IGNITION TIMING BASICS
An example: This image offers a good example of basic ignition timing advance. Volvo B21FT (1981-85).
 
Volvo Ignition System Manual TP30432 pg 16.
I altered the above map to make it more simple. It only shows RPM and degrees of ignition timing. It does not show static base timing at idle or vacuum/boost timing adjustments. It only shows the timing advance the bare distributor will provide. If you need to know, the base timing at idle for a B21FT is 12º BTDC (Before Top Dead Center).

To keep this simple for now, this map doesn't take into account variable changes in engine manifold vacuum or BOOST. The factory B21FT ignition system will compensate for engine vacuum and boost changes, creating small adjustments in the timing map, which are designed to achieve good performance with regard to power, gas mileage, emissions and safety from too much timing advance during boost (or over-boost). More info about this will be explained later. 

This B21FT timing ramp is generated by a centrifugal (mechanical) advance inside the distributor. The reason why it's max is 24º-28º is because it's not precise enough to choose just one precise setting. Don't expect miracles from 1970s tech. Once it reaches the max time of around  25º to 26º it will maintain that level until redline.

If the above timing map was laid out like a more modern timing map (and nothing is added for base timing at idle), it would look like this chart below. 

RPM	900	1000	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000
Degrees	10º (provided by distributor mechanical advance)	12º	18º	22º	26º	26º	25º	25º	25º	25º	25º	25º

The reason why I made this chart without adding base timing at idle is to show ONLY the timing advance the distributor will provide. If you're questioning why there's 10º shown at idle below, it's because the centrifugal (mechanical) advance in the distributor adds 0º of timing with the engine off, but 10º of timing at 900 RPM. If you wanted to zero that out at idle, you would need to rotate the distributor to change it to 0º at idle. If this was done, then the other numbers on this map would be off by 1 0º. So I made this map above with standard distributor advance only.

But don't worry about the above info if it's confusing. If the B21FT timing map was laid out like a more modern map AND if I included 12º base timing at idle, it would look like this below.  To achieve this, all I would need to do is rotate the distributor + 2º to get 12º at idle.

RPM	900	1000	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000
Degrees	12º	14º	20º	24º	28º	28º	27º	27º	27º	27º	27º	27º

So what you now see above is the base timing for a B21FT.

Ignition timing is a very wide subject with a lot of chapters. We'll get to all of it slowly. HOWEVER, if you have a bone stock engine, then the idea of changing or setting ignition timing for YOU might only concern where to set the base timing at IDLE. Your factory ignition system will have a preset range of ignition timing advance for running above idle and you may not be able to modify that.

So what can you do with a Stock Engine?
For those of you with factory systems that can't really be "tuned," just about the only thing you COULD change if needed would be to alter the ignition timing at idle. So if, for example, your factory ignition timing at idle is 12º BTDC and your preset timing map is maxed out at 26º, if you were to adjust your idle timing from 12º to 16º, then your max total timing would change from 26º to 30º. Adding the extra 4 degrees at the top might help or it might not help. We'll discuss this later.

From my old Chevy hot rod days: An old hot rod trick for performance was to add as much EARLY initial ignition advance as you can get the engine to handle at lower RPMs, beginning at or just above idle.  For example, suppose you have a factory distributor with a mechanical advance that advances timing 24 degrees maximum. If your factory idle timing is set at 10º BTDC, your total timing would be 34 degrees.  If you were able to modify the distributor to have only 10 degrees of total mechanical advance, then if you set your ignition timing to 24º BTDC, the total timing advance hasn't changed, but hot rodders discovered this mod increased power at a much earlier RPM. This can help the engine launch harder out of the hole and give you stronger starts from light to light. If done correctly, this can offer better throttle response, a better idle vacuum signal, and it can help the engine run cooler. 
 
CAUTIONS TO UNDERSTAND: An engine with timing advance that's too late (too retarded) will have lower idle vacuum, slower throttle response, it can feel like a turd at lower RPMs and it can run hotter than normal. An engine with the timing too soon (too advanced) could have an erratic vacuum signal or it might back-fire through the intake manifold. It might have a snappy throttle response but might not pull well under load. And the engine will be more prone to pre-ignition (detonation) or "pinging" under load, especially if the engine is boosted.

Later Volvo systems made after 1989 have removed the possibility of doing any idle timing adjustments, because these later systems use a crank position sensor to keep track of engine timing, instead of the ignition distributor.  These later systems can be modified with special "chips" and custom programming to the ECU, but otherwise it's pretty hard to make adjustments on a later car apart from leaving it stock.

If you're interested in custom tuned chips for LH 2.4 and EZK ignition for later Volvos, they're available in the U.S. here: https://www.redblockpowered.com/

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COMBUSTION BASICS

Your familiar with the basic functions of a 4-stroke engine and the four cycles; INTAKE, COMPRESSION, COMBUSTION and EXHAUST. When concentrating on IGNITION TIMING, we only need to focus on COMBUSTION. 
An animated gif of a 4-stroke engine can be seen at: https://energyeducation.ca/encyclopedia/Four_stroke_engine

FOCUSING ON TIMING IN COMBUSTION

Picking the right place to create IGNITION SPARK in the combustion cycle is about matching the peak in-cylinder combustion pressure to a point in the cycle where it has the most leverage on the crankshaft during the POWER STROKE, which begins just after TDC (Top Dead Center).

What is peak in-cylinder combustion pressure?
In-cylinder pressure is the pressure in your cylinder during the combustion stroke. The pressure is caused by the ignition of fuel mixed with air. The fuel in the combustion chamber doesn't really explode. It's not instantaneous. It BURNS. This burn can be measured in milliseconds, so it takes time. The PEAK pressure happens when the fuel combustion has made the biggest "flash" possible in the burn process. If you were to video the fuel combustion and then watch it in ultra slow-motion, you might see when that brightest "flash" occurs. That will be your peak in-cylinder combustion pressure. This PEAK offers the BEST benefit if it's timed to happen at a certain point.

  Back to the discussion of leverage.
Experience tells us the best crank leverage occurs around 12° to 15° ATDC (After Top Dead Center). If we understand that, then we need to move ignition spark timing ahead to compensate for different fuel burn rates associated with engine speed, cylinder filling, temperatures, various air/fuel mixtures, and more. Whatever timing value we come up with to get this "perfect" formula is called MBT Timing (Maximum Brake Torque Timing), because it results in the best possible torque, or leverage, when pushing the crankshaft DOWN during the power stroke.

If we decided to just time the spark at 12° to 15° ATDC it would be too late, because the fuel needs time to burn. We need to ignite the fuel ahead of time. So when you see that an engine at idle is set to spark at 12° BTDC, you can get the picture that the fuel needs time to burn before it reaches the best combustion pressure at 12° to 15° ATDC. If you're trying to determine precisely where MBT Timing is for your engine, that can be done on a dyno, even at lower RPMS if necessary. Have a look at the video below. That's a technique used to find MBT Timing.

From 12º BTDC to 12º ATDC is 24º of engine rotation; If you're a math nerd, you can figure out how much time it takes for the engine to rotate 24º at 900 RPM (15 revolutions per second). Nevermind, That's 0.004 seconds (4 milliseconds). If you search hard you'll find info suggesting that a gasoline/air mixture can take as low as 0.002 seconds (2 milliseconds) to light. The key to all this is timing the PEAK of that burn to push the piston at the right time (see MBT above). This burn might occur a bit faster or slower depending on air/fuel ratio, engine compression ratio (affected also by boost level), etc. Then as the engine speeds up, it needs a bit more spark advance to get the fuel burning in time. So an ignition system will take advantage of this and add a little more spark advance after idle.

This image below will offer some comparisons of fuel burn speed. Different fuels are shown, as well as different CRs (compression ratios). Higher CRs burn faster than lower CRs. This also means a turbo boosted fuel mixture burns faster. This is an easy thing to understand, since we already know that compressing the fuel mixture makes a more dense mixture, which releases more power when it burns. The higher compression the better. It explained why a 20:1 CR diesel engine can create huge torque numbers.

The OTHER thing we can discover from this image is the fuel burn speed compared to the mixture. 1.0 Lambda, or stoich, for gasoline has an air/fuel mixture of about 14.7 to 1. This image shows us that the fastest combustion occurs near stoich (1.0 Lambda) or maybe slighter richer than stoich for some fuels.  If you go too lean or too rich, the combustion speed slows down and these graphs support that, BUT the speed drop-off is not dramatic. It's a very small drop. So if you're wondering what the point of this part is in relation to TIMING, here it is. There are some theories out there (and some videos and articles) which suggest the speed of fuel combustion should be compensated by the amount of ignition advance: i.e; a LEANER mixture should get MORE timing advance and a RICHER mixture should get LESS timing advance. Maybe there's a small point to that reasoning, but it's my opinion the burn speed differences are way too small to allow this micro-crap to take up your valuable time and energy.  Feel free to email me a comment if you disagree.

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For this next timing section, I want to go beyond the basics. We can call it Basics Plus.
I want to discuss ignition timing in your engine for the broad range of engine RPM, which you might choose to set if you COULD change settings for more power, better economy, etc.
The below video offers a good look at the basics of how tuning the ignition timing can be done for the benefit of more power.
Dyno testing ignition timing for maximum power through the engine RPM range. Naturally aspirated (non-boosted).
https://www.youtube.com/watch?v=AX2bn_4GEDI

When you're deciding what to do with spark timing as the RPM increases, videos like this can help.
There are a few fundamentals you should know.
1.  The timing map should generally be created from idle to redline (redline being your max usable RPM).

2. Testing like in the above video should first be done at FULL-THROTTLE with no vacuum spark advance devices and (if a turbo) no boost spark retard device. Also if it's a turbo, no boost should be allowed in this testing.
In the above video, this engine is tested through the RPM range a number of times. In the first tests, the spark is set at a low static setting. This can begin at i.e. 15º BTDC (Before Top Dead Center). This 15º setting is used through the full rev-test. No advance is allowed. This way the engine performance can be measured at all RPMs at that static spark setting.  Then another test is done at a higher spark setting (i.e. 20º BTDC). Then another at i.e. 25º BTDC, etc. Watch how that's done in the video. This will eventually give you a good idea how the engine runs at full power at various RPMs with each spark setting.  Then you can use that info to begin creating a basic spark advance map.   

3. The total timing upper limit in most cases for a naturally aspirated engine should be approximately between 25 and 35 degrees, including the base ignition timing at idle. This is the max spark timing that you will allow at any RPM. This means that if we assume your base idle timing is 12 degrees BTDC, then you have another 13 to 23 degrees to figure out after idle. Since I'm sticking to BASIC info for now, I won't go into any detail about a boosted engine yet, except to say that most boosted engines will always have a way to reduce total timing when under boost.  More info later on boosted engines.

There is some accepted theory in the small aviation engine world about base ignition timing numbers that I want to share. The reason I have such obscure info in my head is because I use SDS EFI ( www.sdsefi.com) in my 242. Besides being used in racing engines, SDS EFI is also used in aircraft engines. In the aviation world the accepted maximum total timing advance for a PARALLEL VALVE engine (Lycoming for example) is 25 degrees.  The max for an ANGLE VALVE engine is 20-22 degree s. I can't vouch for the relevance of this for a Volvo engine. It's just more random info to help keep your mind cluttered with "stuff."  A Volvo B21, B23, or B230 is a parallel valve engine. A Volvo B234 (16 valve DOHC) is an angle valve engine.   


4. Most of the timing increase in your ignition map will happen between IDLE and CRUISING RPM. Anywhere from 2500 to 3000 RPM would be appropriate, however I'll use 2500 RPM for cruising, which represents a good general mid-range cruising speed. So if we look at the B21FT example, the timing between idle and 2500 RPM is from 12º to about 27º. Then if there's any timing left in your maximum, that can be added between 2500 RPM and redline.

Here's a simple example base map below of how this might look using the numbers in steps #3 and #4 above. This gives you a base, or a beginning point to start, before you introduce other things that will add to the confusion and make more changes to timing. If you don't know what numbers to use when you're starting an ignition project, the best numbers will mimic what the stock factory ignition system used until you know what changes to make.

900	1000	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000
12º	14º	20º	24º	28º	28º	27º	27º	27º	27º	27º	27º

If you have no idea how to actually test your engine to get this information or if you just want a basic timing map to begin with and can't do real testing, then try to find the factory timing map, or you can try the above base map to get you started.

PRE-MADE BASE MAPS
If you're using an aftermarket engine management system, you should know that some systems will have pre-made base maps for lots of different engines, which you can use as a beginning point if needed. Haltech and MaxxECU are some that offer base maps in their programs for almost any engine ever made.


WARNING TO NOT PURSUE MORE TIMING UNLESS YOU KNOW.
You WILL be tempted to add more total timing, because you'll read or hear somewhere that more timing is more power. This is only true until more timing reduces power. You will probably see spark maps made by other people who tell you 30 or 35 degrees or more of total timing works for their engine. Before you copy those maps, let's have another discussion about this image so you can have a better understanding of why more timing might not be a good idea.
 
The above image shows spark occurring at TDC (Top Dead Center). If you've been reading, you know that the spark actually needs to begin the fuel burn before TDC for the fuel to burn in time. Now look at the image above with "12º- 15º ATDC". It shows burning fuel is pushing the piston down. The best location for the best leverage for that burn energy to PEAK is at 12º-15º ATDC. That's a pretty specific place, isn't it?
This should give you a clue that precision is king here.

So if you add more timing and the fuel burn begins too soon, then some of that energy gets used up before the piston can get into the best position to be pushed down. In simple terms, that results in less power available to push it down. It's easy for this to happen without you noticing if you choose more timing than the engine needs. The ultimate test of course is dyno testing. So this is why I recommend staying with FACTORY NUMBERS for this map until you test it or until you find a real reason to change.

Take a few minutes to watch this video. This guy is dyno testing a 400 plus HP V8 that's mostly stock with a few mods. The factory ignition has maxxed out timing at only 18º BTDC. He decides to test the power at 18º, 20º, 22º, 24º and 26º BTDC. He finds the most power with a max of 26º. If he had used someone else's map with total timing different than 26º, he would have risked getting less power.
https://www.youtube.com/watch?v=kRdHfMgkCwk

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Beyond the Basics. Using Vacuum to further alter Timing (part-throttle cruising).
The next step for your ignition is to find out if slightly different spark timing could improve things at certain RPMs under PART-THROTTLE (when your engine sees the most manifold vacuum).
Remember, the above spark advance map and discussion was all about finding the best FULL-THROTTLE settings. Now we can look at part-throttle. The best example of part-throttle is cruising on the highway is at around 2500 rpm. This is not the same as the 2500 RPM timing we discussed in the previous section (that was for POWER). Now we're discussing cruising only.

Some engines will become more efficient if the timing is advanced a bit during part-throttle cruise. This was the reason why many cars (including old Volvos) had vacuum diaphragms on distributors. These improvements in the engine might be harder to measure. It's still possible using a dynamometer, but most people just do these tests on a highway. If you goof in this area then you probably won't hurt the engine. You might only get less gas mileage. 

For example, if your using the 2500 RPM setting of 28º from the below table, you might find that during part-throttle cruise the engine would do better with a little extra spark advance. An ignition system that can read changing manifold vacuum levels and use info that to make timing adjustments during part-throttle cruise for better economy is a good thing.

	900	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000
0 inHG	12º	15º	20º	28º	28º	27º	27º	27º	27º	27º	27º
10 inHG				35º

In the above table I have added a parameter for vacuum.  It's measures in inHG in this example. This means Inches of Mercury, which is a very common way to measure vacuum. It's the most common method used by auto vacuum gauges. Zero inHG is basically ambient atmospheric pressure at sea level. This is the same reading you'll get with your engine off. On the other hand your engine might reach up to around 25 inHG vacuum under closed-throttle deceleration (engine braking). The maximum vacuum possible is about 30 inHg, but that will be higher than your engine will ever actually get.

The above vacuum-boost gauges are pretty common. The first one here uses inHG to measure engine vacuum.  The other uses inHG to measure vacuum and PSI to measure boost.

kPa (Kilopascal) for Vacuum/Boost Measurement
You'll find that there are lots of spark advance tuning maps out there that use OTHER parameters for vacuum or boost. kPa (Kilopascal) is a very common one for ignition maps (but not normally used for a vacuum gauge). kPa is commonly used to measure both vacuum AND boost. If you need to convert from one parameter to another, there are easy to find conversion calculators on-line.

With most engine tuning tables that use kPa, 100 kPa will basically represent ambient atmospheric pressure at sea level. Anything under 100 kPa is vacuum. Anything over 100 kPa would be pressure, or boost. 
 
An automotive gauge measuring vacuum in kPa is not as common, but if you had one like ABOVE, it would read near ZERO with your engine off. Your engine might reach up to around 850 kPa vacuum (8.5 x100) on this gauge under closed-throttle deceleration (engine braking). The maximum vacuum possible will be about 1000 kPa (10 x100) on this gauge, although your engine will never actually get that high.

Most engine tuning tables using kPa will measure it differently than the above kPa gauge does. On such a table, 100 kPa will usually represent normal pressure at sea level. This would be the reading you see with your engine off. Once you begin working with tuning tables like this, you get used to it really fast. Here's another example:

	900	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000
100 kPa	12º	15 º	20º	28º	28º	27º	27º	27º	27º	27º	27º
65 kPa				35º

In the above table I have changed the vacuum parameters from inHG to kPa.  In this table, 100 kPa will be what you see with your engine OFF. Less than 100 kPa is vacuum. The maximum vacuum possible is ZERO kPa, although that will be more vacuum than your engine will ever actually have.  So, if your using the 2500 RPM setting of 25º BTDC, like in the above table, you might find that during PART-THROTTLE cruise the engine might read around 65 kPa vacuum, and at that vacuum level the engine might run better with some extra spark advance. The above 65 kPa row with 35º instead of 28º is an example of an adjustment that could be added for better part-throttle performance.

It's not uncommon for cars to add 15 degrees or more to the standard timing map just for this kind of freeway economy arrangement.  Many OEM ignitions using a vacuum diaphragm or programming have added such an increase of 15 degrees or more.

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Using Vacuum AND Boost to Alter Timing.
An example: A Volvo B21FT (1981-85) will typically have an ignition timing advance map like this one below.
 
Volvo Ignition System Manual TP30432 pg 16.
This is the same distributor timing map I showed you before for the B21FT, except I have now added information for the dual vacuum/boost diaphragm on the distributor. This dual diaphragm was considered to be pretty advanced in 1981 and it worked well for a B21FT at the time. It looks similar to diaphragms on non-turbo engines of the era, but those control timing advance only during part throttle cruise. 
A detailed explanation of this diaphragm is below.

  This Diaphragm when under VACUUM.
This diaphragm advances ignition timing when vacuum is present, except at idle (more details on this in the Ported Vacuum section). This advance function is primarily for economy. The diaphragm can advance timing up to about 12 to 16 degrees over the normal timing map during part-throttle cruise. An increase like this offers more part-throttle engine torque at a point when it's safe, because the engine is not under a great load during part-throttle cruise. This helps to increase fuel economy.

This Diaphragm when under BOOST.
This part of the diaphragm retards ignition timing when BOOST is present. This is primarily for engine safety reasons. An engine under boost and load develops a lot more torque and more cylinder pressure. More torque is awesome, but it comes with a lot more stress and heat, which can damage the engine. So this is designed to protect the engine by retarding the timing curve as boost increases. The amount of boost retard with this diaphragm was set to max out at about 6 to 10 degrees at about 6 PSI for early 240s. Volvo info suggests this changed to about 3 to 7 degrees at full boost for later versions with the intercooler boost system (late 1984). Full boost for an early B21FT was set at 6 PSI (42 kPa), or 7.1 to 8.2 PSI (49 to 56 kPa) for later versions with the intercooler boost system (late 1984). Maximum power for the early B21FT was about 131 HP and 155 ft lbs torque, or about 162 HP and 181 ft lbs. torque for later versions with the intercooler boost system (late 1984). 

As a bit of 240 Turbo trivia, the European version B21ET offered more turbo boost than a U.S.A. car. The B21ET  developed about 9.2 to 10.1 PSI (64 to 70 kPa), which gave the B21ET more power: 145 HP and 167 ft lbs.torque. Later B21ET cars were never officially fitted with intercoolers from the factory, but later B19ET cars were (for the Italian market). The B19ET was the same as a B21ET, except with a smaller bore. The later intercooled B19ET cars had 11 to 12.1 PSI (76 to 82 kPa) and developed 170 HP and 184 ft lbs. torque.

If the above B21FT distributor timing map was laid out like a more modern map, it would look like this below. Note that the above map uses 0 kPa (zero kPa) for ambient atmospheric pressure. So this sample scale will do the same.
In this example, nothing is added for base timing at idle. Idle will be 0º. You would not use this to set your timing. I'm showing this to demonstrate ONLY what the timing advance that the distributor providing using it's advance or retard mechanism. If you're questioning why some numbers on the vacuum side seem to be off by 10º, this is because the centrifugal (mechanical) advance in a B21FT distributor adds up to 10º of timing at 900 RPM, so the distributor would need to be rotated BACKWARD to achieve 0º.

Boost

900

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

+40 kPa

-8º

-8º

-6º

0º

4º

8º

8º

7º

7º

7º

7º

7º

7º

+30 kPa

-6º

-6º

-4º

2º

6º

10º

10º

9º

9º

9º

9º

9º

9º

+20 kPa

-2º

-2º

0º

6º

10º

14º

14º

13º

13º

13º

13º

13º

13º

+10 kPa

-0º

0º

2º

8º

12º

16º

16º

15º

15º

15º

15º

15º

15º

0 kPa

0º

2º

8º

12º

16º

16º

15º

15º

15º

15º

15º

15º

-10 kPa

+0º

0º

2º

8º

12º

16º

16º

15º

15º

15º

15º

15º

15º

-20 kPa

+5º

5º

7º

13º

17º

21º

21º

20º

20º

20º

20º

20º

20º

-30 kPa

+14º

14º

16º

22º

26º

30º

30º

29º

29º

29º

29º

29º

29º

-40 kPa

+15º

15º

17º

23º

27º

31º

31º

30º

30º

30º

30º

30º

30º

Vacuum

If the large number of boxes in this image above seems like too much to grasp, you can just concentrate first on the base map in the middle inside the GREEN boxes. That base map will be the spark timing advance for this engine before it sees any changes caused by vacuum or boost.

Here's another example below. In this example, I have included base IDLE timing, which is 12º BTDC at 900 RPM. Since the centrifugal (mechanical) advance in a B21FT distributor adds 10 degree automatically at 900 RPM,setting this idle timing to 12º means the distributor gets adjusted +2º.
So the purpose of this map below is to show you the actual, real world timing that a B21FT sees with all devices in working order.  

Boost

900

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

+40 kPa

-8º

4º

6º

12º

16º

20º

20º

19º

19º

19º

19º

19º

19º

+30 kPa

-6º

6º

8º

14º

18º

22º

22º

21º

21º

21º

21º

21º

21º

+20 kPa

-2º

10º

12º

18º

22º

26º

26º

25º

25º

25º

25º

25º

25º

+10 kPa

-0º

12º

14º

20º

24º

28º

28º

27º

27º

27º

27º

27º

27º

0 kPa

12º

14º

20º

24º

28º

28º

27º

27º

27º

27º

27º

27º

-10 kPa

+0º

12º

14º

20º

24º

28º

28º

27º

27º

27º

27º

27º

27º

-20 kPa

+5º

17º

19º

25º

29º

33º

33º

32º

32º

32º

32º

32º

32º

-30 kPa

+14º

26º

28º

34º

38º

42º

42º

41º

41º

41º

41º

41º

41º

-40 kPa

+15º

27º

29º

35º

39º

43º

43º

42º

42º

42º

42º

42º

42º

Vacuum

If the large number of boxes in this image above seems like too much to grasp, you can just concentrate first on the base map in the middle inside the GREEN boxes. That base map will be the spark timing advance for this engine before it sees any changes caused by vacuum or boost.

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PORTED VACUUM versus MANIFOLD VACUUM
In a paragraph above I mentioned that the B21FT distributor diaphragm modifies ignition timing when vacuum is detected, except at idle. In this section I want to discuss further what happens at idle and just above idle in an engine using PORTED VACUUM.
  Engines like the the B21FT have a throttle body with special ports on the side. The vacuum line to the distributor diaphragm does not come from a manifold vacuum source. It comes from a port on the throttle body. You can see BELOW this throttle body has two very small vacuum ports on the side facing the cam cover. I've labeled them PORT 1 and PORT 2 in the below images.  The opening in PORT 1 is slightly smaller than PORT 2.
Both ports are in a position near the edge of the throttle valve which opens toward the incoming air. Due to the positions of these ports, they do not see vacuum at idle. They see only ambient atmospheric air pressure at idle.  When the throttle valve begins to open, the ports will then be below the throttle valve and that's when they'll see PORTED VACUUM. So this ported vacuum will advance timing in the diaphragm under part throttle (initial part throttle push or during part throttle cruise). 
 

<<< The vacuum diagram at left is from TP30727 Volvo greenbook discussing 240 emissions. It shows PORT 1 (or line A) going to the distributor diaphragm. 
Let's continue looking at how this vacuum line affects ignition timing in a B21FT. The initial timing at idle is set to 12º BTDC by setting or rotating the distributor. The above B21FT ignition table shows us that additional timing advance will occur when the diaphragm sees vacuum, but with this configuration, this PORT 1 vacuum circuit has no vacuum at idle. So when the throttle is slightly opened, that's when the diaphragm sees vacuum and begins advancing timing. This increase in ignition timing will help produce more engine torque at part throttle. Besides an improvement in fuel economy during part throttle cruise, this cab be especially helpful for the B21FT, because the low static compression ratio (7.5 to 1) is generally not very optimal for low RPM acceleration.

If you have a 240 Turbo, there's something you might need to know about the above under hood vacuum hose label. All year 240 Turbo labels are similar (1981-85). If you look closely, you'll notice the throttle body ports are reversed compared to the greenbook diagram above left. The above label shows Port 2 going to the distributor diaphragm. Since I believe the Volvo GREENBOOK diagram is correct (and the actual TB plumbing has been confirmed by several 240 Turbo owners), then this means all of the vacuum hose routing labels Volvo placed under the hood for 240 Turbos are INCORRECT.  

To help reinforce this PORTED VACUUM concept into our feeble brains, here are a couple good videos below from someone showing us the difference between ported and manifold vacuum. This video shows an old Ford carbureted engine, but the concept is 100% the same with our old Volvos.
https://www.youtube.com/watch?v=ohw1oRlcC4c


https://www.youtube.com/watch?v=sLwXlsHBd4k


ONE LAST THING ABOUT PORTED versus MANIFOLD VACUUM
If you search around much on this topic, you'll undoubtedly come across some videos or articles which will argue about whether or not to used ported vacuum for a vacuum advance distributor.  You'll find people who will defend using normal MANIFOLD vacuum for that purpose or they'll insist THEIR testing proves THEIR engine runs BEST using normal manifold vacuum.  When you see that, feel free to analyze it and work out precisely what the timing advance looks and when that vacuum appears compared to when it appear in a ported vacuum source. Try to see whether it seems real or not. Some people will insist that ported vacuum was only for emissions. I'll argue that maybe it was for emissions PARTLY, but I believe it's also about economy and that's never a bad thing to have. Sometimes there's a fine line between running better or worse with too much timing at the wrong RPM. Hopefully you can make your own informed decision on this and not just take the word of others without a good look at it.

Feel free to email me if you feel like discussing this at greater length.

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Modern ignition system timing maps using vacuum AND boost.
These maps below use kPa for vacuum and boost reference. Unlike the factory B21FT map image that uses 0 kPa for ambient atmospheric pressure, Microsquirt and many other systems use 100 kPa to designate ambient atmospheric pressure. So then in these maps any number below 100 kPa is vacuum and any number above 100 kPa is boost.  200 kPa equals 1 bar of boost.

Here's an CAUTIONARY example below of a spark map you might find that was made in Microsquirt.  Someone made and shared this online for a Volvo 4 cylinder (with turbo). This is not a typical B21FT or B230FT map. This one was custom made for an unknown custom Volvo turbo and it uses a lot more total timing than a typical Volvo map. I have no idea if these numbers can actually work ok on a Volvo. The engine would run, but the big question is would it run well or safely?

This map was shared in TB as an "example" to help others set their timing. The problem is no info was offered about it's use. Pump gas or Ethanol? Nope. Was it verified with dyno testing?  Nope.
 
Please don't assume that the factory timing numbers no longer apply to your engine, just because you changed a cam, added a turbo or increased boost. Volvo engineers probably put in lots of work to come up with those numbers. Copying a map like this above for your use should not be done, unless you know the engine and fuel used and you trust the dyno tests that helped create it.  

Here's another modern timing map BELOW that shows much more conservative max timing numbers (27 degrees max in the base map in the red bordered box). These numbers came from a known and trusted friend and they're backed up by real dyno tests for a Volvo B230 Turbo producing over 500 hp at the wheels. This map below was using pump gas, however the highest power was using ethanol. The ethanol map was very similar, but it added about 2 degrees extra timing under boost. 

If you're newer to this stuff and the large number of boxes in images like this seem like too much to grasp, try concentrating on just the base map in the middle inside the red border. That base map will be the basic spark timing advance for this engine before any changes are made by vacuum or boost. That's a good place to begin. Once you have an understanding of a base map, then you can begin to better understand how small changes below 100 kPa (VACUUM) make changes, and also how small changes above 100 kPa (BOOST) make changes. 

If you've had experience with Megasquirt, Microsquirt, Haltech, Motec, MaxxECU or other aftermarket systems, then you're familiar with using kPa for vacuum and boost values. When I first moved from Bosch K-Jet to programmable EFI in the 1990s, my first experiences began with SDS EFI (Simple Digital Systems), which I still use on my 1984 242 Turbo. More info on that HERE. SDS EFI is primarily designed as a race car engine management system. It doesn't use kPa at all. It uses old school PSI and inHG values for boost and vacuum, but the programming is otherwise pretty much like modern systems. 

If you've made your own timing maps using Megasquirt, Microsquirt, Haltech, Motec or other aftermarket systems for a Volvo, I would love to hear from you. Please email me.
CONTACT ME

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Using ignition timing to help achieve a stable idle speed.
This section will discuss IDLE ONLY. Getting a decent idle may seem a bit over-simplified. You just make sure the air/fuel is decent and your idle timing is set for something like 12º BTDC and move on, right?  No, not necessarily. If you find that your ignition system offers more capabilities that might help improve a stable idle, then try using them. I'm not talking about using idle valves or controlling an electronic throttle body if you have one.  I'm just talking about using ignition timing to optimize your idle.

Let's assume you have a 4 cylinder that has an idle target of 900 RPM and base idle timing is  12º BTDC. If your system will allow you to control ignition timing at idle independently from timing for other uses, then a good way to help maintain a stable idle is to make it so that it increases timing advance if the idle dips below your 900 RPM target. And it reduces timing advance if your idle rises above your target. Some systems will allow you to program such a timing change if the idle deviates more than 50 RPM under or over your target idle. Some systems may not have as much resolution and it may only allow changes if it deviates more than 100 or 250 RPM. A timing change for a deviation of over 50 RPM could feasibly be 5 to 8 degrees. For a 250 RPM deviation it could be 10 to 15 degrees. With this type of system in place, when idle speed dips, ignition timing is increased, which will increase the RPM. When idle speed rises too high, ignition timing is retarded, which will decrease the RPM. This can have a nice stabilizing effect. If you have a programmable system, have a closer look and see if this kind of scheme can be used.

Doing something like this will not usually take the place of your main idle speed controls, such as an air bypass or idle valve. It's only intended as an enhancement, however it's not uncommon for a race car to use something like this without an idle valve or any other device, since a perfect idle is not usually a priority for a race car.

There are some limits to observe if you're planning to set the base idle timing higher than normal. I consider normal to be around 12º to 18º BTDC. If you plan to set your idle at over 20º, then you may find that advancing the timing with this scheme may not increase RPM as easily. The reason is that you may be getting close to MBT Timing (I discussed MBT earlier HERE). Pushing timing advance beyond MBT will not produce more torque or more RPM. If this happens, you could still use the retard side of this scheme to reduce RPM spikes. 
 

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  Here's a good article that will give you some more useful things.
https://www.autospeed.com/cms/a_109132/article

I welcome your comments or questions.  If you can add to any info here, please email me. CONTACT ME.
 

123 IGNITION 123 Ignition in the USA offers standalone ignition and distributor systems for a number of classic European and Japanese cars, including VOLVO. These systems offer several different levels of programming, including Bluetooth support for IOS/Android and tablets.They have distributors for Volvo B21, B23, B230, for naturally aspirated and TURBO. https://123ignitionusa.com/b21-b23-volvo-engines-tune-bluetooth/ They have a tutorial that will help to explain how the timing maps are programmed, including centrifugal advance, vacuum advance and boost retard: https://123ignitionusa.com/curve-tutorial/

Ignition conversion for 1985-88 240 (convert from Chrysler to EZK Ignition). This is a conversion harness for any 240 using LH 2.2 (1985 to 88).  The 1985-88 240 originally came with an undesirable Chrysler ignition system with the fender mounted Ignition Control Unit. This conversion harness eliminates that Chrysler ignition. It's designed to allow you to instead use the more desirable EZ 117 K ignition system. The new ignition box mounts under your dash. The EZ 117 K ignition ECU came in the Volvo 740 series during these same years. Your existing main engine harness is not disturbed for this conversion. All new connections under the hood are PLUG AND PLAY. Some wires will need to be spliced under the dash near the ECU.  This conversion harness is designed to work with your existing LH 2.2 system and it serves to bridge the necessary connections to the more modern EZ 117 K ignition system.  This may be found HERE.

If you own a 240 Turbo and it has the factory ignition system, you might discover that the ignition spark is rather weak.  It's usually ok if there are no performance modifications, but if you increase turbo boost, you'll discover that the spark is not strong enough.  It's called spark blow-out and it'll appear as a strong miss or sudden engine cut-out when under boost. It happens to weak sparks when an elevated level of boost is added. High compression or more boost pressure in the combustion chamber makes it more difficult for a spark plug to ignite the fuel mixture. If this happens to you, a band-aide cure is to reduce the spark plug gap to about half of normal.

Installing a high-performance coil doesn't cure this. Installing better plugs or better plug wires doesn't cure it. BUT INSTALLING AN MSD IGNITION AMPLIFIER DOES CURE IT.
The MSD Digital 6A ignition amplifier (PN 6201) and the MSD Tach Adapter (PN 8910) are needed.  These are added to the existing Volvo ignition. No wires from your existing ignition are removed.
You can read more about what the MSD box does at their site, but in short, it seriously amplifies the spark for your factory ignition and it will fix spark blow-out.


If you need any of these tabs for your coil, you can search for 6 mm Faston Tabs.  They're easily available as single or double tabs like these below.


Here's a diagram showing the same MSD 6A if you have a 1985-88 240 with LH 2.2 and standard ignition (perhaps you have added a turbo).