Ignition advance

[Bitsyncmaster]

You fire the spark plugs before TDC (top dead center) to get the burning flue/air to start expanding around TDC. It would seem to me that the amount of advance should be proportional to the engine RPM. So why not just have the entire advance built into the distributor? Why do they use vacuum advance in addition to mechanical advance?

My guess is they could not get enough advance to work reliably using only mechanical advance.

[Ron]

There is no problem getting enough advance using only mechanical advance. What you are missing is that the total advance is a result, not necessarily by design -- They have separate jobs. The mechanical advance is based on engine speed and the vacuum advance is based on engine load. Mechanical advance is set at full throttle (most important), it varies though out the RPM range. Vacuum advance does nothing at idle if connected to a ported vacuum source -- Once the throttle is slightly open, ported and manifold vacuum are the same. Note at high RPMs when you might expect both to be adding advance, there is little manifold vacuum, so there is no vacuum advance! "Performance" wise, the only time a significant amount of advance is added by the vacuum advance is during part throttle, low load. [Here the manifold pressure is low>>volumetric efficiency is low>> cylinder pressure is low>>the fuel mixture burns slower. So, we need to ignite the fuel sooner to reach peak cylinder pressure at the optimum moment.
Bottom line- The vacuum advance is for driveability (and gas mileage).

[Bitsyncmaster]

I don't think the vacuum advance servo is a linear function. I think it just gives full or no advance.

[Rich]

Seen below are descriptions of the vacuum ignition advance taken from the DMC Technical Info Manual. The specs from p.28 show a few of the timing advance values for both the vacuum and mechanical advance systems. This vacuum advance theory is the same as on any pre-ECU-ignition engine before or after this engine.

On some distributor designs one can see an external linkage from the vacuum dashpot outside the distributor. While gunning the engine at idle the linkage can be seen to move across a range in direct proportion to manifold vacuum, not in an on-off fashion. Don't bother looking for the vacuum advance linkage on the PRV unless you are good with a mirror down in the clutch slave area.
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There are aspects of the vacuum advance that are on-off. Two valves are in series in the vacuum circuit between the intake manifold and the distributor vacuum advance. First, the thermal vacuum control valve (sensing from one branch of the coolant y-pipe in the VOD) disables vac advance until the coolant temperature at that valve exceeds 40degC/104degF. This aids the catalytic converter warm-up to meet emissions rules.

Second, the solenoid valve in this same vacuum circuit and located just before the distributor will close when a microswitch at the throttle linkage signals closed-throttle. So during idle or engine deceleration the vacuum advance is defeated by this valve.

To summarize, whenever the engine is warm and the throttle isn't closed the vacuum advance continuously adjusts the ignition timing depending on engine load (manifold vacuum), this being additional to the engine speed-dependent mechanical advance.

[David T]

Refer to M:01:04. The advance is not "on-off". It is affected by vacuum and engine speed. These values are meant for sea level with regular gas and an engine in good tune.
David Teitelbaum

[Ron]

I don't think the vacuum advance servo is a linear function. I think it just gives full or no advance.

No. The servos are spring loaded and the vacuum varies with RPM-Load.
Maybe you are getting this from observations while the vehicle is parked (no load)..or pulling the vacuum line when setting timing???

Look at this chart for GM vacuum advance canisters.
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Vacuum drives the advance servo -- vacuum varies, so advance varies.

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You might find this interesting (and it relates to the vacuum switch discussion in the other thread)
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I want to map how the D advance acts normally and with the advance swapped to the manifold vacuum...but I'm thinking it is a bad idea more and more anyway...and I don't think I can get enough inputs into my scanner and too lazy to dump it to a database and piece it together lol ...I might after I compare boost gauge with another gauge hooked to the ported vac, and if by surprise, I see nothing obvious...

[Bitsyncmaster]

so the spec shows 5", 10" and 15" of vacuum. So I assume 15" give the maximum advance.
What manifold vacuum would I expect at wide open throttle on flat roadway?

[LEVY]

On low compression engines it might not make a difference but on high compression engines you don't want full advance before the engine fires or you will have a very hard time starting the engine, by using vacuum advance you will not have full advance until the engine is running.




You fire the spark plugs before TDC (top dead center) to get the burning flue/air to start expanding around TDC. It would seem to me that the amount of advance should be proportional to the engine RPM. So why not just have the entire advance built into the distributor? Why do they use vacuum advance in addition to mechanical advance?

My guess is they could not get enough advance to work reliably using only mechanical advance.

[Bitsyncmaster]

On low compression engines it might not make a difference but on high compression engines you don't want full advance before the engine fires or you will have a very hard time starting the engine, by using vacuum advance you will not have full advance until the engine is running.

What I'm thinking is the mechanical advance could add more advance according to RPM so at speeds below 1000 RPM it would not add any advance.

I guess a high load on the engine requires less advance than a lightly loaded engine which is what the vacuum advance can do. So why does the total advance not keep increasing up to red line?

[Spittybug]

so the spec shows 5", 10" and 15" of vacuum. So I assume 15" give the maximum advance.
What manifold vacuum would I expect at wide open throttle on flat roadway?

Not too much. With wide open throttle, by definition you are open to atmosphere on two big bores. The little tiny bore of the vacuum advance line pales in comparison. When only slightly open, the ratio between the two is much closer obviously. So, at low RPM such as idle, the engine is pulling a big vacuum but as the throttles open up, the afore mentioned ratio of throttle barrel air to advance air gets huge, satisfying the engine's desire for air and the "pull" on the vacuum advance line goes away. By now however your engine speed has increased, so the centrifugal advance has not only replaced the vacuum advance but added even more.

To visual thinkers, the centrifugal advance is linear with RPM (X axis) and the vacuum advance is like a bell curve.(Y axis). The attached picture is a 3D table view of the advance table from Megasquirt. Base timing of 17deg advance, big plateau for RPM based advance and a increasing then decreasing vacuum advance.

By no means optimized, but it is modeled after our stock values (ok, a little more aggressive).

[Bitsyncmaster]

I just remember watching a vacuum gage on a running engine and don't remember seeing the vacuum drop that drastically with increasing engine RPM. Guess I need to try that test again.

[David T]

None of this is important unless you are mapping an EFI system or are trying to install forced aspiration. Set the motor to the stock specs and check it with a timing light. Typically you want to get all of your advance in as fast as the motor can handle it, it makes the most power. You can't start the motor with all of that advance and you can't put it all in till the motor warms up. If you modify the cams you can increase the speed you add the advance and you can put more in. You also want to tune the advance to coincide with the RPM's your shift points are at.
David Teitelbaum

[Ron]

None of this is important unless...But, but, inquiring minds want to know, David :deviltail:

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Seriously, if you are cruising above ~3000 RPM up or down hill, typical mechanical advance it maxed out ~20 and the vacuum is high so you get another ~20, at most. Go WOT and your vacuum goes to ~zero causing vacuum advance to do the same.
You go from low load, lean, slow burning fuel, and high vacuum, so 20° vacuum advance is added, starting the burn sooner (13+20+20°) , TO, high load, rich, fast burning fuel, and ~no vacuum, so there is no vacuum advance, starting the burn later (13+20°)...Both reaching max burn/pressure at a sweet spot just after TDC.

Do you also remember the in dash economy gauges (vac gauge). If you pressed the pedal much past WHEREVER you put it and let it settle down, it would drop faster than a camera shutter LOL

[Bitsyncmaster]

None of this is important unless you are mapping an EFI system or are trying to install forced aspiration.David Teitelbaum

If everybody thought like that, we would still be running steam engines in our cars:confused2:

This is the idea. Since it looks like the engine will idle faster with more advance (I get about 500 RPM increase adding vacuum advance with a hot engine with no load, open loop idle), why not sense when the AC is on and turn on the vacuum advance. If the engine is running more efficiently that way it should increased MPG for city driving and maybe keep the engine cooler.:wink: