Ignition Timing Theory - Port Source vs Manifold Source

[Captain Obvious]

Our Z's use ported vacuum for timing advance. So what's the theory difference between port based vacuum advance and simple manifold vacuum advance? I'm not a timing guy, but I got the basics:

Manifold vacuum is highest at idle, lowest at WOT, and in-between when in-between.

Port vacuum on the other hand, is zero at idle, highest just above idle, and the same as manifold vacuum everywhere else.

I guess the question is "why"? Why do they eliminate the vacuum advance at idle? What's the advantages?

Normally, the vacuum source is used as sort of an engine load sense device. In other words, when the engine load is low (like at light cruise) the mixture is lean and you need more time for burn, but when the engine load is high (like at WOT) the rich mixture takes less time for burn. No problem.

But following that concept... Wouldn't the load be lowest at idle?

I've done some web searching and have seen some theories that draw a link to emissions reductions and port vacuum, but I'm a simple follower and can't tell established truth from speculation.

So, we got any timing experts in the house?

[LeonV]

Good question!

"Ported vacuum" is used for emissions reduction, among other things. Retarded spark timing reduces peak cylinder pressures, which reduces peak combustion temperatures. This makes for a reduction in NOx emissions, since NOx is dependent on combustion temperature. Conversely, exhaust temperature increases with retarded timing, thus a reduction in unburned HC is also realized.

A side-benefit that I see is stall protection. Since the vacuum advance quickly activates as the throttle is cracked open, off idle engine torque is considerably more than at idle. This gives the driver an effective buffer, or aid, in getting the car going from a stop. This can be seen in modern spark maps as well.

Also, overly-advanced idle timing is more prone to misfire. Taking a stock '72 240Z, with base timing at 17deg and max vac advance giving another 11deg, that's 28deg of spark advance at idle!

[doradox]

Good question!

"Ported vacuum" is used for emissions reduction, among other things. Retarded spark timing reduces peak cylinder pressures, which reduces peak combustion temperatures. This makes for a reduction in NOx emissions, since NOx is dependent on combustion temperature. Conversely, exhaust temperature increases with retarded timing, thus a reduction in unburned HC is also realized.

A side-benefit that I see is stall protection. Since the vacuum advance quickly activates as the throttle is cracked open, off idle engine torque is considerably more than at idle. This gives the driver an effective buffer, or aid, in getting the car going from a stop. This can be seen in modern spark maps as well.

Also, overly-advanced idle timing is more prone to misfire. Taking a stock '72 240Z, with base timing at 17deg and max vac advance giving another 11deg, that's 28deg of spark advance at idle!

Not much, if any, NOX is made at idle as combustion pressure and heat is very low. HC is lowered mostly because of the more stable idle you allude to. An old trick back in the day to get a marginal car to pass the HC requirement on an emissions test was to retard the timing, plug the vacuum advance hose, and jack the idle back up. Engines with hot cams also benefited from that trick.

Steve

[LeonV]

Not much, if any, NOX is made at idle as combustion pressure and heat is very low. HC is lowered mostly because of the more stable idle you allude to. An old trick back in the day to get a marginal car to pass the HC requirement on an emissions test was to retard the timing, plug the vacuum advance hose, and jack the idle back up. Engines with hot cams also benefited from that trick.

Steve

I agree that there is less NOx emissions at idle than with the engine loaded, but you will see a reduction in NOx through retarded timing. Even if you have a stable idle, retarding the timing will increase exhaust temperatures which then burns up more unburned HC coming from the combustion chamber.

At this point, we're getting into the finer details. Point being, for the OP, is that certain emissions will be reduced and off-idle torque will jump making take-offs easier (and stalling harder, unless you let off the throttle).

[doradox]

At this point, we're getting into the finer details. Point being, for the OP, is that certain emissions will be reduced and off-idle torque will jump making take-offs easier (and stalling harder, unless you let off the throttle).

I agree about the finer details but exactly where does this torque increase come from? Both ported and manifold vacuum are the same just off idle so where is the advantage? Both engines would be producing the same torque at the same rpm and throttle opening. They are both effectively operating with manifold vacuum just off idle.

Steve

[LeonV]

I agree about the finer details but exactly where does this torque increase come from? Both ported and manifold vacuum are the same just off idle so where is the advantage? Both engines would be producing the same torque at the same rpm and throttle opening. They are both effectively operating with manifold vacuum just off idle.

Steve

I suppose my phrasing was a bit awkward. Retarded timing at idle, and advanced everywhere else has a stabilizing effect on idle speed. But now that I think about it, the effect isn't really there when using a distributor.

There could be some dynamics involving how much the throttle must be open under retarded/advanced timing, but I think these are all secondary effects and not really noticeable. It's more beneficial for EFI with programmable spark maps.

[Captain Obvious]

No, no... Go on with the finer points. I'm cool with that.

Retarded spark timing reduces peak cylinder pressures, which reduces peak combustion temperatures. This makes for a reduction in NOx emissions, since NOx is dependent on combustion temperature. Conversely, exhaust temperature increases with retarded timing, thus a reduction in unburned HC is also realized.

Why do the exhaust temps go up? Because the exhaust valve opens before the burn has completed and you're dumping still burning gasses into the exhaust manifold?

And about those unburned HC? Why don't you just give the burn more time to complete in the cylinder by advancing the timing? That way you could get the energy out of it too instead of just wasting it in the exhaust manifold? I'm assuming there's a reason this doesn't work, but I just have to ask.

Before I asked here, I did do a little digging and turned up this document about ignition timing:

http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

This guy doesn't like port advance at all. He talks about it on page four:

The key to making this system work at maximum efficiency was retarded spark at idle; with retarded idle spark timing, the "burn" begins late, and is not complete when the exhaust valve opens, which does two things which were important for emissions. The incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and the significant increase in exhaust gas temperature ensured rapid light-off and combustion of the hydrocarbons in the exhaust gas stream when the fresh oxygen-carrying air was introduced from the air pump.

This makes sense to me, but seems to require an air pump in order to work (which we have - or used to have).

But then the question becomes: If you're not running an air pump, does port advance do anything for the unburned HC?

[LeonV]

No, no... Go on with the finer points. I'm cool with that.

Why do the exhaust temps go up? Because the exhaust valve opens before the burn has completed and you're dumping still burning gasses into the exhaust manifold?

Somewhat, you're getting close. The burn is long completed by the time the valve opens but the burn does begin later, thus the maximum combustion temperature occurs later in the power stroke. This gives less time for heat transfer to occur between the end-gas and the cylinder walls, thus the exhaust coming out of the cylinder is hotter but the combustion temperature is cooler (lower cylinder pressure).

And about those unburned HC? Why don't you just give the burn more time to complete in the cylinder by advancing the timing? That way you could get the energy out of it too instead of just wasting it in the exhaust manifold? I'm assuming there's a reason this doesn't work, but I just have to ask.

As spark timing is advanced, exhaust temperature drops leading to a decrease in HC oxidation, thus HC emissions increase. HC are a product of combustion, you're not going to run your engine without HC coming out. However, HC emissions are not highly influenced by spark timing, not nearly as much as NOx emissions.

Before I asked here, I did do a little digging and turned up this document about ignition timing:

http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

This guy doesn't like port advance at all. He talks about it on page four:

This makes sense to me, but seems to require an air pump in order to work (which we have - or used to have).

But then the question becomes: If you're not running an air pump, does port advance do anything for the unburned HC?

Yes, however, an air pump will catalyze the oxidation of HC.

As you can see, that guy is spreading misinformation, at least in the quote you posted as I didn't have a chance to read more. The burn is long done before the valve opens and there is no incomplete combustion to speak of. In fact, retarded timing stabilizes engine running (to a point, obviously), as discussed earlier.

[doradox]

I was a State certified Emission tech in AZ for many years. In my experience the below is generally true. YYMV.

In the real world on an engine in good condition and otherwise good tune the base timing alone doesn't make much difference in HC.

However.

At idle vacuum leaks and poorly metering carbs, contribute to cause poor combustion and cause high HC. Bad oil control introduces an HC that doesn't combust well and ends up adding a few PPM to the readings.

If we assume the idle speed will be set back to spec after the timing is changed....

Retarding the timing AND the resultant opening of the throttle to regain correct idle speed helps mask those problems to a great extent.

For the vacuum leak opening the throttle at lower timing reduces vacuum, thus the magnitude of the leak, and increases the amount of A/F mixture the engine is consuming thus lowering the relative size, and effect, of the vacuum leak. Lean misfire a very common cause for high HC.

Some worn carbs meter inconsistently, causing misfires, when throttle openings are very small and so the increase in throttle opening and airflow required to bump the idle back up helps as well.

Ported vacuum advance helps lower HC in worn engines as I have described above. My experience is that, in general, applying manifold vacuum to a vacuum advance unit on an engine designed for ported vacuum advance reduces idle quality. But again YYMV.

Steve

Edited January 14, 2012 by doradox

[Captain Obvious]

Thanks for the additional info. Looking at that chart I see what you mean about the HC. Almost flat with respect to timing while NOx continues to climb as timing is advanced.

So our idle timing is on that chart, but once the mechanical and vacuum advance mechanisms kick in at higher RPM's we quickly end up off the right side of that chart. Do the NOx continue to climb, and do the unburned HC still stay pretty much flat? That would explain why EGR only kicks in above idle. Well that, and it would probably completely ruin your idle stability if it was active at idle.

So throwing emissions to the wind and timing for power only... You want your timing to be as far advanced as possible just before you start to ping, right? That would ensure that you're eeeking out the most energy from every charge. How effective are knock sensors? Do they pick up each and every knock, or is more of a "average" thing?

If I bolted a knock sensor to the center of my L6 and then went out for a drive with my base timing set five degrees higher than normal, what do you think the sensor pick up?

[Captain Obvious]

Steve, that all makes perfect sense to me. Thanks for that.

PA separates their inspections into "safety" and "emissions" and I've had my state safety inspection license for years. I never bothered to get my emissions cert, and I've been kicking myself over it ever since. Now the prices have gone way up on the training classes, and I can't justify the cost for "hobby" use.

Also, these days I suspect there's a whole lot less theory being taught in the classes, and a whole lot more of "Where to find the OBD II port", and "How to change an O2 sensor". :sick:

I should have got my emissions cert years ago back when they were teaching stuff like what we're talking about here.

[LeonV]

Thanks for the additional info. Looking at that chart I see what you mean about the HC. Almost flat with respect to timing while NOx continues to climb as timing is advanced.

So our idle timing is on that chart, but once the mechanical and vacuum advance mechanisms kick in at higher RPM's we quickly end up off the right side of that chart. Do the NOx continue to climb, and do the unburned HC still stay pretty much flat? That would explain why EGR only kicks in above idle. Well that, and it would probably completely ruin your idle stability if it was active at idle.

NOx climbs with increasing combustion temperatures. Therefore, in a nutshell, NOx does climb as spark timing is advanced (higher cylinder pressure = higher temp).

NOx emissions and EGR are very interesting subjects. I have a summary/explanation posted on HybridZ, it can be found here.

So throwing emissions to the wind and timing for power only... You want your timing to be as far advanced as possible just before you start to ping, right? That would ensure that you're eeeking out the most energy from every charge. How effective are knock sensors? Do they pick up each and every knock, or is more of a "average" thing?

NO! Absolutely not! I don't mean to yell, just want to emphasize the point. I'll try to summarize why this is so.

Once the mixture is ignited, it burns and releases its energy over a certain period of time. The whole point of having the spark timing advance is for the mixture burn time to match the engine speed. Burn time is essentially constant (depends on AFR, combustion chamber geometry, fuel composition, etc.), but engine speed is variable. Therefore, the burn must start earlier at higher engine speeds.

There is an optimal point at each engine speed, where the burn must release a large part of its energy. In relation to crank angle, this is at about 15 degrees ATDC, if I remember right.

If you ignite too late, the burn happens late, and your maximum energy is less (piston is dropping = bigger volume = less pressure) since it occurs at a less favorable crank angle, thus you make less power.

If you ignite too early, your peak energy release occurs too early, when there is less leverage on the crankshaft (rod almost, if not completely vertical).

Your goal when tuning a spark map is to find the "happy point", the technical term being Max Brake Torque (MBT) timing. The knock limit comes into play if your fuel does not match your engine (too low octane, too high compression, high intake temps, or combination thereof).

The thing is, the knock limit can stop you before you reach MBT. However, if you reach MBT but have not reached the knock limit, then you can easily over-advance the timing. Therefore, just because you advance to just before the knock limit does not mean that you will be at MBT (the optimal spark advance for that specific engine speed/load).

If you are interested in more, here is a great paper I just found on the topic: http://www.vehicular.isy.liu.se/Publications/MSc/06_EX_3809_JM.pdf

Another great reference for those truly interested, is "Internal Combustion Engine Fundamentals" by Heywood. This is the ICE bible.

If I bolted a knock sensor to the center of my L6 and then went out for a drive with my base timing set five degrees higher than normal, what do you think the sensor pick up?

You usually need to calibrate those sensors so that you're making sure they don't give false alarms. However, once you reach high enough rpm, these sensors tend to pick up a lot of noise and give false alarms, at least on our solid-rocker adjuster L-series engines.

Edited January 14, 2012 by LeonV
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