacuum mechanism should provide 4-6 degrees. tii's (mechanical advance only) should have 30-34 degrees of total advance @ 3500 RPM, whereas a non-tii 02 should have 38-42 total degrees, peaking at 2700 RPM. It looks to me like these distributors with vacuum retard units compensate for lack of a vacuum advance mechanism by having a more aggressive centrifugal advance curve.

Chris B.

'73 ex-Malaga

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Guest Anonymous

Mine does advance.

I'm the moron who's got the 1600 distributor in my 2002 motor (081 vs 078), but still bought the 2002 pertronix, but that's a different story.

With stock internals, I'm running waaay more advance than the book says without too much trouble on 87 octane.

When checking my mechanical advance, I discovered that it doesn't advance quite enough. Yeah, maybe I need a distributor, but I ain't gonna shell out for one right now.

Instead, a guy can do a lot with the vacuum advance diaphragm. Take your distributor out of the car and clamp it in a vice in your shop. Open a beer and start studying it. Take the diaphragm off. Notice that you can adjust it for total travel (advance) and, (how do I say it?) diaphragm effectiveness.

Pretty cool, eh?

Now plot out spec advance.

Plot out your actual advance.

Notice how you suddenly have a new feeling for how you can put the plot more where you want it without doing a single thing to the weights and springs if you don't feel like it.

I'm running 55 to 57 degrees total advance, with about 13 degrees coming from the diaphragm.

Nuts? I dunno. Seems great to me.

Hope that helps.

-g

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Guest Anonymous

moderately around time since there is only the mechanical advance. Do those motors run hotter and less well if you're not standing on it?

sorry, I'm a mechanical idiot

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Guest Anonymous

If I am not mistaken, '75s and '76s have two vacuum lines- one for retard and one for advance. My '73 has one line, and it retards only.

Chris B.

'73 ex-Malaga

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Guest Anonymous

The TII's fuel injection probably makes up for some of the timing inefficiency by providing a more appropriate mixture than either the normal single-carbed 2002 or the TI, so the mileage and driveability of the TII around town is probably better than either of the other systems. Still, the TII was designed for performance driving, and I'll bet that when it came to deciding whether to give the edge to open-throttle and high-rev performance vs. slow-poke around-town performance, the engineers probably went for high-end performance over low-end efficiency.

The TI probably doesn't behave as nicely around town as a standard carbed 02. The around-town gas mileage is worse, and that energy must be going somewhere; if not turned into mechanical energy, it leaves as heat. High-performance, big-cam, high-compression sidedraft setups are not at their best around town, because the design favors high performance over utilitarian efficiency.

That's not to say that a TI or TII would suffer overheating problems or what have you, I'm sure the engineers saw to it that the cooling systems were appropriate. But if you take a given 2002, say an ordinary mild-compression 2-bbl downdraft car with properly adjusted vacuum advance distributor, and throw in a mechanical-only distributor designed for a high-compression motor, your gas mileage will probably suffer and your radiator will need to dump more heat.

Mike

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Guest Anonymous

Great explanation! It made me wonder if a Tii distributor with a vac advance would be the best of both worlds, because under full throttle/low vacuum conditions, it would act like a pure mechanical advance Tii distributor, but when cruising on the freeway at small throttle opening/high vac conditions, it would advance the timing for better fuel economy. Is there a reason why they didn't do this on the Tii?

Colin

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