Mach 1 Club

Full Version: Vacuum Advance - Timing is everything
You're currently viewing a stripped down version of our content. View the full version with proper formatting.
In a perfect world there would be no need for ignition advance curves. As soon as the piston reached top dead center and the engine built maximum compression, you'd be able to light the fire. Kaboom. Job done. But this isn't a perfect world, and neither is the case of ignition advance curves.

The reason for ignition advance is that the air/fuel mix doesn't burn instantaneously - it takes a little bit of time from the moment the spark is set off to the moment when peak cylinder pressure is reached. During that time, the crankshaft keeps rotating. So if you lit off the mix at TDC, the piston will be well down the bore by the time peak cylinder pressure is reached, and you'll get lousy horsepower and lots of unburned fuel out the tailpipe. The cure is to light off the air/fuel mix *before* the piston reaches TDC, so that the peak cylinder pressure is achieved at just about the time the piston is positioned to take full advantage of it. This is what we call ignition advance.

Okay, let's back up a bit. Why do we need any ignition timing advance at all? Why not light the fire off when the piston is at TDC? The answer is that it takes the flame some time to grow to fill the whole combustion chamber, and during the time the flame is growing, the piston keeps moving. The only way to get the flame to finish burning and produce lots of pressure on the piston when it's ready to be pushed down the bore, is to light it off early, while the piston is still moving up the bore. That is why ignition timing is needed.

How much advance do you need? Clearly, it depends on how fast the air/fuel mix burns, and how fast your engine is turning. Roughly speaking, if your engine is turning faster, you want more advance; this is the why distributors have mechanical advance in them, which puts out the spark earlier and earlier in the cycle as the engine rpm's climb. Once you reach a high enough rpm, the air-fuel mixture begins to whoosh into the cylinder with so much velocity that it becomes turbulent, and consequently the flame spreads very fast; increase the rpm, and the mixture becomes more turbulent in direct proportion, and the flame spreads even faster. This means that once you exceed a certain high rpm, the mixture tends to burn in about the same number of crankshaft degrees, no matter what the rpm. Now you no longer need the ignition timing to keep advancing with increasing rpm, so the distributor is designed to level off the advance above some rpm.

The somewhat large overall advance numbers seem too large to believe, and it freaked me out also. However, when I began to understand the role of ignition timing, I began to understand why this much timing is okay in certain cases.

Your distributor contains a mechanism (mechanical/centrifigal advance) of springs and weights that advances the spark timing as engine rpm goes up. This is to compensate for the fact that the engine turns faster, so it goes through more degrees of rotation in the time the flame spreads through the chamber. To compensate the flame is lit off earlier. The distributor also contains a vacuum advance canister. It's role is to add a bit of advance during certain high vacuum phases of driving, like cruise, mainly for fuel economy.

The reason for vacuum advance is that the rate at which the air/fuel mix burns also depends on how much of it is packed into the cylinder in the first place - i.e., how dense the mix is. When the engine is driving around at part-throttle, the almost-closed carburetor throttle blades restrict the amount of air/fuel mix entering the engine (compared to wide open throttle). This low-density mixture burns more slowly. To compensate, the ignition needs to be fired off earlier when the engine is at part throttle (than when it is at full throttle). The clever solution is vacuum advance. When the engine is at part throttle, manifold vacuum is high, and this sucks on the vacuum advance diaphragm and advances the spark. If everything is set up correctly, the extra advance compensates for the slower-burning mixture.

Vacuum advance during WOT acceleration is a common myth. More than one mechanic has told me this one. Part throttle yes, but not all out acceleration.

When the engine is at part-throttle (cruising at constant speed on level road, for instance), the intake manifold vacuum is high and engine load is low. That is to say, the resulting air/fuel mix is lean. And it turns out the speed at which a flame spreads in an air/fuel mix decreases when the air/fuel mix is thin. So the flame takes longer to spread from the spark-plug and fill the whole combustion chamber. If we want the combustion process timed right, then, we need more ignition advance at part throttle, to compensate for the slower flame burn in the leaner air/fuel mix. And this is exactly what the vacuum advance does: the vacuum cannister measures engine vacuum, which is to say, it measures engine load. The lesser the engine load or need, the more the spark gets advanced. Exactly what the engine wants, and a very clever idea!

During acceleration, there is less vacuum signal. Vacuum advance is less when there is less vacuum, using either manifold or ported sources, as I explained above. Most vacuum advance units are not adjustable - the factory figures out what works to meet emissions requirements, then presets that amount in the vacuum cannister design. Some Mopar units are adjustable from the factory. For Fords and GM cars you can buy aftermarket adjustable vac. advance units (Accel, Crane, etc).

VACUUM ADVANCE: If you didn't disconnect the vacuum advance when setting the timing, it is probably bumping up your advance as you hit third gear, since your vacuum starts to come up as you approach top speed. It dials in greater ignition advance, which is necessary when you've got a thin or lean air-fuel mixture such as at part-throttle; you have to start that mixture burning quite early in order to get all that fuel combusted by the time the piston is passing TDC. It greatly improves part-throttle fuel-economy. You can disable it temporarily with a golf-tee plugging the line to the canister. It also bumps your idle speed up. That's it. Makes little to no difference for actual driving, only for idle emissions quality. Bob Barry

The total amount of advance or overall advance depends on many things (compression ratio, head design, rear-end ratio, weight of your car, etc) but I've been told numbers of around 10° to 15° advance at idle, around 36° full mechanical advance (with the vacuum advance disconnected). At part throttle, high rpm, with vacuum advance, the ignition timing should be somewhere in the range of 50°. That number surprises many, but that's what's needed for maximum fuel economy at part throttle (cruising).

Total advance at high rpm and wide open throttle = initial timing + mechanical advance

Total advance at high rpm and part-throttle = initial timing + mechanical advance + vacuum advance

One rule supersedes everything else: if the engine detonates, reduce the timing immediately till all traces of detonation are gone. Detonation will kill your engine in a very short time (it breaks piston rings, crumbles pistons, etc.).

The best way to set ignition timing is to modify the initial advance and advance curve to get the best power at WOT at all rpms. Do this with the vacuum advance disconnected. Once the mechanical advance is dialed in, connect the vacuum advance, and dial it in for best *part-throttle* power with no pinging or surging. This last step is universally omitted when the car magazines write about engine buildups.

A 2.56:1 rear is a pretty darn stiff rear ratio, which means the engine sees a much bigger load (less torque multiplication through the rear gear). A bigger engine load means increased cylinder pressure and increased tendency to detonate. Translation: be cautious in going to lighter and lighter distributor springs, as your engine is working harder than most due to the very stiff rear end. Again what I've heard for Mopars is to use full advance by roughly 2500 - 3000 rpm.

If you can get away with full advance at lower rpm with no pinging, fine. Just be very careful not to run into even mild "silent detonation" which can still break piston rings and damage pistons without being loud enough for you to hear over the sound of the car. Once you find the point where the thing pings, back off a few degrees to give it a safety margin. Also a hotter day or a tank of bad gas might come your way and needs a safety cushion. Better an engine a few % down on torque than one that needs a rebuild due to detonation.

Race engines don't need vacuum advance, because they're never at part-throttle anyway. Any street engine spends more time at part-throttle than WOT, and can always benefit from having vacuum advance. Magazines like Hot Rod test engines at WOT (Wide Open Throttle) on a dyno, where vacuum advance plays no role, so they leave it off the engines. Then they tell you that you need 36 deg, or 32 deg, or whatever, of mechanical advance. They totally fail to inform you that your engine will run better on the street with additional vacuum advance over and beyond that 36° or whatever.

If you take a look at the chart, you can see that the variables can change throughout the range of the engine operation. The timing mechanism of the distributor must make timing changes based upon these factors. It's also easy to see there is no one perfect curve. Each engine will be different and, consequently, each curve will be different. More on curving the distributor later.


Early, pre-emission vacuum advance units were typically linked to a manifold vacuum source. This meant that the vacuum was most often taken from a location below the carburetor throttle body. During idle and part throttle operation, manifold vacuum is high. This advances the ignition timing under those conditions and improves fuel economy. When the engine is operated at wide-open throttle, manifold vacuum is low. This means the vacuum mechanism does not advance ignition timing. As a result, there is no chance of detonation (or pinging). In the mid-'60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened. This is completely opposite to manifold vacuum. At idle, a spark ported vacuum system has no vacuum advance (in contrast, a manifold vacuum advance might have as much as 12-degrees extra timing).

Reworking the Advance Curve

The results of tinkering with the advance curve on any engine can be remarkable. For example, many stock production line distributors were setup to bring the advance all in at engine speeds of 4,000 rpm or more. Bringing the curve in sooner can result in startling performance improvements. The function of the advance curve is to match the ignition timing to the burning rate of the fuel and the speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed can cause a need for an ignition timing change.

- Use as much initial advance as possible without encountering excessive starter load or engine kickback.
- Start the centrifugal advance just above the idle rpm.
- The starting point of the centrifugal advance curve is controlled by the installed length and tension of the spring.
- How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness. The stiffer the spring, the slower the advance curve.
- The amount of advance is controlled by the advance bushing. The bigger the bushing, the smaller the amount of advance.

Here are a couple of extra curve tips I’ve come up with over the years: - Automatic transmission cars almost always need a quicker, but shorter, curve than stick shift cars, however, the total timing should still be the same.
- Automatic transmission cars use more initial advance than stick shift cars.
- By using a separate starter and ignition switch, you can overcome adverse starter load by spinning the engine first, then clicking on the ignition switch.

So far so good. The very best re-curves are those that use the car as a testbed. All that is required is a degreed damper (or a timing tape), a timing light, a reliable tach, a notepad, and a bit of patience. The main idea when recurving a distributor is to bring the curve in as quickly as possible without the engine detonating. In other words, play with the springs until you reach the optimum curve for your application. Some cars may require one very light spring and a heavy spring; certain combination's will require a pair of medium springs, while others can get away with a pair of light springs. I've even seen Corvette Delco applications that required but one spring. The other weight was used "springless."

When playing with the curve, set the initial. Make a note of the initial timing. Then make a note of the spring combination in the distributor. Increase the engine speed and make a note of the speed at which the curve begins along with the speed at which the curve ends (where the curve is "all in"). You can also graph the results by checking the timing at 200-rpm intervals (correlating the advance shown on the harmonic damper to the engine speed). Test the results and begin again. Trial and error plays a major role in the selection of a proper curve. What you have to obtain is good throttle response along with detonation free timing. As mentioned earlier, some engines will "like" more initial timing than others, while some combinations will want more total timing. In any case, you can adjust where the advance starts, the rate of advance (slope), as well as the total amount of advance. Take the time to sort through the timing maze and be certain you record all changes in the notebook. It will become a valuable guide when setting up the curve for your particular combination.

As you can see, in the world of ignition timing, it's not a perfect world. Even when cars are seemingly equal, they might need a different curve. It's all a matter of trial and error.