I have read all I could find on re-curving and setting timing, but I have yet to come upon why. The time it takes for the system to be triggered and produce a spark is constant. So as the RPMs go up you have to start that process earlier? Why is that so? Does it have to do with spark flame front being affected by the smaller amount of time it has to propagate?
It would help me time my new rebuilt/modified SL 6.

bwhitejr

Hey

the process ain't start earlier, the distributer advances the triggering mechanish (points, magnetic pickup) so it's triggered earlier. The process itself always starts at the same place and ends at the same place. In our cars, with pretty much any stocker or "normal" aftermarket units, the distributer is the one that takes care of deciding when it's gonna happen. That's why is so important to understand what's going on and be able to modify it.

Actually, we're putting together a recurving article with Slant6Ram so stay tuned....

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

That isn´t too far off

You exactly nailed the reason that MECHANICAL advance is needed- as RPM increases, the flame front still burns at the same speed so you have to start the process earlier in order to achieve (as near as possible) total combustion and peak cylinder pressure while the piston is moving down on the power stroke and before the exhaust valve opens.

Vacuum advance is a different story. If engines operated at wide-open throttle all the time, mechanical advance is all that they would need (and they wouldn't need any variable advance mechanism if they ran at WOT and constant RPM). But any engine that spends a lot of time running throttled back (example, cruising down the highway at 55-70 mph) also benefits from vacuum advance. The reason for this is a little more complicated. When the engine is running throttled, there is a partial vacuum in the intake manifold. That means that when the cylinder fills and the intake valve closes, there's still a partial vacuum in the cylinder before the compression stroke begins. At the end of the compression stroke, the cylinder pressure is lower than it woul d have been if compression had started from atmospheric pressure instead of a partial vacuum. OK, so now what? Well, the lower the pressure when burning begins, the slower the flame front. So, following the train of thought from beginning to end, you need to start the flame burning even sooner than you would just based on RPM if the engine is running throttled back.

So- mechanical advance matches combustion speed to engine speed, and vacuum advance compensates for slower combustion speed at high vacuum.

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I found this on the Internet this morning:

Ignition Advance
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 supercedes 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.

See the Tuning section for how-to details.

[ Thanks to John Carri, others for this information. ]

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Ok, but in a race car, who cares, as long as it is fast?

Not for long enough to matter.

Also, I've been told that vacuum advance causes motors to be less consistent at the track. Some guys even run "fixed" timing on pure race cars. (it seems to work for Ron Parker?)

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1980 Aspen 225 super six

Dumbasss... it means that starts earlier...

lucky me I was'nt driving when I posted this....

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

What would anyone here suggest about having the choice of mechanical
or vacuum advance for a street/strip car?? I keep getting opinions
from the "go mech.advance" side of things. But I do need "some"
streetability

Right now I'm coming thru carburetion issues but when I had my previous setup, I tried plain mech and mech with vac, and even though my recurved dist was a 7R and I'm running 15º BTDC initial, fuel economy hurted bad w/o vacuum advance. Even given the fact that I drive with a lot of throttle opening (leadfoot) the vac advance helped in town and I believe that under normal cruise condition (never cared to check up on that).

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

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No, no, no, no, and no.... but then it doesn't matter if they're sub-optimal under those conditions. Vacuum advance on a race-only car is just something else to fail, and can reduce the precision of the mechanical advance.

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Define "some." If gas mileage on the street doesn't matter or is less important than getting the ultimate at the track, then don't worry about adding vacuum advance..

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You just answered what I was looking for , thanks! "ultimate" will do